Commit 9abb8973 authored Sep 30, 2020 by Peter Oskolkov Committed by Peter Zijlstra Oct 03, 2020

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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guitttot@linaro.org>
Link: https://lkml.kernel.org/r/20200930173532.1069092-1-posk@google.com

parent f166b111

kernel/sched/fair.c

+10 −10

Original line number	Diff line number	Diff line
		@@ -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;

		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.
		*/
		if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
		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);

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