linux-kernel - [PATCH] sched/fair: revert force wakeup preemption

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Message-ID: <20260123102858.52428-1-vincent.guittot@linaro.org>
Date: Fri, 23 Jan 2026 11:28:58 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	vschneid@...hat.com,
	linux-kernel@...r.kernel.org,
	ryan.roberts@....com,
	mgorman@...hsingularity.net,
	vineethr@...ux.ibm.com,
	clm@...a.com,
	Christian.Loehle@....com
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH] 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: e837456fdca8 ("sched/fair: Reimplement NEXT_BUDDY to align with EEVDF goals")
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04993c763a06..16ecc3475fe2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8822,16 +8822,6 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
 	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)) {
-- 
2.43.0