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: 678d5718d8
("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>
This commit is contained in:
parent
9a92e3dc6a
commit
54d27365ca
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@ -5322,18 +5322,21 @@ again:
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* entity, update_curr() will update its vruntime, otherwise
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* entity, update_curr() will update its vruntime, otherwise
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* forget we've ever seen it.
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* forget we've ever seen it.
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*/
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*/
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if (curr && curr->on_rq)
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if (curr) {
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update_curr(cfs_rq);
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if (curr->on_rq)
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else
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update_curr(cfs_rq);
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curr = NULL;
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else
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curr = NULL;
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/*
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/*
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* This call to check_cfs_rq_runtime() will do the throttle and
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* This call to check_cfs_rq_runtime() will do the
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* dequeue its entity in the parent(s). Therefore the 'simple'
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* throttle and dequeue its entity in the parent(s).
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* nr_running test will indeed be correct.
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* Therefore the 'simple' nr_running test will indeed
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*/
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* be correct.
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if (unlikely(check_cfs_rq_runtime(cfs_rq)))
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*/
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goto simple;
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if (unlikely(check_cfs_rq_runtime(cfs_rq)))
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goto simple;
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}
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se = pick_next_entity(cfs_rq, curr);
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se = pick_next_entity(cfs_rq, curr);
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cfs_rq = group_cfs_rq(se);
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cfs_rq = group_cfs_rq(se);
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