Commit 49ee5768 authored Jan 19, 2017 by Peter Zijlstra Committed by Ingo Molnar Jan 30, 2017

sched/core: Optimize pick_next_task() for idle_sched_class



Steve noticed that when we switch from IDLE to SCHED_OTHER we fail to
take the shortcut, even though all runnable tasks are of the fair
class, because prev->sched_class != &fair_sched_class.

Since I reworked the put_prev_task() stuff, we don't really care about
prev->class here, so removing that condition will allow this case.

This increases the likely case from 78% to 98% correct for Steve's
workload.

Reported-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170119174408.GN6485@twins.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 3a09b8d4

kernel/sched/core.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -3321,15 +3321,14 @@ static inline void schedule_debug(struct task_struct *prev)
		static inline struct task_struct *
		pick_next_task(struct rq rq, struct task_struct prev, struct rq_flags *rf)
		{
		const struct sched_class *class = &fair_sched_class;
		const struct sched_class *class;
		struct task_struct *p;

		/*
		* Optimization: we know that if all tasks are in
		* the fair class we can call that function directly:
		*/
		if (likely(prev->sched_class == class &&
		rq->nr_running == rq->cfs.h_nr_running)) {
		if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
		p = fair_sched_class.pick_next_task(rq, prev, rf);
		if (unlikely(p == RETRY_TASK))
		goto again;

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