From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-186.mta1.migadu.com (out-186.mta1.migadu.com [95.215.58.186]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B0CE5202F8D for ; Mon, 26 May 2025 13:15:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.186 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748265316; cv=none; b=r3phSsoC96KmNBfhgdkkOkH7w7zVjFGCTOhVZiv+3lEr3XE2V8JTaL8Ennkkn8QRwSdnRkdaHxGuv9KuGAv1s2e+2x+u5M8u53hktMTvy08yciybMFIoPcvYc9FoZIvHuvyth0N3Q65hbPlu9o0x5i3sVvfXqCPl2+DcKn/L4F8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748265316; c=relaxed/simple; bh=5cEag7n/qT/o4g/Ip4j9kwDbg2D33oMxgHBLz+BmT7g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=R6qcfVMQQdEL76wflHlW6vw6fh5myZThT5TEpvlLHJF6VeoZCV5cF3Vo6Q8Zdwjn7QnTDo3imbD6a/wYM4rUWsheJ5CfMGMHqlnpoV0St5IRFb+bNOpDzWKt+5y+QPn63gU4ewbcDfnFRPwzuNvZoRzH6B0ac/6e9cF9ozTV10I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=lOHsH8r0; arc=none smtp.client-ip=95.215.58.186 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="lOHsH8r0" Message-ID: <16ab4d89-89ea-464b-812a-172f971c4627@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1748265310; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CfDsj/vggLw1N/iYXFLN6UBaW5wDtfTF6K8viv4sf0Y=; b=lOHsH8r0rAxYasqquR5evScp3j0sdXz2qMSwPB1laENDgaNWr6cDRiUG0m8Y8i7rI/Gqft Lr2tcMj0UsGgcM/29lHTUHt4sGnMver8GCjndfw1ZIYysb67b14Cm9POvw6nFFjt48RJt4 G/NKSms/TR+GZP1W2K3gpWI8x/Dtf04= Date: Mon, 26 May 2025 21:14:58 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched_class change for throttled task To: Aaron Lu Cc: Valentin Schneider , Ben Segall , K Prateek Nayak , Peter Zijlstra , Josh Don , Ingo Molnar , Vincent Guittot , Xi Wang , linux-kernel@vger.kernel.org, Juri Lelli , Dietmar Eggemann , Steven Rostedt , Mel Gorman , Chuyi Zhou , Jan Kiszka , Florian Bezdeka References: <20250520104110.3673059-1-ziqianlu@bytedance.com> <20250520104110.3673059-5-ziqianlu@bytedance.com> <63237b23-ae10-45f9-abdd-8ea4adb4d15e@linux.dev> <20250523075640.GA1168183@bytedance> <20250523094106.GA1210419@bytedance> <20250523115921.GB1240558@bytedance> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Chengming Zhou In-Reply-To: <20250523115921.GB1240558@bytedance> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2025/5/23 19:59, Aaron Lu wrote: > On Fri, May 23, 2025 at 05:53:55PM +0800, Chengming Zhou wrote: >> On 2025/5/23 17:42, Aaron Lu wrote: >>> On Fri, May 23, 2025 at 05:13:35PM +0800, Chengming Zhou wrote: >>>> On 2025/5/23 15:56, Aaron Lu wrote: >>>>> On Fri, May 23, 2025 at 10:43:53AM +0800, Chengming Zhou wrote: >>>>>> On 2025/5/20 18:41, Aaron Lu wrote: >>>>>>> On task group change, for tasks whose on_rq equals to TASK_ON_RQ_QUEUED, >>>>>>> core will dequeue it and then requeued it. >>>>>>> >>>>>>> The throttled task is still considered as queued by core because p->on_rq >>>>>>> is still set so core will dequeue it, but since the task is already >>>>>>> dequeued on throttle in fair, handle this case properly. >>>>>>> >>>>>>> Affinity and sched class change is similar. >>>>>> >>>>>> How about setting p->on_rq to 0 when throttled? which is the fact that >>>>>> the task is not on cfs queue anymore, does this method cause any problem? >>>>>> >>>>> >>>>> On task group change/affinity change etc. if the throttled task is >>>>> regarded as !on_rq, then it will miss the chance to be enqueued to the >>>>> new(and correct) cfs_rqs, instead, it will be enqueued back to its >>>>> original cfs_rq on unthrottle which breaks affinity or task group >>>> >>>> Yeah, this is indeed a problem, I was thinking to delete the throttled task >>>> from the cfs_rq limbo list, then add it to another cfs_rq limbo list or cfs_rq >>>> runnable tree based on the new cfs_rq's throttle status. >>> >>> Only work when the task is still handled by fair :) >>> >>>> >>>> But it's much complex compared with your current method. >>>> >>>>> settings. We may be able to do something in tg_unthrottle_up() to take >>>>> special care of these situations, but it seems a lot of headaches. >>>>> >>>>> Also, for task group change, if the new task group does not have throttle >>>>> setting, that throttled task should be allowed to run immediately instead >>>>> of waiting for its old cfs_rq's unthrottle event. Similar is true when >>>>> this throttled task changed its sched class, like from fair to rt. >>>>> >>>>> Makes sense? >>>> >>>> Ok, the another problem of the current method I can think of is the PELT maintenance, >>>> we skip the actual dequeue_task_fair() process, which includes PELT detach, we just >>>> delete it from the cfs_rq limbo list, so it can result in PELT maintenance error. >>>> >>> >>> There are corresponding callbacks that handle this, e.g. for task group >>> change, there is task_change_group_fair() that handles PELT detach; for >>> affinity change, there is migrate_task_rq_fair() does that and for sched >>> class change, there is switched_from/to() does that. >>> >>> Or do I miss anything? >> >> migrate_task_rq_fair() only do it when !task_on_rq_migrating(p), which is wakeup migrate, >> because we already do detach in dequeue_task_fair() for on_rq task migration... >> You can see the DO_DETACH flag in update_load_avg() called from dequeue_entity(). >> > > Understood, thanks for catching this! > > So the code was initially developed on top of v5.15 and there is a > detach in migrate_task_rq_fair(): > > if (p->on_rq == TASK_ON_RQ_MIGRATING) { > /* > * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old' > * rq->lock and can modify state directly. > */ > lockdep_assert_rq_held(task_rq(p)); > detach_entity_cfs_rq(&p->se); > } > > But looks like it's gone now by commit e1f078f50478("sched/fair: Combine > detach into dequeue when migrating task") and I failed to notice this > detail... Yeah.. > > Anyway, the task is already dequeued without TASK_ON_RQ_MIGRATING being > set when throttled and it can't be dequeued again, so perhaps something > like below could cure this situation?(just to illustrate the idea, not > even compile tested) Ok, seems reasonable to me! > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 89afa472299b7..dc2e9a6bf3fd7 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5868,6 +5868,9 @@ static void dequeue_throttled_task(struct task_struct *p, int flags) > WARN_ON_ONCE(flags & DEQUEUE_SLEEP); > > list_del_init(&p->throttle_node); > + > + if (task_on_rq_migrating(p)) > + detach_task_cfs_rq(p); > } > >