OpenCloudOS-Kernel/kernel/locking/rwsem-xadd.c

514 lines
14 KiB
C
Raw Normal View History

/* rwsem.c: R/W semaphores: contention handling functions
*
* Written by David Howells (dhowells@redhat.com).
* Derived from arch/i386/kernel/semaphore.c
rwsem: Implement writer lock-stealing for better scalability Commit 5a505085f043 ("mm/rmap: Convert the struct anon_vma::mutex to an rwsem") changed struct anon_vma::mutex to an rwsem, which caused aim7 fork_test performance to drop by 50%. Yuanhan Liu did the following excellent analysis: https://lkml.org/lkml/2013/1/29/84 and found that the regression is caused by strict, serialized, FIFO sequential write-ownership of rwsems. Ingo suggested implementing opportunistic lock-stealing for the front writer task in the waitqueue. Yuanhan Liu implemented lock-stealing for spinlock-rwsems, which indeed recovered much of the regression - confirming the analysis that the main factor in the regression was the FIFO writer-fairness of rwsems. In this patch we allow lock-stealing to happen when the first waiter is also writer. With that change in place the aim7 fork_test performance is fully recovered on my Intel NHM EP, NHM EX, SNB EP 2S and 4S test-machines. Reported-by: lkp@linux.intel.com Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com> Signed-off-by: Alex Shi <alex.shi@intel.com> Cc: David Howells <dhowells@redhat.com> Cc: Michel Lespinasse <walken@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Anton Blanchard <anton@samba.org> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: paul.gortmaker@windriver.com Link: https://lkml.org/lkml/2013/1/29/84 Link: http://lkml.kernel.org/r/1360069915-31619-1-git-send-email-alex.shi@intel.com [ Small stylistic fixes, updated changelog. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-02-05 21:11:55 +08:00
*
* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
* and Michel Lespinasse <walken@google.com>
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
*
* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
*/
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/export.h>
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
#include <linux/sched/rt.h>
#include "mcs_spinlock.h"
/*
* Guide to the rw_semaphore's count field for common values.
* (32-bit case illustrated, similar for 64-bit)
*
* 0x0000000X (1) X readers active or attempting lock, no writer waiting
* X = #active_readers + #readers attempting to lock
* (X*ACTIVE_BIAS)
*
* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
* attempting to read lock or write lock.
*
* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
* X = #active readers + # readers attempting lock
* (X*ACTIVE_BIAS + WAITING_BIAS)
* (2) 1 writer attempting lock, no waiters for lock
* X-1 = #active readers + #readers attempting lock
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
* (3) 1 writer active, no waiters for lock
* X-1 = #active readers + #readers attempting lock
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
*
* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
* (WAITING_BIAS + ACTIVE_BIAS)
* (2) 1 writer active or attempting lock, no waiters for lock
* (ACTIVE_WRITE_BIAS)
*
* 0xffff0000 (1) There are writers or readers queued but none active
* or in the process of attempting lock.
* (WAITING_BIAS)
* Note: writer can attempt to steal lock for this count by adding
* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
*
* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
*
* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
* the count becomes more than 0 for successful lock acquisition,
* i.e. the case where there are only readers or nobody has lock.
* (1st and 2nd case above).
*
* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
* acquisition (i.e. nobody else has lock or attempts lock). If
* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
* are only waiters but none active (5th case above), and attempt to
* steal the lock.
*
*/
/*
* Initialize an rwsem:
*/
void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/*
* Make sure we are not reinitializing a held semaphore:
*/
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
sem->count = RWSEM_UNLOCKED_VALUE;
raw_spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
sem->owner = NULL;
osq_lock_init(&sem->osq);
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
#endif
}
EXPORT_SYMBOL(__init_rwsem);
enum rwsem_waiter_type {
RWSEM_WAITING_FOR_WRITE,
RWSEM_WAITING_FOR_READ
};
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
};
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
enum rwsem_wake_type {
RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
RWSEM_WAKE_READERS, /* Wake readers only */
RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
};
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then:
* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
* - there must be someone on the queue
* - the spinlock must be held by the caller
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only woken if downgrading is false
*/
static struct rw_semaphore *
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
{
struct rwsem_waiter *waiter;
struct task_struct *tsk;
struct list_head *next;
long oldcount, woken, loop, adjustment;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
if (wake_type == RWSEM_WAKE_ANY)
/* Wake writer at the front of the queue, but do not
* grant it the lock yet as we want other writers
* to be able to steal it. Readers, on the other hand,
* will block as they will notice the queued writer.
