From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 821973F58CC; Wed, 20 May 2026 17:07:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779296848; cv=none; b=ROcuUhQlkUnYNTc8F2ER02Kuvea35j3NiND3uWvtBmce0J84+22w68uV5njtVC0qj2ERbsNWEkRAuEcrrh10+JOIKTvfqRvzndyCv0H03ylrKptQC+dxVs5Y5Zp33PHwYdJL7A3Fa2QwK0vjPaBVJgD+9I0LSzukwQiJ6zbuEGc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779296848; c=relaxed/simple; bh=jp3andc19psJ27ZAEnIXgu9rPS2lqGttJ8OiKC+wr0k=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=RYKGrUTKMD8xDdtA02OtK45FqorMX9ct6wKlyxnGtca5Bi2+uOvJr3eq6Ux63ATKYT9Qjo+Y0kFq61XY3tbXn1maN/CsyZbDk1/7xNTgqO5YbjwI5Sq+oR7UwtxK4hg5pqT3Zj63BuQH1XfrKuEni0BgtfCQ4r/s/ajHXoGth44= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=HstDPP1h; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="HstDPP1h" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B3281F000E9; Wed, 20 May 2026 17:07:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1779296846; bh=2pc8QnsUhhSMZMfdchLNgzC/fKTdfyia5D1dqZgWuqo=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=HstDPP1hzcSE2uRPv2Ny3nLPyeAd3AZ+z8kf4VdrDNDOzrij1VEafuaLQoFE8vLyz qZajIknH6yuwnMkfW73RbwTopayBBXrilz4THJ7wGR9Bu2kocohomSna5TwduiIi0b h4BeArYVYVBK0OHEy3bcA11UrA7hLS4T3UvRvrZ0= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Vincent Guittot , "Peter Zijlstra (Intel)" , Sasha Levin Subject: [PATCH 7.0 0952/1146] sched/fair: Fix wakeup_preempt_fair() vs delayed dequeue Date: Wed, 20 May 2026 18:20:02 +0200 Message-ID: <20260520162209.778326966@linuxfoundation.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260520162148.390695140@linuxfoundation.org> References: <20260520162148.390695140@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 7.0-stable review patch. If anyone has any objections, please let me know. ------------------ From: Vincent Guittot [ Upstream commit ac8e69e693631689d74d8f1ebee6f84f737f797f ] Similar to how pick_next_entity() must dequeue delayed entities, so too must wakeup_preempt_fair(). Any delayed task being found means it is eligible and hence past the 0-lag point, ready for removal. Worse, by not removing delayed entities from consideration, it can skew the preemption decision, with the end result that a short slice wakeup will not result in a preemption. tip/sched/core tip/sched/core +this patch cyclictest slice (ms) (default)2.8 8 8 hackbench slice (ms) (default)2.8 20 20 Total Samples | 22559 22595 22683 Average (us) | 157 64( 59%) 59( 8%) Median (P50) (us) | 57 57( 0%) 58(- 2%) 90th Percentile (us) | 64 60( 6%) 60( 0%) 99th Percentile (us) | 2407 67( 97%) 67( 0%) 99.9th Percentile (us) | 3400 2288( 33%) 727( 68%) Maximum (us) | 5037 9252(-84%) 7461( 19%) Fixes: f12e148892ed ("sched/fair: Prepare pick_next_task() for delayed dequeue") Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260422093400.319251-1-vincent.guittot@linaro.org Signed-off-by: Sasha Levin --- kernel/sched/fair.c | 27 ++++++++++++++------------- 1 file changed, 14 insertions(+), 13 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ab4114712be74..87200a22b3169 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1007,7 +1007,7 @@ static inline void cancel_protect_slice(struct sched_entity *se) * * Which allows tree pruning through eligibility. */ -static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq, bool protect) +static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq, bool protect) { struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node; struct sched_entity *se = __pick_first_entity(cfs_rq); @@ -1078,11 +1078,6 @@ static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq, bool protect) return best; } -static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq) -{ - return __pick_eevdf(cfs_rq, true); -} - struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq) { struct rb_node *last = rb_last(&cfs_rq->tasks_timeline.rb_root); @@ -5540,11 +5535,11 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags); * 4) do not run the "skip" process, if something else is available */ static struct sched_entity * -pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq) +pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq, bool protect) { struct sched_entity *se; - se = pick_eevdf(cfs_rq); + se = pick_eevdf(cfs_rq, protect); if (se->sched_delayed) { dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED); /* @@ -8809,7 +8804,7 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f { enum preempt_wakeup_action preempt_action = PREEMPT_WAKEUP_PICK; struct task_struct *donor = rq->donor; - struct sched_entity *se = &donor->se, *pse = &p->se; + struct sched_entity *nse, *se = &donor->se, *pse = &p->se; struct cfs_rq *cfs_rq = task_cfs_rq(donor); int cse_is_idle, pse_is_idle; @@ -8920,11 +8915,17 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f } pick: + nse = pick_next_entity(rq, cfs_rq, preempt_action != PREEMPT_WAKEUP_SHORT); + /* If @p has become the most eligible task, force preemption */ + if (nse == pse) + goto preempt; + /* - * If @p has become the most eligible task, force preemption. + * Because p is enqueued, nse being null can only mean that we + * dequeued a delayed task. */ - if (__pick_eevdf(cfs_rq, preempt_action != PREEMPT_WAKEUP_SHORT) == pse) - goto preempt; + if (!nse) + goto pick; if (sched_feat(RUN_TO_PARITY)) update_protect_slice(cfs_rq, se); @@ -8959,7 +8960,7 @@ static struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf) throttled |= check_cfs_rq_runtime(cfs_rq); - se = pick_next_entity(rq, cfs_rq); + se = pick_next_entity(rq, cfs_rq, true); if (!se) goto again; cfs_rq = group_cfs_rq(se); -- 2.53.0