From mboxrd@z Thu Jan 1 00:00:00 1970 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 DAA34C433EF for ; Wed, 20 Jul 2022 13:41:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235726AbiGTNlI (ORCPT ); Wed, 20 Jul 2022 09:41:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34896 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231751AbiGTNlF (ORCPT ); Wed, 20 Jul 2022 09:41:05 -0400 Received: from mail-pg1-x52f.google.com (mail-pg1-x52f.google.com [IPv6:2607:f8b0:4864:20::52f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B6B8B49B71 for ; Wed, 20 Jul 2022 06:41:02 -0700 (PDT) Received: by mail-pg1-x52f.google.com with SMTP id e132so16427406pgc.5 for ; Wed, 20 Jul 2022 06:41:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:content-language:to :cc:references:from:in-reply-to:content-transfer-encoding; bh=qKe9h167RYgDmgLdVVflY53zcuYISuUI/T7O6NAAcQo=; b=fEpNyoOJmqPx+KdDKO0aU8SwtybOSy1CQFXNp5twY5elBZZGTGRF9PMWOhqBF2p3ST J5xqW+ZEPA+ivJ2Hf6FMMfEmf8S7LdAGn0qmDWs/xxDxoniBbfsfav0xkLizqrkLV1kt UX/qaR/y1jPdPPFptdL2ivJa2wXNasOmXyR14T3WZ641sgLlep2kINNCOkD+cnWAsxu/ pnlMQIMxUkkj/VXwKv2aAUyrDeotFYoTSPwj7hsreshZ8IKXzrEO83HchgYkaewLzB36 0dzu4ci1qFleOZNNneA8N+2yh/Y3XscPXWFeX3gZZBOSw7RsJRj0ZeoAbBOBXkk+Shj0 pblg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:in-reply-to :content-transfer-encoding; bh=qKe9h167RYgDmgLdVVflY53zcuYISuUI/T7O6NAAcQo=; b=DFR3abuolq2ECQlePhfA2qEpH0uVixE2rsO1cB9jRGM2A8rQr6fV+KdfrrFK7LFpXb VlGxZu0rR5Z1uSVkaI5w3r1kA9umZ5Y83AVK/FEQjkhXhoouD3rh89r7bssAobmJetVp yuG1Qkdzg41mxlrqnxssDHildRHX8IjQgNgc0Upg4zjQHds3qzBh+4OdpePbE1W8qeCt 2ycXOpbkDeYyWExOwD5vYEGMKK+A0YyP7bF36NTjq/hhgmqNk/oMKCAaWwHA6L18EtoY CpSdjrGKZ+Kdfe5XFWDaXxWuih2CxuFjIiJSKrjwtVt0DaxtIMF0Xyytsx/sOiOhpA14 +E2A== X-Gm-Message-State: AJIora8qBZDHAyTq+kXivbyXQgBFfGUvMfiKWingdyeasF3t/AriKfTs ZQ+IOeoB5yDc/Uyd6ccznVUmWg== X-Google-Smtp-Source: AGRyM1tUi6jETJCs6pHwnH/XK6Avxt+a6VkWA0CZgkMs3kUa5aR5NFJ2OnvKb2oLVbvOnpf9CK5TNQ== X-Received: by 2002:a05:6a00:16c7:b0:520:6ede:2539 with SMTP id l7-20020a056a0016c700b005206ede2539mr38729118pfc.46.1658324462145; Wed, 20 Jul 2022 06:41:02 -0700 (PDT) Received: from [10.4.100.137] ([139.177.225.240]) by smtp.gmail.com with ESMTPSA id w63-20020a623042000000b0052ab8646500sm13605434pfw.56.2022.07.20.06.40.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Jul 2022 06:41:01 -0700 (PDT) Message-ID: <107763fc-7eab-a807-fa15-024178bdf57e@bytedance.com> Date: Wed, 20 Jul 2022 21:40:55 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.0.1 Subject: Re: [External] Re: [PATCH v2 07/10] sched/fair: use update_load_avg() to attach/detach entity load_avg Content-Language: en-US To: Vincent Guittot Cc: Dietmar Eggemann , mingo@redhat.com, peterz@infradead.org, rostedt@goodmis.org, bsegall@google.com, vschneid@redhat.com, linux-kernel@vger.kernel.org References: <20220713040430.25778-1-zhouchengming@bytedance.com> <20220713040430.25778-8-zhouchengming@bytedance.com> <88062fb6-e2fe-cf4e-10b5-7694c4d30941@bytedance.com> From: Chengming Zhou In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/7/19 18:29, Vincent Guittot wrote: > On Fri, 15 Jul 2022 at 18:21, Chengming Zhou > wrote: >> >> On 2022/7/15 19:18, Dietmar Eggemann wrote: >>> On 13/07/2022 06:04, Chengming Zhou wrote: >>>> Since update_load_avg() support DO_ATTACH and DO_DETACH now, we can >>>> use update_load_avg() to implement attach/detach entity load_avg. >>>> >>>> Another advantage of using update_load_avg() is that it will check >>>> last_update_time before attach or detach, instead of unconditional >>>> attach/detach in the current code. >>>> >>>> This way can avoid some corner problematic cases of load tracking, >>>> like twice attach problem, detach unattached NEW task problem. >>> >>> This explanation is somewhat hard to follow for me. Since both issues >>> have been fixed already (you mention this further below) you're saying >>> that with you change you don't reintroduce them? >> >> Sorry for this not very clear explanation. >> >> Yes, both issues have been fixed already, what I want to say is that bugfix >> brings its own problem and limitation mentioned below. > > As Dietmar said, the commit message is misleading because someone can > think you fix these bugs whereas