*/
wake_up_process(waiter->task);
goto out;
}
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
/* Writers might steal the lock before we grant it to the next reader.
* We prefer to do the first reader grant before counting readers
* so we can bail out early if a writer stole the lock.
*/
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
adjustment = 0;
if (wake_type != RWSEM_WAKE_READ_OWNED) {
adjustment = RWSEM_ACTIVE_READ_BIAS;
try_reader_grant:
oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
/* A writer stole the lock. Undo our reader grant. */
if (rwsem_atomic_update(-adjustment, sem) &
RWSEM_ACTIVE_MASK)
goto out;
/* Last active locker left. Retry waking readers. */
goto try_reader_grant;
}
}
/* Grant an infinite number of read locks to the readers at the front
* of the queue. Note we increment the 'active part' of the count by
* the number of readers before waking any processes up.
*/
woken = 0;
do {
woken++;
if (waiter->list.next == &sem->wait_list)
break;
waiter = list_entry(waiter->list.next,
struct rwsem_waiter, list);
} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
if (waiter->type != RWSEM_WAITING_FOR_WRITE)
/* hit end of list above */
adjustment -= RWSEM_WAITING_BIAS;
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
if (adjustment)
rwsem_atomic_add(adjustment, sem);
next = sem->wait_list.next;
loop = woken;
do {
waiter = list_entry(next, struct rwsem_waiter, list);
next = waiter->list.next;
tsk = waiter->task;
smp_mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
} while (--loop);
sem->wait_list.next = next;
next->prev = &sem->wait_list;
out:
return sem;
rwsem: Implement writer lock-stealing for better scalability Commit 5a505085f043 ("mm/rmap: Convert the struct anon_vma::mutex to an rwsem") changed struct anon_vma::mutex to an rwsem, which caused aim7 fork_test performance to drop by 50%. Yuanhan Liu did the following excellent analysis: https://lkml.org/lkml/2013/1/29/84 and found that the regression is caused by strict, serialized, FIFO sequential write-ownership of rwsems. Ingo suggested implementing opportunistic lock-stealing for the front writer task in the waitqueue. Yuanhan Liu implemented lock-stealing for spinlock-rwsems, which indeed recovered much of the regression - confirming the analysis that the main factor in the regression was the FIFO writer-fairness of rwsems. In this patch we allow lock-stealing to happen when the first waiter is also writer. With that change in place the aim7 fork_test performance is fully recovered on my Intel NHM EP, NHM EX, SNB EP 2S and 4S test-machines. Reported-by: lkp@linux.intel.com Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com> Signed-off-by: Alex Shi <alex.shi@intel.com> Cc: David Howells <dhowells@redhat.com> Cc: Michel Lespinasse <walken@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Anton Blanchard <anton@samba.org> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: paul.gortmaker@windriver.com Link: https://lkml.org/lkml/2013/1/29/84 Link: http://lkml.kernel.org/r/1360069915-31619-1-git-send-email-alex.shi@intel.com [ Small stylistic fixes, updated changelog. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
2013-02-05 21:11:55 +08:00
}
/*
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
* Wait for the read lock to be granted
*/
__visible
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
{
long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
struct rwsem_waiter waiter;
struct task_struct *tsk = current;
/* set up my own style of waitqueue */
waiter.task = tsk;
waiter.type = RWSEM_WAITING_FOR_READ;
get_task_struct(tsk);
raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list))
adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
count = rwsem_atomic_update(adjustment, sem);
/* If there are no active locks, wake the front queued process(es).
*
* If there are no writers and we are first in the queue,
* wake our own waiter to join the existing active readers !
