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sched,rcu: Make cond_resched() report RCU quiescent states

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Given a CPU running a loop containing cond_resched(), with no
other tasks runnable on that CPU, RCU will eventually report RCU
CPU stall warnings due to lack of quiescent states.  Fortunately,
every call to cond_resched() is a perfectly good quiescent state.
Unfortunately, invoking rcu_note_context_switch() is a bit heavyweight
for cond_resched(), especially given the need to disable preemption,
and, for RCU-preempt, interrupts as well.

This commit therefore maintains a per-CPU counter that causes
cond_resched(), cond_resched_lock(), and cond_resched_softirq() to call
rcu_note_context_switch(), but only about once per 256 invocations.
This ratio was chosen in keeping with the relative time constants of
RCU grace periods.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney
2014-03-16 21:36:25 -07:00
parent 0e980234c9
commit ac1bea8578
3 changed files with 60 additions and 1 deletions
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18
kernel/rcu/update.c
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@@ -338,3 +338,21 @@ static int __init check_cpu_stall_init(void)
early_initcall(check_cpu_stall_init);
#endif /* #ifdef CONFIG_RCU_STALL_COMMON */
/*
* Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
*/
DEFINE_PER_CPU(int, rcu_cond_resched_count);
/*
* Report a set of RCU quiescent states, for use by cond_resched()
* and friends. Out of line due to being called infrequently.
*/
void rcu_resched(void)
{
preempt_disable();
__this_cpu_write(rcu_cond_resched_count, 0);
rcu_note_context_switch(smp_processor_id());
preempt_enable();
}

7
kernel/sched/core.c
View File

@@ -4051,6 +4051,7 @@ static void __cond_resched(void)
int __sched _cond_resched(void)
{
rcu_cond_resched();
if (should_resched()) {
__cond_resched();
return 1;
@@ -4069,15 +4070,18 @@ EXPORT_SYMBOL(_cond_resched);
*/
int __cond_resched_lock(spinlock_t *lock)
{
bool need_rcu_resched = rcu_should_resched();
int resched = should_resched();
int ret = 0;
lockdep_assert_held(lock);
if (spin_needbreak(lock) || resched) {
if (spin_needbreak(lock) || resched || need_rcu_resched) {
spin_unlock(lock);
if (resched)
__cond_resched();
else if (unlikely(need_rcu_resched))
rcu_resched();
else
cpu_relax();
ret = 1;
@@ -4091,6 +4095,7 @@ int __sched __cond_resched_softirq(void)
{
BUG_ON(!in_softirq());
rcu_cond_resched(); /* BH disabled OK, just recording QSes. */
if (should_resched()) {
local_bh_enable();
__cond_resched();