Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E08D6C433FE for ; Fri, 7 Jan 2022 10:46:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237590AbiAGKq4 (ORCPT ); Fri, 7 Jan 2022 05:46:56 -0500 Received: from foss.arm.com ([217.140.110.172]:39382 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237653AbiAGKqy (ORCPT ); Fri, 7 Jan 2022 05:46:54 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6F49C13D5; Fri, 7 Jan 2022 02:46:54 -0800 (PST) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A639C3F66F; Fri, 7 Jan 2022 02:46:52 -0800 (PST) Subject: Re: [RT] BUG in sched/cpupri.c To: Valentin Schneider , John Keeping Cc: linux-rt-users@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , linux-kernel@vger.kernel.org References: <71ddbe51-2b7f-2b13-5f22-9013506471dc@arm.com> <87zgou6iq1.mognet@arm.com> <20211221164528.3c84543f.john@metanate.com> <31a47e99-6de3-76ec-62ad-9c98d092ead5@arm.com> <87r1a4775a.mognet@arm.com> From: Dietmar Eggemann Message-ID: Date: Fri, 7 Jan 2022 11:46:45 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <87r1a4775a.mognet@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22/12/2021 20:48, Valentin Schneider wrote: > On 22/12/21 18:46, Dietmar Eggemann wrote: >> On 21.12.21 17:45, John Keeping wrote: >>> On Tue, 21 Dec 2021 16:11:34 +0000 >>> Valentin Schneider wrote: >>> >>>> On 20/12/21 18:35, Dietmar Eggemann wrote: [...] >> switched_from_rt() -> rt_queue_pull_task(, pull_rt_task) >> pull_rt_task()->tell_cpu_to_push()->irq_work_queue_on(&rq->rd->rto_push_work,) >> rto_push_irq_work_func() -> push_rt_task(rq, true) >> >> seems to be the only way with pull=true. >> >> In my tests, rq->rt.rt_nr_running seems to be 0 when it happens. >> >> [ 22.288537] CPU3 switched_to_rt: p=[ksoftirqd/3 35] >> [ 22.288554] rt_mutex_setprio: CPU3 p=[ksoftirqd/3 35] pi_task=[rcu_preempt 11] queued=1 running=0 prio=98 oldprio=120 >> [ 22.288636] CPU3 switched_from_rt: p=[ksoftirqd/3 35] rq->rt.rt_nr_running=0 >> ^^^^^^^^^^^^^^^^^^^^^^ >> [ 22.288649] rt_mutex_setprio: CPU3 p=[ksoftirqd/3 35] queued=1 running=1 prio=120 oldprio=98 >> [ 22.288681] CPU3 push_rt_task: next_task=[rcu_preempt 11] migr_dis=1 rq->curr=[ksoftirqd/3 35] pull=1 >> ^^^^^^^^^^ ^^^^^^ > > mark_wakeup_next_waiter() first deboosts the previous owner and then > wakeups the next top waiter. Seems like you somehow have the wakeup happen > before the push_rt_task IRQ work is run. Also, tell_cpu_to_push() should > only pick a CPU that is in rq->rd->rto_mask, which requires having at least > 2 RT tasks there... True, this_cpu has rt_nr_running = 0 and *cpu* has rt_nr_running >= 2: mark_wakeup_next_waiter() (1) /* deboost */ rt_mutex_adjust_prio() rt_mutex_setprio(current, ...) rq = __task_rq_lock(current, ); check_class_changed(rq, p, prev_class, oldprio) switched_from_rt() if (!task_on_rq_queued(p) || rq->rt.rt_nr_running) return; rt_queue_pull_task(rq) queue_balance_callback(rq, ..., pull_rt_task); pull_rt_task() tell_cpu_to_push() *cpu* = rto_next_cpu(rq->rd) irq_work_queue_on(&rq->rd->rto_push_work, *cpu*) rto_push_irq_work_func() push_rt_task(rq, true) <-- !!! (2) /* waking the top waiter */ rt_mutex_wake_q_add(wqh, waiter); > Now, that wakeup from the rtmutex unlock would give us a resched_curr() via > check_preempt_curr() if required, which is good, though I think we are > still missing some for sched_setscheduler() (there are no wakeups > there). So if we just have to live with an IRQ work popping in before we > get to preempt_schedule_irq() (or somesuch), then perhaps the below would > be sufficient. I think that's the case here but we are on the RT overloaded CPU (*cpu*). > >> What about slightly changing the layout in switched_from_rt() (only lightly tested): >> >> >> @@ -2322,7 +2338,15 @@ static void switched_from_rt(struct rq *rq, struct task_struct *p) >> * we may need to handle the pulling of RT tasks >> * now. >> */ >> - if (!task_on_rq_queued(p) || rq->rt.rt_nr_running) >> + if (!task_on_rq_queued(p)) >> + return; >> + >> + if (task_current(rq, p) && (p->sched_class < &rt_sched_class)) { >> + resched_curr(rq); >> + return; >> + } >> + >> + if (rq->rt.rt_nr_running) >> return; >> >> rt_queue_pull_task(rq); > > If !rq->rt.rt_nr_running then there's no point in issuing a reschedule (at > least from RT's perspective; p->sched_class->switched_to() takes care of > the rest) Right. [...] > /* > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index ef8228d19382..8f3e3a1367b6 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1890,6 +1890,16 @@ static int push_rt_task(struct rq *rq, bool pull) > if (!next_task) > return 0; > > + /* > + * It's possible that the next_task slipped in of higher priority than > + * current, or current has *just* changed priority. If that's the case > + * just reschedule current. > + */ > + if (unlikely(next_task->prio < rq->curr->prio)) { > + resched_curr(rq); > + return 0; > + } IMHO, that's the bit which prevents the BUG. But this would also prevent the case in which rq->curr is an RT task with lower prio than next_task. Also `rq->curr = migration/X` goes still though which is somehow fine since find_lowest_rq() bails out for if (task->nr_cpus_allowed == 1). And DL tasks (like sugov:X go through and they can have task->nr_cpus_allowed > 1 (arm64 slow-switching boards with shared freuency domains with schedutil). cpupri_find_fitness()->convert_prio() can handle task_pri, p->prio = -1 (CPUPRI_INVALID) although its somehow by coincidence. So maybe something like this: @ -1898,6 +1898,11 @@ static int push_rt_task(struct rq *rq, bool pull) if (!pull || rq->push_busy) return 0; + if (rq->curr->sched_class != &rt_sched_class) { + resched_curr(rq); + return 0; + } + cpu = find_lowest_rq(rq->curr); [...]