*/
if (count == RWSEM_WAITING_BIAS ||
(count > RWSEM_WAITING_BIAS &&
adjustment != -RWSEM_ACTIVE_READ_BIAS))
rwsem: implement support for write lock stealing on the fastpath When we decide to wake up readers, we must first grant them as many read locks as necessary, and then actually wake up all these readers. But in order to know how many read shares to grant, we must first count the readers at the head of the queue. This might take a while if there are many readers, and we want to be protected against a writer stealing the lock while we're counting. To that end, we grant the first reader lock before counting how many more readers are queued. We also require some adjustments to the wake_type semantics. RWSEM_WAKE_NO_ACTIVE used to mean that we had found the count to be RWSEM_WAITING_BIAS, in which case the rwsem was known to be free as nobody could steal it while we hold the wait_lock. This doesn't make sense once we implement fastpath write lock stealing, so we now use RWSEM_WAKE_ANY in that case. Similarly, when rwsem_down_write_failed found that a read lock was active, it would use RWSEM_WAKE_READ_OWNED which signalled that new readers could be woken without checking first that the rwsem was available. We can't do that anymore since the existing readers might release their read locks, and a writer could steal the lock before we wake up additional readers. So, we have to use a new RWSEM_WAKE_READERS value to indicate we only want to wake readers, but we don't currently hold any read lock. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-05-07 21:45:59 +08:00
sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
raw_spin_unlock_irq(&sem->wait_lock);
/* wait to be given the lock */
while (true) {
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (!waiter.task)
break;
schedule();
}
tsk->state = TASK_RUNNING;
return sem;
}
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
if (!(count & RWSEM_ACTIVE_MASK)) {
/* try acquiring the write lock */
if (sem->count == RWSEM_WAITING_BIAS &&
cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
if (!list_is_singular(&sem->wait_list))
rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
return true;
}
}
return false;
}
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
* Try to acquire write lock before the writer has been put on wait queue.
*/
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
long old, count = ACCESS_ONCE(sem->count);
while (true) {
if (!(count == 0 || count == RWSEM_WAITING_BIAS))
return false;
old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
if (old == count)
return true;
count = old;
}
}
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner;
locking/rwsem: Allow conservative optimistic spinning when readers have lock Commit 4fc828e24cd9 ("locking/rwsem: Support optimistic spinning") introduced a major performance regression for workloads such as xfs_repair which mix read and write locking of the mmap_sem across many threads. The result was xfs_repair ran 5x slower on 3.16-rc2 than on 3.15 and using 20x more system CPU time. Perf profiles indicate in some workloads that significant time can be spent spinning on !owner. This is because we don't set the lock owner when readers(s) obtain the rwsem. In this patch, we'll modify rwsem_can_spin_on_owner() such that we'll return false if there is no lock owner. The rationale is that if we just entered the slowpath, yet there is no lock owner, then there is a possibility that a reader has the lock. To be conservative, we'll avoid spinning in these situations. This patch reduced the total run time of the xfs_repair workload from about 4 minutes 24 seconds down to approximately 1 minute 26 seconds, back to close to the same performance as on 3.15. Retesting of AIM7, which were some of the workloads used to test the original optimistic spinning code, confirmed that we still get big performance gains with optimistic spinning, even with this additional regression fix. Davidlohr found that while the 'custom' workload took a performance hit of ~-14% to throughput for >300 users with this additional patch, the overall gain with optimistic spinning is still ~+45%. The 'disk' workload even improved by ~+15% at >1000 users. Tested-by: Dave Chinner <dchinner@redhat.com> Acked-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Jason Low <jason.low2@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1404532172.2572.30.camel@j-VirtualBox Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-07-05 11:49:32 +08:00
bool on_cpu = false;
