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rcu: Remove local_irq_disable() in rcu_preempt_note_context_switch() 

The rcu_preempt_note_context_switch() function is on a scheduling fast
path, so it would be good to avoid disabling irqs.  The reason that irqs
are disabled is to synchronize process-level and irq-handler access to
the task_struct ->rcu_read_unlock_special bitmask.  This commit therefore
makes ->rcu_read_unlock_special instead be a union of bools with a short
allowing single-access checks in RCU's __rcu_read_unlock().  This results
in the process-level and irq-handler accesses being simple loads and
stores, so that irqs need no longer be disabled.  This commit therefore
removes the irq disabling from rcu_preempt_note_context_switch().

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2014-08-14 16:01:53 -07:00
parent 4ff475ed4c
commit 1d082fd061
4 changed files with 26 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
include/linux/init_task.h
View File

@ -111,7 +111,7 @@ extern struct group_info init_groups;
#ifdef CONFIG_PREEMPT_RCU
#define INIT_TASK_RCU_PREEMPT(tsk) \
.rcu_read_lock_nesting = 0, \
.rcu_read_unlock_special = 0, \
.rcu_read_unlock_special.s = 0, \
.rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry), \
INIT_TASK_RCU_TREE_PREEMPT()
#else

16
include/linux/sched.h
View File

@ -1212,6 +1212,13 @@ struct sched_dl_entity {
struct hrtimer dl_timer;
};
union rcu_special {
struct {
bool blocked;
bool need_qs;
} b;
short s;
};
struct rcu_node;
enum perf_event_task_context {
@ -1264,7 +1271,7 @@ struct task_struct {
#ifdef CONFIG_PREEMPT_RCU
int rcu_read_lock_nesting;
char rcu_read_unlock_special;
union rcu_special rcu_read_unlock_special;
struct list_head rcu_node_entry;
#endif /* #ifdef CONFIG_PREEMPT_RCU */
#ifdef CONFIG_TREE_PREEMPT_RCU
@ -2005,16 +2012,11 @@ extern void task_clear_jobctl_trapping(struct task_struct *task);
extern void task_clear_jobctl_pending(struct task_struct *task,
unsigned int mask);
#ifdef CONFIG_PREEMPT_RCU
#define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
#endif /* #ifdef CONFIG_PREEMPT_RCU */
static inline void rcu_copy_process(struct task_struct *p)
{
#ifdef CONFIG_PREEMPT_RCU
p->rcu_read_lock_nesting = 0;
p->rcu_read_unlock_special = 0;
p->rcu_read_unlock_special.s = 0;
p->rcu_blocked_node = NULL;
INIT_LIST_HEAD(&p->rcu_node_entry);
#endif /* #ifdef CONFIG_PREEMPT_RCU */

32
kernel/rcu/tree_plugin.h
View File

@ -155,9 +155,8 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
* not in a quiescent state. There might be any number of tasks blocked
* while in an RCU read-side critical section.
*
* Unlike the other rcu_*_qs() functions, callers to this function
* must disable irqs in order to protect the assignment to
* ->rcu_read_unlock_special.
* As with the other rcu_*_qs() functions, callers to this function
* must disable preemption.
*/
static void rcu_preempt_qs(int cpu)
{
@ -166,7 +165,7 @@ static void rcu_preempt_qs(int cpu)
if (rdp->passed_quiesce == 0)
trace_rcu_grace_period(TPS("rcu_preempt"), rdp->gpnum, TPS("cpuqs"));
rdp->passed_quiesce = 1;
current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
current->rcu_read_unlock_special.b.need_qs = false;
}
/*
@ -190,14 +189,14 @@ static void rcu_preempt_note_context_switch(int cpu)
struct rcu_node *rnp;
if (t->rcu_read_lock_nesting > 0 &&
(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
!t->rcu_read_unlock_special.b.blocked) {
/* Possibly blocking in an RCU read-side critical section. */
rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
rnp = rdp->mynode;
raw_spin_lock_irqsave(&rnp->lock, flags);
smp_mb__after_unlock_lock();
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
t->rcu_read_unlock_special.b.blocked = true;
t->rcu_blocked_node = rnp;
/*
@ -239,7 +238,7 @@ static void rcu_preempt_note_context_switch(int cpu)
: rnp->gpnum + 1);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
} else if (t->rcu_read_lock_nesting < 0 &&
t->rcu_read_unlock_special) {
t->rcu_read_unlock_special.s) {
/*
* Complete exit from RCU read-side critical section on
@ -257,9 +256,7 @@ static void rcu_preempt_note_context_switch(int cpu)
* grace period, then the fact that the task has been enqueued
* means that we continue to block the current grace period.
*/
local_irq_save(flags);
rcu_preempt_qs(cpu);
local_irq_restore(flags);
}
/*
@ -340,7 +337,7 @@ void rcu_read_unlock_special(struct task_struct *t)
bool drop_boost_mutex = false;
#endif /* #ifdef CONFIG_RCU_BOOST */
struct rcu_node *rnp;
int special;
union rcu_special special;
/* NMI handlers cannot block and cannot safely manipulate state. */
if (in_nmi())
@ -350,12 +347,13 @@ void rcu_read_unlock_special(struct task_struct *t)
/*
* If RCU core is waiting for this CPU to exit critical section,
* let it know that we have done so.
* let it know that we have done so. Because irqs are disabled,
* t->rcu_read_unlock_special cannot change.
*/
special = t->rcu_read_unlock_special;
if (special & RCU_READ_UNLOCK_NEED_QS) {
if (special.b.need_qs) {
rcu_preempt_qs(smp_processor_id());
if (!t->rcu_read_unlock_special) {
if (!t->rcu_read_unlock_special.s) {
local_irq_restore(flags);
return;
}
@ -368,8 +366,8 @@ void rcu_read_unlock_special(struct task_struct *t)
}
/* Clean up if blocked during RCU read-side critical section. */
if (special & RCU_READ_UNLOCK_BLOCKED) {
t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
if (special.b.blocked) {
t->rcu_read_unlock_special.b.blocked = false;
/*
* Remove this task from the list it blocked on. The
@ -658,7 +656,7 @@ static void rcu_preempt_check_callbacks(int cpu)
}
if (t->rcu_read_lock_nesting > 0 &&
per_cpu(rcu_preempt_data, cpu).qs_pending)
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
t->rcu_read_unlock_special.b.need_qs = true;
}
#ifdef CONFIG_RCU_BOOST
@ -941,7 +939,7 @@ void exit_rcu(void)
return;
t->rcu_read_lock_nesting = 1;
barrier();
t->rcu_read_unlock_special = RCU_READ_UNLOCK_BLOCKED;
t->rcu_read_unlock_special.b.blocked = true;
__rcu_read_unlock();
}

2
kernel/rcu/update.c
View File

@ -93,7 +93,7 @@ void __rcu_read_unlock(void)
barrier(); /* critical section before exit code. */
t->rcu_read_lock_nesting = INT_MIN;
barrier(); /* assign before ->rcu_read_unlock_special load */
if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special.s)))
rcu_read_unlock_special(t);
barrier(); /* ->rcu_read_unlock_special load before assign */
t->rcu_read_lock_nesting = 0;