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Mon, 25 Sep 2023 05:21:58 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHAm4ifKXpzDtyUPhGIDZDGdGBDvG3dNyF9RNLXRMXlOTPyatpeH0NuSCukQ0AYeGIPawefuA== X-Received: by 2002:adf:e3ce:0:b0:317:dadc:4a63 with SMTP id k14-20020adfe3ce000000b00317dadc4a63mr6997860wrm.8.1695644518330; Mon, 25 Sep 2023 05:21:58 -0700 (PDT) Received: from vschneid.remote.csb ([80.214.159.242]) by smtp.gmail.com with ESMTPSA id bw10-20020a0560001f8a00b0032326908972sm3230747wrb.17.2023.09.25.05.21.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 25 Sep 2023 05:21:57 -0700 (PDT) From: Valentin Schneider To: Peter Zijlstra , linux-kernel@vger.kernel.org Cc: linux-tip-commits@vger.kernel.org, Sebastian Andrzej Siewior , Ingo Molnar , x86@kernel.org Subject: Re: [tip: sched/core] sched/rt: Make rt_rq->pushable_tasks updates drive rto_mask In-Reply-To: <20230925101127.GB31921@noisy.programming.kicks-ass.net> References: <20230811112044.3302588-1-vschneid@redhat.com> <169563211069.27769.17070510461354463740.tip-bot2@tip-bot2> <20230925101127.GB31921@noisy.programming.kicks-ass.net> Date: Mon, 25 Sep 2023 14:21:56 +0200 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25/09/23 12:11, Peter Zijlstra wrote: > On Mon, Sep 25, 2023 at 08:55:10AM -0000, tip-bot2 for Valentin Schneider wrote: >> The following commit has been merged into the sched/core branch of tip: >> >> Commit-ID: 612f769edd06a6e42f7cd72425488e68ddaeef0a >> Gitweb: https://git.kernel.org/tip/612f769edd06a6e42f7cd72425488e68ddaeef0a >> Author: Valentin Schneider >> AuthorDate: Fri, 11 Aug 2023 12:20:44 +01:00 >> Committer: Ingo Molnar >> CommitterDate: Mon, 25 Sep 2023 10:25:29 +02:00 >> >> sched/rt: Make rt_rq->pushable_tasks updates drive rto_mask >> >> Sebastian noted that the rto_push_work IRQ work can be queued for a CPU >> that has an empty pushable_tasks list, which means nothing useful will be >> done in the IPI other than queue the work for the next CPU on the rto_mask. >> >> rto_push_irq_work_func() only operates on tasks in the pushable_tasks list, >> but the conditions for that irq_work to be queued (and for a CPU to be >> added to the rto_mask) rely on rq_rt->nr_migratory instead. >> >> nr_migratory is increased whenever an RT task entity is enqueued and it has >> nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a >> rt_rq's current task. This means a rt_rq can have a migratible current, N >> non-migratible queued tasks, and be flagged as overloaded / have its CPU >> set in the rto_mask, despite having an empty pushable_tasks list. >> >> Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task(). >> Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them. >> >> Note that the case where the current task is pushed away to make way for a >> migration-disabled task remains unchanged: the migration-disabled task has >> to be in the pushable_tasks list in the first place, which means it has >> nr_cpus_allowed > 1. >> >> Reported-by: Sebastian Andrzej Siewior >> Signed-off-by: Valentin Schneider >> Signed-off-by: Ingo Molnar >> Tested-by: Sebastian Andrzej Siewior >> Link: https://lore.kernel.org/r/20230811112044.3302588-1-vschneid@redhat.com >> --- >> kernel/sched/debug.c | 3 +-- >> kernel/sched/rt.c | 70 ++++++------------------------------------- >> kernel/sched/sched.h | 2 +- >> 3 files changed, 10 insertions(+), 65 deletions(-) >> > >> @@ -358,53 +357,6 @@ static inline void rt_clear_overload(struct rq *rq) >> cpumask_clear_cpu(rq->cpu, rq->rd->rto_mask); >> } >> >> -static void update_rt_migration(struct rt_rq *rt_rq) >> -{ >> - if (rt_rq->rt_nr_migratory && rt_rq->rt_nr_total > 1) { >> - if (!rt_rq->overloaded) { >> - rt_set_overload(rq_of_rt_rq(rt_rq)); >> - rt_rq->overloaded = 1; >> - } >> - } else if (rt_rq->overloaded) { >> - rt_clear_overload(rq_of_rt_rq(rt_rq)); >> - rt_rq->overloaded = 0; >> - } >> -} >> - >> -static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq) >> -{ >> - struct task_struct *p; >> - >> - if (!rt_entity_is_task(rt_se)) >> - return; >> - >> - p = rt_task_of(rt_se); >> - rt_rq = &rq_of_rt_rq(rt_rq)->rt; >> - >> - rt_rq->rt_nr_total++; >> - if (p->nr_cpus_allowed > 1) >> - rt_rq->rt_nr_migratory++; >> - >> - update_rt_migration(rt_rq); >> -} >> - >> -static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq) >> -{ >> - struct task_struct *p; >> - >> - if (!rt_entity_is_task(rt_se)) >> - return; >> - >> - p = rt_task_of(rt_se); >> - rt_rq = &rq_of_rt_rq(rt_rq)->rt; >> - >> - rt_rq->rt_nr_total--; >> - if (p->nr_cpus_allowed > 1) >> - rt_rq->rt_nr_migratory--; >> - >> - update_rt_migration(rt_rq); >> -} > > sched/deadline.c has something very similar, does that need updating > too? Hm I think so yes: - push_dl_task() is an obvious noop if the pushable tree is empty - pull_dl_task() can be kicked if !rq->dl.overloaded, which similarly to rt is driven by nr_migratory but could be boiled down to having pushable tasks (due to the nr_cpus_allowed > 1 constraint). Lemme poke at it.