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
if (need_resched())
locking/rwsem: Allow conservative optimistic spinning when readers have lock Commit 4fc828e24cd9 ("locking/rwsem: Support optimistic spinning") introduced a major performance regression for workloads such as xfs_repair which mix read and write locking of the mmap_sem across many threads. The result was xfs_repair ran 5x slower on 3.16-rc2 than on 3.15 and using 20x more system CPU time. Perf profiles indicate in some workloads that significant time can be spent spinning on !owner. This is because we don't set the lock owner when readers(s) obtain the rwsem. In this patch, we'll modify rwsem_can_spin_on_owner() such that we'll return false if there is no lock owner. The rationale is that if we just entered the slowpath, yet there is no lock owner, then there is a possibility that a reader has the lock. To be conservative, we'll avoid spinning in these situations. This patch reduced the total run time of the xfs_repair workload from about 4 minutes 24 seconds down to approximately 1 minute 26 seconds, back to close to the same performance as on 3.15. Retesting of AIM7, which were some of the workloads used to test the original optimistic spinning code, confirmed that we still get big performance gains with optimistic spinning, even with this additional regression fix. Davidlohr found that while the 'custom' workload took a performance hit of ~-14% to throughput for >300 users with this additional patch, the overall gain with optimistic spinning is still ~+45%. The 'disk' workload even improved by ~+15% at >1000 users. Tested-by: Dave Chinner <dchinner@redhat.com> Acked-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Jason Low <jason.low2@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1404532172.2572.30.camel@j-VirtualBox Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-07-05 11:49:32 +08:00
return false;
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
rcu_read_lock();
owner = ACCESS_ONCE(sem->owner);
if (owner)
on_cpu = owner->on_cpu;
rcu_read_unlock();
/*
locking/rwsem: Allow conservative optimistic spinning when readers have lock Commit 4fc828e24cd9 ("locking/rwsem: Support optimistic spinning") introduced a major performance regression for workloads such as xfs_repair which mix read and write locking of the mmap_sem across many threads. The result was xfs_repair ran 5x slower on 3.16-rc2 than on 3.15 and using 20x more system CPU time. Perf profiles indicate in some workloads that significant time can be spent spinning on !owner. This is because we don't set the lock owner when readers(s) obtain the rwsem. In this patch, we'll modify rwsem_can_spin_on_owner() such that we'll return false if there is no lock owner. The rationale is that if we just entered the slowpath, yet there is no lock owner, then there is a possibility that a reader has the lock. To be conservative, we'll avoid spinning in these situations. This patch reduced the total run time of the xfs_repair workload from about 4 minutes 24 seconds down to approximately 1 minute 26 seconds, back to close to the same performance as on 3.15. Retesting of AIM7, which were some of the workloads used to test the original optimistic spinning code, confirmed that we still get big performance gains with optimistic spinning, even with this additional regression fix. Davidlohr found that while the 'custom' workload took a performance hit of ~-14% to throughput for >300 users with this additional patch, the overall gain with optimistic spinning is still ~+45%. The 'disk' workload even improved by ~+15% at >1000 users. Tested-by: Dave Chinner <dchinner@redhat.com> Acked-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Jason Low <jason.low2@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1404532172.2572.30.camel@j-VirtualBox Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-07-05 11:49:32 +08:00
* If sem->owner is not set, yet we have just recently entered the
* slowpath, then there is a possibility reader(s) may have the lock.
* To be safe, avoid spinning in these situations.
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
*/
return on_cpu;
}
static inline bool owner_running(struct rw_semaphore *sem,
struct task_struct *owner)
{
if (sem->owner != owner)
return false;
/*
* Ensure we emit the owner->on_cpu, dereference _after_ checking
* sem->owner still matches owner, if that fails, owner might
* point to free()d memory, if it still matches, the rcu_read_lock()
* ensures the memory stays valid.