it's not the case Hi Vincent, thanks for your review! I will refactor the commit message to avoid this misleading, sorry for my bad English expression. > >> >> So I want to use another way to solve these problems better. >> >>> >>>> 1. switch to fair class (twice attach problem) >>>> >>>> p->sched_class = fair_class; --> p.se->avg.last_update_time = 0 >>>> if (queued) >>>> enqueue_task(p); >>>> ... >>>> enqueue_entity() >>>> update_load_avg(UPDATE_TG | DO_ATTACH) >>>> if (!se->avg.last_update_time && (flags & DO_ATTACH)) --> true >>>> attach_entity_load_avg() --> attached, will set last_update_time >>>> check_class_changed() >>>> switched_from() (!fair) >>>> switched_to() (fair) >>>> switched_to_fair() >>>> attach_entity_load_avg() --> unconditional attach again! >>>> >>>> 2. change cgroup of NEW task (detach unattached task problem) >>>> >>>> sched_move_group(p) >>>> if (queued) >>>> dequeue_task() >>>> task_move_group_fair() >>>> detach_task_cfs_rq() >>>> detach_entity_load_avg() --> detach unattached NEW task >>>> set_task_rq() >>>> attach_task_cfs_rq() >>>> attach_entity_load_avg() >>>> if (queued) >>>> enqueue_task() >>>> >>>> These problems have been fixed in commit 7dc603c9028e >>>> ("sched/fair: Fix PELT integrity for new tasks"), which also >>>> bring its own problems. >>>> >>>> First, it add a new task state TASK_NEW and an unnessary limitation >>>> that we would fail when change the cgroup of TASK_NEW tasks. >> >> This is the limitation that bugfix has brought. >> >> We can't change cgroup or switch to fair for task with last_update_time=0 >> if we don't have conditional detach/attach. >> >> So we have to: >> >> 1. !fair task also need to set last_update_time. >> 2. cpu_cgroup_can_attach() have to wait for TASK_NEW to fully attached. >> >>>> >>>> Second, it attach entity load_avg in post_init_entity_util_avg(), >>>> in which we only set sched_avg last_update_time for !fair tasks, >>>> will cause PELT integrity problem when switched_to_fair(). >>> >>> I guess those PELT integrity problems are less severe since we have the >>> enqueue_task_fair() before the switched_to_fair() for enqueued tasks. So >>> we always decay the time the task spend outside fair. >> >> enqueue_task_fair() >> enqueue_entity() >> update_load_avg() >> if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD)) --> true >> __update_load_avg_se(now, cfs_rq, se); --> (1) >> >> We can see above, for queued !fair task, (1) will deay the delta time >> (now - se.avg.last_update_time) even for a NEW !fair task. >> >>> >>> Looks to me that you want to replace this by your `freeze PELT when >>> outside fair` model. >> >> Yes, want to freeze PELT for two !fair cases: >> >> 1. !fair task hasn't been fair before: will still have its init load_avg >> when switch to fair. > > But I'm not sure it makes sense to keep these init values. As an > example, the util_avg is set according to the cpu utilization at the > time of the task creation. I would tend to decay them as these init > values become less and less relevant. > > so we should return early in post_init_entity_util_avg() and don't set > util_avg if sched class is not cfs Yes, this indeed is a problem if we attach this init sched_avg of !fair task. I'm also not sure whether it make sense to keep them to 0 ? Will it cause unfairness problem between cfs_rqs? > >> >> 2. !fair task has been switched_from_fair(): will still keep its lastest >> load_avg when switch to fair. >> >>> >>>> This patch make update_load_avg() the only location of attach/detach, >>>> and can handle these corner cases like change cgroup of NEW tasks, >>>> by checking last_update_time before attach/detach. >>> >>> [...] >>> >>>> @@ -11527,9 +11522,7 @@ static void detach_entity_cfs_rq(struct sched_entity *se) >>>> struct cfs_rq *cfs_rq = cfs_rq_of(se); >>>> >>>> /* Catch up with the cfs_rq and remove our load when we leave */ >>>> - update_load_avg(cfs_rq, se, 0); >>>> - detach_entity_load_avg(cfs_rq, se); >>>> - update_tg_load_avg(cfs_rq); >>>> + update_load_avg(cfs_rq, se, UPDATE_TG | DO_DETACH); >>> >>> IMHO, the DO_[DE|AT]TACH comments in update_load_avg() would have to be >>> updated in this case. >> >> Correct, will do. >> >> Thanks. >> >>> >>> [...]