Commit 15257cc2 authored by Vincent Guittot's avatar Vincent Guittot Committed by Peter Zijlstra
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sched/fair: Revert force wakeup preemption 



This agressively bypasses run_to_parity and slice protection with the
assumpiton that this is what waker wants but there is no garantee that
the wakee will be the next to run. It is a better choice to use
yield_to_task or WF_SYNC in such case.

This increases the number of resched and preemption because a task becomes
quickly "ineligible" when it runs; We update the task vruntime periodically
and before the task exhausted its slice or at least quantum.

Example:
2 tasks A and B wake up simultaneously with lag = 0. Both are
eligible. Task A runs 1st and wakes up task C. Scheduler updates task
A's vruntime which becomes greater than average runtime as all others
have a lag == 0 and didn't run yet. Now task A is ineligible because
it received more runtime than the other task but it has not yet
exhausted its slice nor a min quantum. We force preemption, disable
protection but Task B will run 1st not task C.

Sidenote, DELAY_ZERO increases this effect by clearing positive lag at
wake up.

Fixes: e837456f ("sched/fair: Reimplement NEXT_BUDDY to align with EEVDF goals")
Signed-off-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260123102858.52428-1-vincent.guittot@linaro.org
parent 4f70f106

kernel/sched/fair.c

+0 −10
Original line number Diff line number Diff line
@@ -8828,16 +8828,6 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int 
	if ((wake_flags & WF_FORK) || pse->sched_delayed)

		return;



	/*

	 * If @p potentially is completing work required by current then

	 * consider preemption.

	 *

	 * Reschedule if waker is no longer eligible. */

	if (in_task() && !entity_eligible(cfs_rq, se)) {

		preempt_action = PREEMPT_WAKEUP_RESCHED;

		goto preempt;

	}



	/* Prefer picking wakee soon if appropriate. */

	if (sched_feat(NEXT_BUDDY) &&

	    set_preempt_buddy(cfs_rq, wake_flags, pse, se)) {