*/
barrier();
return owner->on_cpu;
}
static noinline
bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
{
rcu_read_lock();
while (owner_running(sem, owner)) {
if (need_resched())
break;
arch, locking: Ciao arch_mutex_cpu_relax() The arch_mutex_cpu_relax() function, introduced by 34b133f, is hacky and ugly. It was added a few years ago to address the fact that common cpu_relax() calls include yielding on s390, and thus impact the optimistic spinning functionality of mutexes. Nowadays we use this function well beyond mutexes: rwsem, qrwlock, mcs and lockref. Since the macro that defines the call is in the mutex header, any users must include mutex.h and the naming is misleading as well. This patch (i) renames the call to cpu_relax_lowlatency ("relax, but only if you can do it with very low latency") and (ii) defines it in each arch's asm/processor.h local header, just like for regular cpu_relax functions. On all archs, except s390, cpu_relax_lowlatency is simply cpu_relax, and thus we can take it out of mutex.h. While this can seem redundant, I believe it is a good choice as it allows us to move out arch specific logic from generic locking primitives and enables future(?) archs to transparently define it, similarly to System Z. Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Anton Blanchard <anton@samba.org> Cc: Aurelien Jacquiot <a-jacquiot@ti.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Bharat Bhushan <r65777@freescale.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Chen Liqin <liqin.linux@gmail.com> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Chris Zankel <chris@zankel.net> Cc: David Howells <dhowells@redhat.com> Cc: David S. Miller <davem@davemloft.net> Cc: Deepthi Dharwar <deepthi@linux.vnet.ibm.com> Cc: Dominik Dingel <dingel@linux.vnet.ibm.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Guan Xuetao <gxt@mprc.pku.edu.cn> Cc: Haavard Skinnemoen <hskinnemoen@gmail.com> Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Helge Deller <deller@gmx.de> Cc: Hirokazu Takata <takata@linux-m32r.org> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: James E.J. Bottomley <jejb@parisc-linux.org> Cc: James Hogan <james.hogan@imgtec.com> Cc: Jason Wang <jasowang@redhat.com> Cc: Jesper Nilsson <jesper.nilsson@axis.com> Cc: Joe Perches <joe@perches.com> Cc: Jonas Bonn <jonas@southpole.se> Cc: Joseph Myers <joseph@codesourcery.com> Cc: Kees Cook <keescook@chromium.org> Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com> Cc: Lennox Wu <lennox.wu@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mark Salter <msalter@redhat.com> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Matt Turner <mattst88@gmail.com> Cc: Max Filippov <jcmvbkbc@gmail.com> Cc: Michael Neuling <mikey@neuling.org> Cc: Michal Simek <monstr@monstr.eu> Cc: Mikael Starvik <starvik@axis.com> Cc: Nicolas Pitre <nico@linaro.org> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Paul Burton <paul.burton@imgtec.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Paul Mackerras <paulus@samba.org> Cc: Qais Yousef <qais.yousef@imgtec.com> Cc: Qiaowei Ren <qiaowei.ren@intel.com> Cc: Rafael Wysocki <rafael.j.wysocki@intel.com> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Richard Henderson <rth@twiddle.net> Cc: Richard Kuo <rkuo@codeaurora.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Steven Miao <realmz6@gmail.com> Cc: Steven Rostedt <srostedt@redhat.com> Cc: Stratos Karafotis <stratosk@semaphore.gr> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Vasily Kulikov <segoon@openwall.com> Cc: Vineet Gupta <vgupta@synopsys.com> Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com> Cc: Waiman Long <Waiman.Long@hp.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Wolfram Sang <wsa@the-dreams.de> Cc: adi-buildroot-devel@lists.sourceforge.net Cc: linux390@de.ibm.com Cc: linux-alpha@vger.kernel.org Cc: linux-am33-list@redhat.com Cc: linux-arm-kernel@lists.infradead.org Cc: linux-c6x-dev@linux-c6x.org Cc: linux-cris-kernel@axis.com Cc: linux-hexagon@vger.kernel.org Cc: linux-ia64@vger.kernel.org Cc: linux@lists.openrisc.net Cc: linux-m32r-ja@ml.linux-m32r.org Cc: linux-m32r@ml.linux-m32r.org Cc: linux-m68k@lists.linux-m68k.org Cc: linux-metag@vger.kernel.org Cc: linux-mips@linux-mips.org Cc: linux-parisc@vger.kernel.org Cc: linuxppc-dev@lists.ozlabs.org Cc: linux-s390@vger.kernel.org Cc: linux-sh@vger.kernel.org Cc: linux-xtensa@linux-xtensa.org Cc: sparclinux@vger.kernel.org Link: http://lkml.kernel.org/r/1404079773.2619.4.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-06-30 06:09:33 +08:00
cpu_relax_lowlatency();
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
}
rcu_read_unlock();
/*
* We break out the loop above on need_resched() or when the
* owner changed, which is a sign for heavy contention. Return
* success only when sem->owner is NULL.
