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Date: Wed, 30 Sep 2020 10:35:32 -0700 From: Peter Oskolkov <posk@...gle.com> To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, linux-kernel@...r.kernel.org Cc: Peter Oskolkov <posk@...k.io>, Peter Oskolkov <posk@...gle.com> Subject: [PATCH] sched/fair: tweak pick_next_entity Currently, pick_next_entity(...) has the following structure (simplified): [...] if (last_buddy_ok()) result = last_buddy; if (next_buddy_ok()) result = next_buddy; [...] The intended behavior is to prefer next buddy over last buddy; the current code somewhat obfuscates this, and also wastes cycles checking the last buddy when eventually the next buddy is picked up. So this patch refactors two 'ifs' above into [...] if (next_buddy_ok()) result = next_buddy; else if (last_buddy_ok()) result = last_buddy; [...] Signed-off-by: Peter Oskolkov <posk@...gle.com> --- kernel/sched/fair.c | 20 ++++++++++---------- 1 file changed, 10 insertions(+), 10 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fc3410b8b990..cec6cf9b2bb3 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4465,17 +4465,17 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr) se = second; } - /* - * Prefer last buddy, try to return the CPU to a preempted task. - */ - if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) - se = cfs_rq->last; - - /* - * Someone really wants this to run. If it's not unfair, run it. - */ - if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) + if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) { + /* + * Someone really wants this to run. If it's not unfair, run it. + */ se = cfs_rq->next; + } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) { + /* + * Prefer last buddy, try to return the CPU to a preempted task. + */ + se = cfs_rq->last; + } clear_buddies(cfs_rq, se); -- 2.28.0.709.gb0816b6eb0-goog
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