Commit 54d27365 authored Apr 06, 2015 by Ben Segall Committed by Ingo Molnar Jun 07, 2015

sched/fair: Prevent throttling in early pick_next_task_fair()



The optimized task selection logic optimistically selects a new task
to run without first doing a full put_prev_task(). This is so that we
can avoid a put/set on the common ancestors of the old and new task.

Similarly, we should only call check_cfs_rq_runtime() to throttle
eligible groups if they're part of the common ancestry, otherwise it
is possible to end up with no eligible task in the simple task
selection.

Imagine:
		/root
	/prev		/next
	/A		/B

If our optimistic selection ends up throttling /next, we goto simple
and our put_prev_task() ends up throttling /prev, after which we're
going to bug out in set_next_entity() because there aren't any tasks
left.

Avoid this scenario by only throttling common ancestors.

Reported-by: Mohammed Naser <mnaser@vexxhost.com>
Reported-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Ben Segall <bsegall@google.com>
[ munged Changelog ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Roman Gushchin <klamm@yandex-team.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: pjt@google.com
Fixes: 678d5718 ("sched/fair: Optimize cgroup pick_next_task_fair()")
Link: http://lkml.kernel.org/r/xm26wq1oswoq.fsf@sword-of-the-dawn.mtv.corp.google.com


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

parent 9a92e3dc

kernel/sched/fair.c

+14 −11

Original line number	Diff line number	Diff line
		@@ -5322,18 +5322,21 @@ pick_next_task_fair(struct rq rq, struct task_struct prev)
		* entity, update_curr() will update its vruntime, otherwise
		* forget we've ever seen it.
		*/
		if (curr && curr->on_rq)
		if (curr) {
		if (curr->on_rq)
		update_curr(cfs_rq);
		else
		curr = NULL;

		/*
		* This call to check_cfs_rq_runtime() will do the throttle and
		* dequeue its entity in the parent(s). Therefore the 'simple'
		* nr_running test will indeed be correct.
		* This call to check_cfs_rq_runtime() will do the
		* throttle and dequeue its entity in the parent(s).
		* Therefore the 'simple' nr_running test will indeed
		* be correct.
		*/
		if (unlikely(check_cfs_rq_runtime(cfs_rq)))
		goto simple;
		}

		se = pick_next_entity(cfs_rq, curr);
		cfs_rq = group_cfs_rq(se);