*/
return sem->owner == NULL;
}
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
struct task_struct *owner;
bool taken = false;
preempt_disable();
/* sem->wait_lock should not be held when doing optimistic spinning */
if (!rwsem_can_spin_on_owner(sem))
goto done;
if (!osq_lock(&sem->osq))
goto done;
while (true) {
owner = ACCESS_ONCE(sem->owner);
if (owner && !rwsem_spin_on_owner(sem, owner))
break;
/* wait_lock will be acquired if write_lock is obtained */
if (rwsem_try_write_lock_unqueued(sem)) {
taken = true;
break;
}
/*
* When there's no owner, we might have preempted between the
* owner acquiring the lock and setting the owner field. If
* we're an RT task that will live-lock because we won't let
* the owner complete.
*/
if (!owner && (need_resched() || rt_task(current)))
break;
/*
* The cpu_relax() call is a compiler barrier which forces
* everything in this loop to be re-loaded. We don't need
* memory barriers as we'll eventually observe the right
* values at the cost of a few extra spins.
*/
arch, locking: Ciao arch_mutex_cpu_relax() The arch_mutex_cpu_relax() function, introduced by 34b133f, is hacky and ugly. It was added a few years ago to address the fact that common cpu_relax() calls include yielding on s390, and thus impact the optimistic spinning functionality of mutexes. Nowadays we use this function well beyond mutexes: rwsem, qrwlock, mcs and lockref. Since the macro that defines the call is in the mutex header, any users must include mutex.h and the naming is misleading as well. This patch (i) renames the call to cpu_relax_lowlatency ("relax, but only if you can do it with very low latency") and (ii) defines it in each arch's asm/processor.h local header, just like for regular cpu_relax functions. On all archs, except s390, cpu_relax_lowlatency is simply cpu_relax, and thus we can take it out of mutex.h. While this can seem redundant, I believe it is a good choice as it allows us to move out arch specific logic from generic locking primitives and enables future(?) archs to transparently define it, similarly to System Z. Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Anton Blanchard <anton@samba.org> Cc: Aurelien Jacquiot <a-jacquiot@ti.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Bharat Bhushan <r65777@freescale.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Chen Liqin <liqin.linux@gmail.com> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Chris Zankel <chris@zankel.net> Cc: David Howells <dhowells@redhat.com> Cc: David S. Miller <davem@davemloft.net> Cc: Deepthi Dharwar <deepthi@linux.vnet.ibm.com> Cc: Dominik Dingel <dingel@linux.vnet.ibm.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Guan Xuetao <gxt@mprc.pku.edu.cn> Cc: Haavard Skinnemoen <hskinnemoen@gmail.com> Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Helge Deller <deller@gmx.de> Cc: Hirokazu Takata <takata@linux-m32r.org> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: James E.J. Bottomley <jejb@parisc-linux.org> Cc: James Hogan <james.hogan@imgtec.com> Cc: Jason Wang <jasowang@redhat.com> Cc: Jesper Nilsson <jesper.nilsson@axis.com> Cc: Joe Perches <joe@perches.com> Cc: Jonas Bonn <jonas@southpole.se> Cc: Joseph Myers <joseph@codesourcery.com> Cc: Kees Cook <keescook@chromium.org> Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com> Cc: Lennox Wu <lennox.wu@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mark Salter <msalter@redhat.com> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Matt Turner <mattst88@gmail.com> Cc: Max Filippov <jcmvbkbc@gmail.com> Cc: Michael Neuling <mikey@neuling.org> Cc: Michal Simek <monstr@monstr.eu> Cc: Mikael Starvik <starvik@axis.com> Cc: Nicolas Pitre <nico@linaro.org> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Paul Burton <paul.burton@imgtec.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Paul Mackerras <paulus@samba.org> Cc: Qais Yousef <qais.yousef@imgtec.com> Cc: Qiaowei Ren <qiaowei.ren@intel.com> Cc: Rafael Wysocki <rafael.j.wysocki@intel.com> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Richard Henderson <rth@twiddle.net> Cc: Richard Kuo <rkuo@codeaurora.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Steven Miao <realmz6@gmail.com> Cc: Steven Rostedt <srostedt@redhat.com> Cc: Stratos Karafotis <stratosk@semaphore.gr> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Vasily Kulikov <segoon@openwall.com> Cc: Vineet Gupta <vgupta@synopsys.com> Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com> Cc: Waiman Long <Waiman.Long@hp.