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 86F13C433EF for ; Fri, 15 Jul 2022 16:21:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232147AbiGOQV4 (ORCPT ); Fri, 15 Jul 2022 12:21:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46248 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229475AbiGOQVx (ORCPT ); Fri, 15 Jul 2022 12:21:53 -0400 Received: from mail-pj1-x1029.google.com (mail-pj1-x1029.google.com [IPv6:2607:f8b0:4864:20::1029]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3BB4E6EEAA for ; Fri, 15 Jul 2022 09:21:52 -0700 (PDT) Received: by mail-pj1-x1029.google.com with SMTP id o15so6092668pjh.1 for ; Fri, 15 Jul 2022 09:21:52 -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=6GrLS/v/mfBqbOvGDktGZkoGmn9hNwjxPNaIK3oKp8A=; b=4g+Uiw6VuhSzOvC0y2K3h8Q+YnMl3dB1+Rpt7zfLKr+TWdz/X0mJKlVjUID/cdHKdI BuPVTobzgZGHbLvu3wWx5Sx+0W9mLI0pkK8oj7WMcsAqC0RTMl5QNHdOYfppScY6StGZ dg0+GBLE5KcobBk+oCu370Ue5mW2HS3CXs7VG3FC8xKFjjeML9v7VevJ6uGO4ydAyGqZ Ozjy4v/qEwfoE7xph9piyelUNf7ZuUwOj1XeLVD4MqB/JwMauEbgJWY9kQlOYj2btTwB rPuZ3Yz6KUPVe7SvNTicWDu5JCVPQBAEhPTMnXzLg4gjtpVP7VlIEz9gzU/ZvwXPb/vq qCNQ== 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=6GrLS/v/mfBqbOvGDktGZkoGmn9hNwjxPNaIK3oKp8A=; b=2mzdsAXlVz/kWvDjsy4e70spv7lMzrfPviTrFL+tyYmo3hw/pT6QubTFGo4lWUpENO X9yMZC3wDxmdFuNAwrRCFQ8GOYxbdOZfrK0Gjz9YwH0DzXQ75MV09VxDzsras8wTSxjQ uFE0tO87BQmyThD8dboHWNbw2YPuLnNZLQLE0MwWzowRBpU0/RHJjxDZmFa2xC8EHMeo W926BkaZ+IHpXGsvEgE/jctPHb6tIOP8papcMwSZrbmP1SLj8UZjA3OuILLjB8q94wXr uAkPLwqS9q3j9MuRwxSWS9+1/T90lex1H5ffEHbydu6Trlnh0plj/8liDjDk9yF/gGP7 4dSw== X-Gm-Message-State: AJIora+ESzYLOkGL2hxoefGQ6LqPmpt+9nR4nVAQJ5NUzjLTV5Zpiv/l 7Ul3paNBc01XhmErZiIcC4Zas/qBZeTrKjhf X-Google-Smtp-Source: AGRyM1t+ku27pWB2PMTzHHmQazYUeerAutMF3sBGOExhghCXEjfLZAWeL+znUfymYkKFKQpgCbJGkg== X-Received: by 2002:a17:90b:1c0b:b0:1f0:23df:5406 with SMTP id oc11-20020a17090b1c0b00b001f023df5406mr16907593pjb.157.1657902111778; Fri, 15 Jul 2022 09:21:51 -0700 (PDT) Received: from [10.255.246.141] ([139.177.225.243]) by smtp.gmail.com with ESMTPSA id o90-20020a17090a0a6300b001f0cece9285sm3399995pjo.18.2022.07.15.09.21.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 15 Jul 2022 09:21:51 -0700 (PDT) Message-ID: <88062fb6-e2fe-cf4e-10b5-7694c4d30941@bytedance.com> Date: Sat, 16 Jul 2022 00:21:46 +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: Dietmar Eggemann , mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org, rostedt@goodmis.org, bsegall@google.com, vschneid@redhat.com Cc: linux-kernel@vger.kernel.org References: <20220713040430.25778-1-zhouchengming@bytedance.com> <20220713040430.25778-8-zhouchengming@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 2022/7/15 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. 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. 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. > > [...]