xen: implement Xen-specific spinlocks
The standard ticket spinlocks are very expensive in a virtual environment, because their performance depends on Xen's scheduler giving vcpus time in the order that they're supposed to take the spinlock. This implements a Xen-specific spinlock, which should be much more efficient. The fast-path is essentially the old Linux-x86 locks, using a single lock byte. The locker decrements the byte; if the result is 0, then they have the lock. If the lock is negative, then locker must spin until the lock is positive again. When there's contention, the locker spin for 2^16[*] iterations waiting to get the lock. If it fails to get the lock in that time, it adds itself to the contention count in the lock and blocks on a per-cpu event channel. When unlocking the spinlock, the locker looks to see if there's anyone blocked waiting for the lock by checking for a non-zero waiter count. If there's a waiter, it traverses the per-cpu "lock_spinners" variable, which contains which lock each CPU is waiting on. It picks one CPU waiting on the lock and sends it an event to wake it up. This allows efficient fast-path spinlock operation, while allowing spinning vcpus to give up their processor time while waiting for a contended lock. [*] 2^16 iterations is threshold at which 98% locks have been taken according to Thomas Friebel's Xen Summit talk "Preventing Guests from Spinning Around". Therefore, we'd expect the lock and unlock slow paths will only be entered 2% of the time. Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Christoph Lameter <clameter@linux-foundation.org> Cc: Petr Tesarik <ptesarik@suse.cz> Cc: Virtualization <virtualization@lists.linux-foundation.org> Cc: Xen devel <xen-devel@lists.xensource.com> Cc: Thomas Friebel <thomas.friebel@amd.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Jeremy Fitzhardinge
Ingo Molnar
parent
56397f8dad
commit
2d9e1e2f58
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@ -15,6 +15,7 @@
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* This does not handle HOTPLUG_CPU yet.
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*/
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#include <linux/sched.h>
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#include <linux/kernel_stat.h>
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#include <linux/err.h>
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#include <linux/smp.h>
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@ -35,6 +36,8 @@
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#include "xen-ops.h"
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#include "mmu.h"
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static void __cpuinit xen_init_lock_cpu(int cpu);
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cpumask_t xen_cpu_initialized_map;
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static DEFINE_PER_CPU(int, resched_irq);
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@ -179,6 +182,8 @@ static void __init xen_smp_prepare_cpus(unsigned int max_cpus)
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{
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unsigned cpu;
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xen_init_lock_cpu(0);
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smp_store_cpu_info(0);
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cpu_data(0).x86_max_cores = 1;
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set_cpu_sibling_map(0);
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@ -301,6 +306,7 @@ static int __cpuinit xen_cpu_up(unsigned int cpu)
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clear_tsk_thread_flag(idle, TIF_FORK);
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#endif
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xen_setup_timer(cpu);
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xen_init_lock_cpu(cpu);
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per_cpu(cpu_state, cpu) = CPU_UP_PREPARE;
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@ -413,6 +419,170 @@ static irqreturn_t xen_call_function_single_interrupt(int irq, void *dev_id)
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return IRQ_HANDLED;
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}
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struct xen_spinlock {
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unsigned char lock; /* 0 -> free; 1 -> locked */
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unsigned short spinners; /* count of waiting cpus */
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};
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static int xen_spin_is_locked(struct raw_spinlock *lock)
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{
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struct xen_spinlock *xl = (struct xen_spinlock *)lock;
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return xl->lock != 0;
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}
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static int xen_spin_is_contended(struct raw_spinlock *lock)
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{
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struct xen_spinlock *xl = (struct xen_spinlock *)lock;
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/* Not strictly true; this is only the count of contended
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lock-takers entering the slow path. */
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return xl->spinners != 0;
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}
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static int xen_spin_trylock(struct raw_spinlock *lock)
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{
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struct xen_spinlock *xl = (struct xen_spinlock *)lock;
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u8 old = 1;
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asm("xchgb %b0,%1"
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: "+q" (old), "+m" (xl->lock) : : "memory");
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return old == 0;
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}
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static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
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static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);
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static inline void spinning_lock(struct xen_spinlock *xl)
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{
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__get_cpu_var(lock_spinners) = xl;
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wmb(); /* set lock of interest before count */
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asm(LOCK_PREFIX " incw %0"
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: "+m" (xl->spinners) : : "memory");
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}
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static inline void unspinning_lock(struct xen_spinlock *xl)
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{
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asm(LOCK_PREFIX " decw %0"
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: "+m" (xl->spinners) : : "memory");
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wmb(); /* decrement count before clearing lock */
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__get_cpu_var(lock_spinners) = NULL;
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}
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static noinline int xen_spin_lock_slow(struct raw_spinlock *lock)
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{
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struct xen_spinlock *xl = (struct xen_spinlock *)lock;
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int irq = __get_cpu_var(lock_kicker_irq);
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int ret;
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/* If kicker interrupts not initialized yet, just spin */
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if (irq == -1)
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return 0;
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/* announce we're spinning */
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spinning_lock(xl);
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/* clear pending */
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xen_clear_irq_pending(irq);
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/* check again make sure it didn't become free while
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we weren't looking */
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ret = xen_spin_trylock(lock);
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if (ret)
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goto out;
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/* block until irq becomes pending */
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xen_poll_irq(irq);
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kstat_this_cpu.irqs[irq]++;
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out:
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unspinning_lock(xl);
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return ret;
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}
