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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-06-20 16:49:01 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-06-23 11:19:32 -0700
commit4a81e8328d3791a4f99bf5b436d050f6dc5ffea3 (patch)
tree9ffd05b47ffd38639ec904d3329eba1060d3acae /kernel/sched
parent546a9d8519ed137b2804a3f5a3659003039dd49c (diff)
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rcu: Reduce overhead of cond_resched() checks for RCU
Commit ac1bea85781e (Make cond_resched() report RCU quiescent states) fixed a problem where a CPU looping in the kernel with but one runnable task would give RCU CPU stall warnings, even if the in-kernel loop contained cond_resched() calls. Unfortunately, in so doing, it introduced performance regressions in Anton Blanchard's will-it-scale "open1" test. The problem appears to be not so much the increased cond_resched() path length as an increase in the rate at which grace periods complete, which increased per-update grace-period overhead. This commit takes a different approach to fixing this bug, mainly by moving the RCU-visible quiescent state from cond_resched() to rcu_note_context_switch(), and by further reducing the check to a simple non-zero test of a single per-CPU variable. However, this approach requires that the force-quiescent-state processing send resched IPIs to the offending CPUs. These will be sent only once the grace period has reached an age specified by the boot/sysfs parameter rcutree.jiffies_till_sched_qs, or once the grace period reaches an age halfway to the point at which RCU CPU stall warnings will be emitted, whichever comes first. Reported-by: Dave Hansen <dave.hansen@intel.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Christoph Lameter <cl@gentwo.org> Cc: Mike Galbraith <umgwanakikbuti@gmail.com> Cc: Eric Dumazet <eric.dumazet@gmail.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> [ paulmck: Made rcu_momentary_dyntick_idle() as suggested by the ktest build robot. Also fixed smp_mb() comment as noted by Oleg Nesterov. ] Merge with e552592e (Reduce overhead of cond_resched() checks for RCU) Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/core.c7
1 files changed, 1 insertions, 6 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3bdf01b494fe..bc1638b33449 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4147,7 +4147,6 @@ static void __cond_resched(void)
int __sched _cond_resched(void)
{
- rcu_cond_resched();
if (should_resched()) {
__cond_resched();
return 1;
@@ -4166,18 +4165,15 @@ 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 || need_rcu_resched) {
+ if (spin_needbreak(lock) || resched) {
spin_unlock(lock);
if (resched)
__cond_resched();
- else if (unlikely(need_rcu_resched))
- rcu_resched();
else
cpu_relax();
ret = 1;
@@ -4191,7 +4187,6 @@ 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();