linux-kernel - [tip: sched/urgent] sched/fair: Revert force wakeup preemption

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Message-ID: <176916639187.510.1647095982279283388.tip-bot2@tip-bot2>
Date: Fri, 23 Jan 2026 11:06:31 -0000
From: "tip-bot2 for Vincent Guittot" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/fair: Revert force wakeup preemption

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     15257cc2f905dbf5813c0bfdd3c15885f28093c4
Gitweb:        https://git.kernel.org/tip/15257cc2f905dbf5813c0bfdd3c15885f28093c4
Author:        Vincent Guittot <vincent.guittot@...aro.org>
AuthorDate:    Fri, 23 Jan 2026 11:28:58 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 23 Jan 2026 11:53:20 +01:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://patch.msgid.link/20260123102858.52428-1-vincent.guittot@linaro.org
---
 kernel/sched/fair.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a148c61..3eaeced 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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)) {