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static void xen_spin_lock(struct raw_spinlock *lock)
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{
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struct xen_spinlock *xl = (struct xen_spinlock *)lock;
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int timeout;
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u8 oldval;
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do {
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timeout = 1 << 10;
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asm("1: xchgb %1,%0\n"
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" testb %1,%1\n"
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" jz 3f\n"
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"2: rep;nop\n"
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" cmpb $0,%0\n"
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" je 1b\n"
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" dec %2\n"
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" jnz 2b\n"
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"3:\n"
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: "+m" (xl->lock), "=q" (oldval), "+r" (timeout)
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: "1" (1)
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: "memory");
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} while (unlikely(oldval != 0 && !xen_spin_lock_slow(lock)));
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}
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static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl)
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{
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int cpu;
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for_each_online_cpu(cpu) {
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/* XXX should mix up next cpu selection */
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if (per_cpu(lock_spinners, cpu) == xl) {
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xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
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break;
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}
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}
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}
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static void xen_spin_unlock(struct raw_spinlock *lock)
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{
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struct xen_spinlock *xl = (struct xen_spinlock *)lock;
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smp_wmb(); /* make sure no writes get moved after unlock */
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xl->lock = 0; /* release lock */
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/* make sure unlock happens before kick */
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barrier();
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if (unlikely(xl->spinners))
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xen_spin_unlock_slow(xl);
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}
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static __cpuinit void xen_init_lock_cpu(int cpu)
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{
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int irq;
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const char *name;
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name = kasprintf(GFP_KERNEL, "spinlock%d", cpu);
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irq = bind_ipi_to_irqhandler(XEN_SPIN_UNLOCK_VECTOR,
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cpu,
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xen_reschedule_interrupt,
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IRQF_DISABLED|IRQF_PERCPU|IRQF_NOBALANCING,
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name,
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NULL);
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if (irq >= 0) {
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disable_irq(irq); /* make sure it's never delivered */
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per_cpu(lock_kicker_irq, cpu) = irq;
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}
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printk("cpu %d spinlock event irq %d\n", cpu, irq);
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}
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static void __init xen_init_spinlocks(void)
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{
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pv_lock_ops.spin_is_locked = xen_spin_is_locked;
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pv_lock_ops.spin_is_contended = xen_spin_is_contended;
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pv_lock_ops.spin_lock = xen_spin_lock;
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pv_lock_ops.spin_trylock = xen_spin_trylock;
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pv_lock_ops.spin_unlock = xen_spin_unlock;
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}
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static const struct smp_ops xen_smp_ops __initdata = {
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.smp_prepare_boot_cpu = xen_smp_prepare_boot_cpu,
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.smp_prepare_cpus = xen_smp_prepare_cpus,
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@ -430,5 +600,5 @@ void __init xen_smp_init(void)
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{
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smp_ops = xen_smp_ops;
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xen_fill_possible_map();
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paravirt_use_bytelocks();
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xen_init_spinlocks();
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}
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@ -734,6 +734,33 @@ static void restore_cpu_ipis(unsigned int cpu)
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}
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}
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/* Clear an irq's pending state, in preparation for polling on it */
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void xen_clear_irq_pending(int irq)
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{
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int evtchn = evtchn_from_irq(irq);
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if (VALID_EVTCHN(evtchn))
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clear_evtchn(evtchn);
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}
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/* Poll waiting for an irq to become pending. In the usual case, the
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irq will be disabled so it won't deliver an interrupt. */
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void xen_poll_irq(int irq)
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{
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evtchn_port_t evtchn = evtchn_from_irq(irq);
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if (VALID_EVTCHN(evtchn)) {
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struct sched_poll poll;
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poll.nr_ports = 1;
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poll.timeout = 0;
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poll.ports = &evtchn;
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if (HYPERVISOR_sched_op(SCHEDOP_poll, &poll) != 0)
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BUG();
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}
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}
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void xen_irq_resume(void)
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{
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unsigned int cpu, irq, evtchn;
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@ -5,6 +5,7 @@ enum ipi_vector {
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XEN_RESCHEDULE_VECTOR,
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XEN_CALL_FUNCTION_VECTOR,
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XEN_CALL_FUNCTION_SINGLE_VECTOR,
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XEN_SPIN_UNLOCK_VECTOR,
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XEN_NR_IPIS,
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};
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@ -44,4 +44,11 @@ extern void notify_remote_via_irq(int irq);
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extern void xen_irq_resume(void);
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/* Clear an irq's pending state, in preparation for polling on it */
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void xen_clear_irq_pending(int irq);
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/* Poll waiting for an irq to become pending. In the usual case, the
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irq will be disabled so it won't deliver an interrupt. */
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void xen_poll_irq(int irq);
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#endif /* _XEN_EVENTS_H */
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