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([2a0c:5a80:3e07:e200:5f92:8384:b0a4:8063]) by smtp.gmail.com with ESMTPSA id v8-20020a05600c214800b003a2cf1535aasm3257593wml.17.2022.07.13.10.18.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 13 Jul 2022 10:18:43 -0700 (PDT) Message-ID: Subject: Re: [PATCH] nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt From: Nicolas Saenz Julienne To: linux-kernel@vger.kernel.org, fweisbec@gmail.com Cc: bristot@redhat.com, vschneid@redhat.com, cmetcalf@ezchip.com, mgorman@suse.de, bsegall@google.com, rostedt@goodmis.org, dietmar.eggemann@arm.com, vincent.guittot@linaro.org, juri.lelli@redhat.com, peterz@infradead.org, mingo@redhat.com, mtosatti@redhat.com Date: Wed, 13 Jul 2022 19:18:42 +0200 In-Reply-To: <20220628092259.330171-1-nsaenzju@redhat.com> References: <20220628092259.330171-1-nsaenzju@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.44.3 (3.44.3-1.fc36) MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2022-06-28 at 11:22 +0200, Nicolas Saenz Julienne wrote: > dequeue_task_rt() only decrements 'rt_rq->rt_nr_running' after having > called sched_update_tick_dependency() preventing it from re-enabling the > tick on systems that no longer have pending SCHED_RT tasks but have > multiple runnable SCHED_OTHER tasks: >=20 > dequeue_task_rt() > dequeue_rt_entity() > dequeue_rt_stack() > dequeue_top_rt_rq() > sub_nr_running() // decrements rq->nr_running > sched_update_tick_dependency() > sched_can_stop_tick() // checks rq->rt.rt_nr_running, > ... > __dequeue_rt_entity() > dec_rt_tasks() // decrements rq->rt.rt_nr_running > ... >=20 > Every other scheduler class performs the operation in the opposite > order, and sched_update_tick_dependency() expects the values to be > updated as such. So avoid the misbehaviour by inverting the order in > which the above operations are performed in the RT scheduler. >=20 > Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mas= k model") > Signed-off-by: Nicolas Saenz Julienne > --- Ping :) --=20 Nicol=C3=A1s S=C3=A1enz