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sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue

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Similar to how pick_next_entity() must dequeue delayed entities, so too must
wakeup_preempt_fair(). Any delayed task being found means it is eligible and
hence past the 0-lag point, ready for removal.

Worse, by not removing delayed entities from consideration, it can skew the
preemption decision, with the end result that a short slice wakeup will not
result in a preemption.

                     tip/sched/core  tip/sched/core    +this patch
cyclictest slice  (ms) (default)2.8             8               8
hackbench slice   (ms) (default)2.8            20              20
Total Samples          |    22559           22595           22683
Average           (us) |      157              64( 59%)        59(  8%)
Median (P50)      (us) |       57              57(  0%)        58(- 2%)
90th Percentile   (us) |       64              60(  6%)        60(  0%)
99th Percentile   (us) |     2407              67( 97%)        67(  0%)
99.9th Percentile (us) |     3400            2288( 33%)       727( 68%)
Maximum           (us) |     5037            9252(-84%)      7461( 19%)

Fixes: f12e148892 ("sched/fair: Prepare pick_next_task() for delayed dequeue")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260422093400.319251-1-vincent.guittot@linaro.org
This commit is contained in:
Vincent Guittot 2026-04-22 11:34:00 +02:00 • committed by Peter Zijlstra
commit ac8e69e693
1 changed files with 14 additions and 13 deletions
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