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Mon, 09 Nov 2026 06:15:53 -0800 (PST) Received: from scheduler.schedules ([85.100.66.184]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-495966f36besm9520949f8f.14.2026.11.09.06.15.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 09 Nov 2026 06:15:52 -0800 (PST) Date: Mon, 9 Nov 2026 17:15:48 +0300 From: Kayra Cizmeci To: Peter Zijlstra Cc: Vincent Guittot , mingo@redhat.com, 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 06/20 v3] sched/eevdf: Add min slice check when selecting CPU Message-ID: References: <20261106151947.3991524-1-vincent.guittot@linaro.org> <20261106151947.3991524-7-vincent.guittot@linaro.org> <20261109135553.GC2668564@noisy.programming.kicks-ass.net> X-Mailing-List: sched-ext@lists.linux.dev 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: <20261109135553.GC2668564@noisy.programming.kicks-ass.net> On Mon, Nov 09, 2026 at 02:55:53PM +0100, Peter Zijlstra wrote: > On Sat, Nov 07, 2026 at 11:57:12AM +0300, Kayra Cizmeci wrote: > > > (My editor got crazy, so I had to copy this diff :<) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 262626455497..8b0dd858f332 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -8876,10 +8876,8 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas > > > > static inline bool choose_sched_slice_rq(struct rq *rq, struct task_struct *p) > > { > > - if (p->se.slice < get_rq_min_slice(rq)) > > - return true; > > - > > - return false; > > + return p->se.slice < get_rq_min_slice(rq) && > > + READ_ONCE(rq->curr)->sched_class == &fair_sched_class; > > } > > that wants to be rcu_dereference_all() or somesuch, rather than > READ_ONCE(). Ah, my bad :-). > > The think I noticed is that it will only select one slice_cpu; it will > not consider another once its has found one. > > This means it picks the oldest option, rather than the most recent one. So you mean by the time CPU loop ends, the CPU that we selected might not be a good option anymore. > And given this is all racy as heck, the most recent one might in fact be > the better choice. It will have had less time to go stale. > Makes sense to me. So then we gonna overwrite slice_cpu everytime we find a new one? Cheers, Kayra