From 6ccdc84b81a0a6c09a7f0427761d2f8cecfc2218 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 24 Apr 2014 12:00:47 +0200 Subject: [PATCH] sched: Skip double execution of pick_next_task_fair() Tim wrote: "The current code will call pick_next_task_fair a second time in the slow path if we did not pull any task in our first try. This is really unnecessary as we already know no task can be pulled and it doubles the delay for the cpu to enter idle. We instrumented some network workloads and that saw that pick_next_task_fair is frequently called twice before a cpu enters idle. The call to pick_next_task_fair can add non trivial latency as it calls load_balance which runs find_busiest_group on an hierarchy of sched domains spanning the cpus for a large system. For some 4 socket systems, we saw almost 0.25 msec spent per call of pick_next_task_fair before a cpu can be idled." Optimize the second call away for the common case and document the dependency. Reported-by: Tim Chen Signed-off-by: Peter Zijlstra Cc: Linus Torvalds Cc: Len Brown Link: http://lkml.kernel.org/r/20140424100047.GP11096@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index e62c65a12d5b..28921ec91b3d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2592,8 +2592,14 @@ pick_next_task(struct rq *rq, struct task_struct *prev) if (likely(prev->sched_class == class && rq->nr_running == rq->cfs.h_nr_running)) { p = fair_sched_class.pick_next_task(rq, prev); - if (likely(p && p != RETRY_TASK)) - return p; + if (unlikely(p == RETRY_TASK)) + goto again; + + /* assumes fair_sched_class->next == idle_sched_class */ + if (unlikely(!p)) + p = idle_sched_class.pick_next_task(rq, prev); + + return p; } again: