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Fri, 06 Nov 2026 10:39:31 -0800 (PST) Received: from scheduler.schedules ([85.100.66.184]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b795038782sm4128115e9.3.2026.11.06.10.39.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2026 10:39:30 -0800 (PST) Date: Fri, 6 Nov 2026 21:39:26 +0300 From: Kayra Cizmeci To: Vincent Guittot Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org, lukasz.luba@arm.com, rafael@kernel.org, linux-pm@vger.kernel.org, tj@kernel.org, void@manifault.com, arighi@nvidia.com, changwoo@igalia.com, sched-ext@lists.linux.dev, qyousef@layalina.io, christian.loehle@arm.com, pierre.gondois@arm.com, sshegde@linux.ibm.com, tim.c.chen@linux.intel.com Subject: Re: [PATCH 04/20 v3] sched/eevdf: Reset lag when waking up on idle cpu Message-ID: References: <20261106151947.3991524-1-vincent.guittot@linaro.org> <20261106151947.3991524-5-vincent.guittot@linaro.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20261106151947.3991524-5-vincent.guittot@linaro.org> On Fri, Nov 06, 2026 at 04:19:31PM +0100, Vincent Guittot wrote: > When several tasks wake up simultaneously on an idle CPU but their prev > CPUs were not idle, their final vlag will depend on the enqueue ordering > as the first one will lose its lag but not the next ones. > > Reset the lag during the enqueue if no fair task has already been picked > and set as the running task. > > As a typical example: > - CPU0 is idle > - TA with vlag 0ms and TB with vlag 5ms wake up on CPU0 simultaneously. > > Depending which task grabs the lock 1st, the behavior will be different: > - If TA is enqueued 1st, TB will be enqueued with a positive lag and will > be picked 1st. > - If TB is enqueued 1st, it will lose its positive vlag and both TA and > TB will have 0 vlag when eevdf picks next task. > > Signed-off-by: Vincent Guittot > --- > kernel/sched/fair.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 945de0215abb..6ce4162a7703 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -967,6 +967,14 @@ void decay_entity_lag(struct cfs_rq *cfs_rq, struct task_struct *p, int flags) > if (vlag <= 0) > return; > > + rq = rq_of(cfs_rq); > + > + /* You can't claim any lag when waking on idle CPU */ > + if (rq->curr == rq->idle) { > + se->vlag = 0; > + return; > + } > + > /* No fast access to prev rq task clock */ > if (flags & ENQUEUE_MIGRATED) > return; > @@ -977,8 +985,6 @@ void decay_entity_lag(struct cfs_rq *cfs_rq, struct task_struct *p, int flags) > return; > } > > - rq = rq_of(cfs_rq); > - > /* Compute sleep time */ > delta_exec = rq_clock_task(rq) - se->exec_start; > if (unlikely(delta_exec <= 0)) > -- > 2.53.0 > LGTM. Reviewed-by: Kayra Cizmeci Cheers, Kayra