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Wolfram Sang <wsa@the-dreams.de> Cc: adi-buildroot-devel@lists.sourceforge.net Cc: linux390@de.ibm.com Cc: linux-alpha@vger.kernel.org Cc: linux-am33-list@redhat.com Cc: linux-arm-kernel@lists.infradead.org Cc: linux-c6x-dev@linux-c6x.org Cc: linux-cris-kernel@axis.com Cc: linux-hexagon@vger.kernel.org Cc: linux-ia64@vger.kernel.org Cc: linux@lists.openrisc.net Cc: linux-m32r-ja@ml.linux-m32r.org Cc: linux-m32r@ml.linux-m32r.org Cc: linux-m68k@lists.linux-m68k.org Cc: linux-metag@vger.kernel.org Cc: linux-mips@linux-mips.org Cc: linux-parisc@vger.kernel.org Cc: linuxppc-dev@lists.ozlabs.org Cc: linux-s390@vger.kernel.org Cc: linux-sh@vger.kernel.org Cc: linux-xtensa@linux-xtensa.org Cc: sparclinux@vger.kernel.org Link: http://lkml.kernel.org/r/1404079773.2619.4.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-06-30 06:09:33 +08:00
cpu_relax_lowlatency();
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
}
osq_unlock(&sem->osq);
done:
preempt_enable();
return taken;
}
#else
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
return false;
}
#endif
/*
* Wait until we successfully acquire the write lock
*/
__visible
struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
{
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
long count;
bool waiting = true; /* any queued threads before us */
struct rwsem_waiter waiter;
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
/* undo write bias from down_write operation, stop active locking */
count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
/* do optimistic spinning and steal lock if possible */
if (rwsem_optimistic_spin(sem))
return sem;
/*
* Optimistic spinning failed, proceed to the slowpath
* and block until we can acquire the sem.
*/
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
raw_spin_lock_irq(&sem->wait_lock);
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
/* account for this before adding a new element to the list */
if (list_empty(&sem->wait_list))
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
waiting = false;
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
if (waiting) {
count = ACCESS_ONCE(sem->count);
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
/*
* If there were already threads queued before us and there are
* no active writers, the lock must be read owned; so we try to
* wake any read locks that were queued ahead of us.
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
*/
if (count > RWSEM_WAITING_BIAS)
sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
} else
count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
/* wait until we successfully acquire the lock */
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
set_current_state(TASK_UNINTERRUPTIBLE);
while (true) {
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
if (rwsem_try_write_lock(count, sem))
break;
raw_spin_unlock_irq(&sem->wait_lock);
/* Block until there are no active lockers. */
do {
schedule();
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
set_current_state(TASK_UNINTERRUPTIBLE);
} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
raw_spin_lock_irq(&sem->wait_lock);
}
locking/rwsem: Support optimistic spinning We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
2014-05-03 02:24:15 +08:00
__set_current_state(TASK_RUNNING);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
return sem;
}
/*
* handle waking up a waiter on the semaphore
* - up_read/up_write has decremented the active part of count if we come here
*/
__visible
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
{
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
/* do nothing if list empty */
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return sem;
}
/*
* downgrade a write lock into a read lock
* - caller incremented waiting part of count and discovered it still negative
* - just wake up any readers at the front of the queue
*/
__visible
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
unsigned long flags;
raw_spin_lock_irqsave(&sem->wait_lock, flags);
/* do nothing if list empty */
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
return sem;
}
EXPORT_SYMBOL(rwsem_down_read_failed);
EXPORT_SYMBOL(rwsem_down_write_failed);
EXPORT_SYMBOL(rwsem_wake);
EXPORT_SYMBOL(rwsem_downgrade_wake);