From 4a81e8328d3791a4f99bf5b436d050f6dc5ffea3 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 20 Jun 2014 16:49:01 -0700 Subject: 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 Signed-off-by: Paul E. McKenney Cc: Andi Kleen Cc: Christoph Lameter Cc: Mike Galbraith Cc: Eric Dumazet Reviewed-by: Josh Triplett [ 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 --- kernel/sched/core.c | 7 +------ 1 file changed, 1 insertion(+), 6 deletions(-) (limited to 'kernel/sched/core.c') 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(); -- cgit v1.2.3