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Fri, 06 Nov 2026 00:57:06 -0800 (PST) Received: from scheduler.schedules ([85.100.66.184]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-492d7232257sm1905430f8f.22.2026.11.06.00.57.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2026 00:57:05 -0800 (PST) Date: Fri, 6 Nov 2026 11:57:03 +0300 From: Kayra Cizmeci To: Peter Zijlstra Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, joshdon@google.com, tj@kernel.org, jakub@okonski.org Subject: Re: [PATCH 2/3] sched/fair: Make cfs_rq::idle the sum of ancestor tg::idle Message-ID: References: <20261105165525.011876858@infradead.org> <20261105170157.616859418@infradead.org> <20261106083106.GC1779021@noisy.programming.kicks-ass.net> 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: <20261106083106.GC1779021@noisy.programming.kicks-ass.net> On Fri, Nov 06, 2026 at 09:31:06AM +0100, Peter Zijlstra wrote: > On Thu, Nov 05, 2026 at 10:20:00PM +0300, Kayra Cizmeci wrote: > > On Thu, Nov 05, 2026 at 05:55:27PM +0100, Peter Zijlstra wrote: > > > Rather than keeping cfs_rq::idle as a copy of tg::idle, make it the sum of > > > all ancestor tg::idle. This makes it trivial to tell if the current cgroup > > > is marked idle somewhere along its ancestral path. > > > > > > This makes se_id_idle() behave differently. Notably is will now be true > > > when there is a idle cgroup anywhere in the ancestor path. This > > > addresses an issue where preemption of cgroup::idle was overlooked when > > > flattening the runqueue. > > > > How? The diff doesn't touches se_is_idle(). se_is_idle() just looks to > > the policy for tasks: > > > > static int se_is_idle(struct sched_entity *se) > > { > > if (entity_is_task(se)) > > return task_has_idle_policy(task_of(se)); > > return cfs_rq_is_idle(group_cfs_rq(se)); > > } > > > Or am I getting something wrong? > > cfs_rq_is_idle() := cfs_rq->idle > 0 > > and cfs_rq->idle is now getting incremented for the whole subtree. So if > any ancestor is idle, this is now true. Yeah, it is for group path. But we call se_is_idle for tasks in wakeup_preempt_fair(). Not groups, no?