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Sun, 04 Oct 2026 10:30:36 -0700 (PDT) From: Kayra Cizmeci To: vincent.guittot@linaro.org Cc: arighi@nvidia.com, bsegall@google.com, changwoo@igalia.com, christian.loehle@arm.com, dietmar.eggemann@arm.com, juri.lelli@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, lukasz.luba@arm.com, mgorman@suse.de, mingo@redhat.com, peterz@infradead.org, pierre.gondois@arm.com, qyousef@layalina.io, rafael@kernel.org, rostedt@goodmis.org, sched-ext@lists.linux.dev, sshegde@linux.ibm.com, tj@kernel.org, void@manifault.com, vschneid@redhat.com Subject: Re: [PATCH 02/18] sched/eevdf: Reset lag when waking up on idle cpu Date: Sun, 4 Oct 2026 20:30:32 +0300 Message-ID: <20261004173032.18867-1-kayracizmeci@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261002154415.2270586-3-vincent.guittot@linaro.org> References: <20261002154415.2270586-3-vincent.guittot@linaro.org> Precedence: bulk X-Mailing-List: linux-pm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit > When several tasks wake up simultaneously on an idle CPU, their final vlag > will depend of the ordering as the first one will lose its lag but not > the next ones. > Reset the lag when the enqueue happens while no fair task has already been > picked et set as the running task. There is a typo. ('et', also 'depend' and 'loose' Not sure that's all :>) > As a typical example: > CPU0 is idle > TA with vlag 0ms and TB with vlag 5ms wake up on CPU0 simultaneously. > Depending which grab 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. > But if TB is enqueued 1st, it will loose its positive vlag and both TA and > TB will have 0 vlag when fair will pick a task. > diff --git a/include/linux/sched.h b/include/linux/sched.h > index d7cc77181ef9..32d7077ef148 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -592,6 +592,7 @@ struct sched_entity { > u64 vruntime; > /* Approximated virtual lag: */ > s64 vlag; > + u32 vlag_seq; > /* 'Protected' deadline, to give out minimum quantums: */ > u64 vprot; > u64 slice; > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 8cda1d39b037..4c8f12fc8869 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -893,6 +893,7 @@ bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se) > vlag = min(vlag, 0); > } > se->vlag = vlag; > + se->vlag_seq = cfs_rq->idle_seq; > > return avruntime - vlag != se->vruntime; > } > @@ -914,9 +915,22 @@ void decay_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > > 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; > + } > + > + /* Accessing remote rq task clock is a cost */ > if (flags & ENQUEUE_MIGRATED) > return; > > + /* CPU has been idle in between so the lag has been removed */ > + if (se->vlag_seq != cfs_rq->idle_seq) { > + se->vlag = 0; > + return; > + } > + > /* Compute sleep time */ > delta_exec = rq_clock_task(rq) - se->exec_start; > if (unlikely(delta_exec <= 0)) > @@ -8190,6 +8204,9 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags) > > dequeue_hierarchy(p, flags); > > + if (!cfs_rq->h_nr_queued) > + cfs_rq->idle_seq++; Shouldn't this skip DEQUEUE_SAVE? > + > if (sched_feat(PLACE_REL_DEADLINE) && !task_sleep) { > se->deadline -= se->vruntime; > se->rel_deadline = 1; > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index b98084e1f5b0..69a2a749e188 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -689,6 +689,7 @@ struct cfs_rq { > u64 sum_weight; > u64 zero_vruntime; > unsigned int sum_shift; > + u32 idle_seq; > > #ifdef CONFIG_SCHED_CORE > unsigned int forceidle_seq; Yeah, well AFAICT other things seems OK. Also, while trying to get this patch series on my tip branch I had a hard time. I was at the caves of git and b4 figthing with... Everything. Like there were no v2 tags on some of the patches so b4 didn't tracked them. I had to do some things by hand and finally it worked. (Not too greatly tho.) But who cares? :>. Thanks, Kayra