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Fri, 21 Feb 2025 03:12:50 -0800 (PST) From: Abel Wu To: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Josh Don , Tianchen Ding Cc: Abel Wu , linux-kernel@vger.kernel.org (open list:SCHEDULER) Subject: [PATCH 1/2] sched/fair: Do not let idle entities preempt others Date: Fri, 21 Feb 2025 19:12:23 +0800 Message-Id: <20250221111226.64455-2-wuyun.abel@bytedance.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20250221111226.64455-1-wuyun.abel@bytedance.com> References: <20250221111226.64455-1-wuyun.abel@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" A task with SCHED_IDLE policy doesn't preempt others by definition, and the semantics are intended to be preserved when extending to cgroups introduced in commit 304000390f88 ("sched: Cgroup SCHED_IDLE support"). But current implementation allows idle entities to preempt each other on wakeup, which seems not behave as expected especially after commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in st= rict hierarchy") so fix this by explicitly skip wakeup preemption for idle entities. Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support") Signed-off-by: Abel Wu Reviewed-by: Josh Don Reviewed-by: Madadi Vineeth Reddy Reviewed-by: Vincent Guittot --- kernel/sched/fair.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 1c0ef435a7aa..4340178f29b7 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8778,12 +8778,15 @@ static void check_preempt_wakeup_fair(struct rq *rq= , struct task_struct *p, int pse_is_idle =3D se_is_idle(pse); =20 /* - * Preempt an idle entity in favor of a non-idle entity (and don't preempt - * in the inverse case). + * Preempt an idle entity in favor of a non-idle entity. */ if (cse_is_idle && !pse_is_idle) goto preempt; - if (cse_is_idle !=3D pse_is_idle) + + /* + * IDLE entities do not preempt others. + */ + if (unlikely(pse_is_idle)) return; =20 /* --=20 2.37.3 From nobody Mon Feb 9 05:22:19 2026 Received: from mail-pl1-f174.google.com (mail-pl1-f174.google.com [209.85.214.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 99ADB2080F0 for ; 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charset="utf-8" Idle tasks are by definition preempted by non-idle tasks whether feat WAKEUP_PREEMPTION is enabled or not. This isn't true any longer since commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in st= rict hierarchy") which gives priority to WAKEUP_PREEMPTION, so when !FAIR_GROUP_SCHED, SCHED_IDLE tasks do not preempt by non-idle tasks. Fixes: faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in st= rict hierarchy") Signed-off-by: Abel Wu --- kernel/sched/fair.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4340178f29b7..de9a2689de9c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8768,9 +8768,6 @@ static void check_preempt_wakeup_fair(struct rq *rq, = struct task_struct *p, int if (test_tsk_need_resched(rq->curr)) return; =20 - if (!sched_feat(WAKEUP_PREEMPTION)) - return; - find_matching_se(&se, &pse); WARN_ON_ONCE(!pse); =20 @@ -8783,6 +8780,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, = struct task_struct *p, int if (cse_is_idle && !pse_is_idle) goto preempt; =20 + if (!sched_feat(WAKEUP_PREEMPTION)) + return; + /* * IDLE entities do not preempt others. */ --=20 2.37.3