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 X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EDA1BC433E0 for ; Wed, 13 Jan 2021 08:31:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8DAFB2333B for ; Wed, 13 Jan 2021 08:31:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726822AbhAMIbI (ORCPT ); Wed, 13 Jan 2021 03:31:08 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44516 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726288AbhAMIbH (ORCPT ); Wed, 13 Jan 2021 03:31:07 -0500 Received: from mail-lf1-x136.google.com (mail-lf1-x136.google.com [IPv6:2a00:1450:4864:20::136]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0177FC061575 for ; Wed, 13 Jan 2021 00:30:27 -0800 (PST) Received: by mail-lf1-x136.google.com with SMTP id u25so1536395lfc.2 for ; Wed, 13 Jan 2021 00:30:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=XJaTPxEArYKVlZrCQN/uN6ZULj4gM1PQ/CUE3L6eJxU=; b=QQB2oLxgNrUIHQyxcEsn6C89VLuuhBFzbjSH/zkyrZdBnPMU0hM3jQB/cVU0dIx/xZ okSsYHeWA4AGHWLyWeGkhaOGHjYEo2Gw6w4Ou2C4FrFLAiuzHlh85DAb3FYZ6VTfem89 z41/1VcNhfS+p9q9GUAzngqfhUpgatiwI2qYcE9hANBvRw8srX1weIMKayj9EW8U7CHY VVfAXhV6wzNAuX9ZSq6fiuHbCqRi6Mo2fkGIYug+IDktFYHuOtq+3wB0nv8v0c3bbiQU 2RxGSAOyueVkKdPFWTKIZ1niEpZYwkEq5iqL4O9I5ap5rBF8TWKrpvSoG2NRnyeEbFlV I4DQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=XJaTPxEArYKVlZrCQN/uN6ZULj4gM1PQ/CUE3L6eJxU=; b=F+IfbmqMdBY/juKlh8hxz+yX7URGTLMAjGDGda4f73Ja+vjGvGSrauN+2ucycnaS07 MJEj5VBF7q/k5vUSAaCPFz2FgvHQQhHooYMtz57MqmY42xJGEUML7dd1pwU2mmwEZhJZ BzSCWlmfF5BY0ceHBH0NI+5ycjNR3wFA7E88q32V6TcdvxEWRvPTc3g95FybjT7ss473 3Uwd7qk2msihiSWj5h5roLgFpzoFK0A+cT5w+6uHsS91+72LUG0v9mPnAEs1RABlASMC 0zxZjm8FizeljxZv6y6f9qOY/iaTMDp8D8L6fQv1thDbq6tJ+ODkKzwkKqiSPwokUYzk sYEQ== X-Gm-Message-State: AOAM530G7+0uPPjYGhT2XwdcqZWXw3fOMhk/AgYavBXiBb9X6P10k8tt LoLuZ10Iq22m3MVRjhKsbSC7wP/yyyH7WEP2hzF+2A== X-Google-Smtp-Source: ABdhPJyqSaKfwC8QBGkdoh9R8It2LLfON4poxPLPBKa/Og/s3YE+sT0+t1ZifgXnekIt89GsJmKy44QmBoevUDD5p50= X-Received: by 2002:a19:2d0a:: with SMTP id k10mr423526lfj.286.1610526625442; Wed, 13 Jan 2021 00:30:25 -0800 (PST) MIME-Version: 1.0 References: <1608710968-31475-1-git-send-email-ultrachin@163.com> <1fefea2e.70bf.176f08d9fae.Coremail.ultrachin@163.com> <61e22917.538b.176f56231f6.Coremail.ultrachin@163.com> <38c1aeee.2d5f.176f9bb0cfb.Coremail.ultrachin@163.com> In-Reply-To: <38c1aeee.2d5f.176f9bb0cfb.Coremail.ultrachin@163.com> From: Vincent Guittot Date: Wed, 13 Jan 2021 09:30:14 +0100 Message-ID: Subject: Re: [PATCH] sched: pull tasks when CPU is about to run SCHED_IDLE tasks To: chin Cc: linux-kernel , Ingo Molnar , Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , heddchen@tencent.com, =?UTF-8?B?eGlhb2dnY2hlbijpmYjlsI/lhYkp?= Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 13 Jan 2021 at 04:14, chin wrote: > > > > > At 2021-01-12 16:18:51, "Vincent Guittot" wrote: > >On Tue, 12 Jan 2021 at 07:59, chin wrote: > >> > >> > >> > >> > >> At 2021-01-11 19:04:19, "Vincent Guittot" wrote: > >> >On Mon, 11 Jan 2021 at 09:27, chin wrote: > >> >> > >> >> > >> >> At 2020-12-23 19:30:26, "Vincent Guittot" wrote: > >> >> >On Wed, 23 Dec 2020 at 09:32, wrote: > >> >> >> > >> >> >> From: Chen Xiaoguang > >> >> >> > >> >> >> Before a CPU switches from running SCHED_NORMAL task to > >> >> >> SCHED_IDLE task, trying to pull SCHED_NORMAL tasks from other > >> >> > > >> >> >Could you explain more in detail why you only care about this use case > >> >> > >> >> >in particular and not the general case? > >> >> > >> >> > >> >> We want to run online tasks using SCHED_NORMAL policy and offline tasks > >> >> using SCHED_IDLE policy. The online tasks and the offline tasks run in > >> >> the same computer in order to use the computer efficiently. > >> >> The online tasks are in sleep in most times but should responce soon once > >> >> wake up. The offline tasks are in low priority and will run only when no online > >> >> tasks. > >> >> > >> >> The online tasks are more important than the offline tasks and are latency > >> >> sensitive we should make sure the online tasks preempt the offline tasks > >> >> as soon as possilbe while there are online tasks waiting to run. > >> >> So in our situation we hope the SCHED_NORMAL to run if has any. > >> >> > >> >> Let's assume we have 2 CPUs, > >> >> In CPU1 we got 2 SCHED_NORMAL tasks. > >> >> in CPU2 we got 1 SCHED_NORMAL task and 2 SCHED_IDLE tasks. > >> >> > >> >> CPU1 CPU2 > >> >> curr rq1 curr rq2 > >> >> +------+ | +------+ +------+ | +----+ +----+ > >> >> t0 |NORMAL| | |NORMAL| |NORMAL| | |IDLE| |IDLE| > >> >> +------+ | +------+ +------+ | +----+ +----+ > >> >> > >> >> NORMAL exits or blocked > >> >> +------+ | +------+ | +----+ +----+ > >> >> t1 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > >> >> +------+ | +------+ | +----+ +----+ > >> >> > >> >> pick_next_task_fair > >> >> +------+ | +------+ +----+ | +----+ > >> >> t2 |NORMAL| | |NORMAL| |IDLE| | |IDLE| > >> >> +------+ | +------+ +----+ | +----+ > >> >> > >> >> SCHED_IDLE running > >> >> t3 +------+ | +------+ +----+ | +----+ > >> >> |NORMAL| | |NORMAL| |IDLE| | |IDLE| > >> >> +------+ | +------+ +----+ | +----+ > >> >> > >> >> run_rebalance_domains > >> >> +------+ | +------+ | +----+ +----+ > >> >> t4 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > >> >> +------+ | +------+ | +----+ +----+ > >> >> > >> >> As we can see > >> >> t1: NORMAL task in CPU2 exits or blocked > >> >> t2: CPU2 pick_next_task_fair would pick a SCHED_IDLE to run while > >> >> another SCHED_NORMAL in rq1 is waiting. > >> >> t3: SCHED_IDLE run in CPU2 while a SCHED_NORMAL wait in CPU1. > >> >> t4: after a short time, periodic load_balance triggerd and pull > >> >> SCHED_NORMAL in rq1 to rq2, and SCHED_NORMAL likely preempts SCHED_IDLE. > >> >> > >> >> In this scenario, SCHED_IDLE is running while SCHED_NORMAL is waiting to run. > >> >> The latency of this SCHED_NORMAL will be high which is not acceptble. > >> >> > >> >> Do a load_balance before running the SCHED_IDLE may fix this problem. > >> >> > >> >> This patch works as below: > >> >> > >> >> CPU1 CPU2 > >> >> curr rq1 curr rq2 > >> >> +------+ | +------+ +------+ | +----+ +----+ > >> >> t0 |NORMAL| | |NORMAL| |NORMAL| | |IDLE| |IDLE| > >> >> +------+ | +------+ +------+ | +----+ +----+ > >> >> > >> >> NORMAL exits or blocked > >> >> +------+ | +------+ | +----+ +----+ > >> >> t1 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > >> >> +------+ | +------+ | +----+ +----+ > >> >> > >> >> t2 pick_next_task_fair (all se are SCHED_IDLE) > >> >> > >> >> newidle_balance > >> >> +------+ | +------+ | +----+ +----+ > >> >> t3 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > >> >> +------+ | +------+ | +----+ +----+ > >> >> > >> >> > >> >> t1: NORMAL task in CPU2 exits or blocked > >> >> t2: pick_next_task_fair check all se in rbtree are SCHED_IDLE and calls > >> >> newidle_balance who tries to pull a SCHED_NORMAL(if has). > >> >> t3: pick_next_task_fair would pick a SCHED_NORMAL to run instead of > >> >> SCHED_IDLE(likely). > >> >> > >> >> > > >> >> >> CPU by doing load_balance first. > >> >> >> > >> >> >> Signed-off-by: Chen Xiaoguang > >> >> >> Signed-off-by: Chen He > >> >> >> --- > >> >> >> kernel/sched/fair.c | 5 +++++ > >> >> >> 1 file changed, 5 insertions(+) > >> >> >> > >> >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> >> >> index ae7ceba..0a26132 100644 > >> >> >> --- a/kernel/sched/fair.c > >> >> >> +++ b/kernel/sched/fair.c > >> >> >> @@ -7004,6 +7004,11 @@ struct task_struct * > >> >> >> struct task_struct *p; > >> >> >> int new_tasks; > >> >> >> > >> >> >> + if (prev && > >> >> >> + fair_policy(prev->policy) && > >> >> > > >> >> >Why do you need a prev and fair task ? You seem to target the special > >> >> >case of pick_next_task but in this case why not only testing rf!=null > >> >> > to make sure to not return immediately after jumping to the idle > >> >> > >> >> >label? > >> >> We just want to do load_balance only when CPU switches from SCHED_NORMAL > >> >> to SCHED_IDLE. > >> >> If not check prev, when the running tasks are all SCHED_IDLE, we would > >> >> do newidle_balance everytime in pick_next_task_fair, it makes no sense > >> >> and kind of wasting. > >> > > >> >I agree that calling newidle_balance every time pick_next_task_fair is > >> >called when there are only sched_idle tasks is useless. > >> >But you also have to take into account cases where there was another > >> >class of task running on the cpu like RT one. In your example above, > >> >if you replace the normal task on CPU2 by a RT task, you still want to > >> > >> >pick the normal task on CPU1 once RT task goes to sleep. > >> Sure, this case should be taken into account, we should also try to > >> pick normal task in this case. > >> > >> > > >> >Another point that you will have to consider the impact on > >> >rq->idle_stamp because newidle_balance is assumed to be called before > >> > >> >going idle which is not the case anymore with your use case > >> Yes. rq->idle_stamp should not be changed in this case. > >> > >> > >> > >> Actually we want to pull a SCHED_NORMAL task (if possible) to run when a cpu is > >> about to run SCHED_IDLE task. But currently newidle_balance is not > >> designed for SCHED_IDLE so SCHED_IDLE can also be pulled which > >> is useless in our situation. > > > >newidle_balance will pull a sched_idle task only if there is an > >imbalance which is the right thing to do IMO to ensure fairness > >between sched_idle tasks. Being a sched_idle task doesn't mean that > >we should break the fairness > > > >> > >> So we plan to add a new function sched_idle_balance which only try to > >> pull SCHED_NORMAL tasks from the busiest cpu. And we will call > >> sched_idle_balance when the previous task is normal or RT and > >> hoping we can pull a SCHED_NORMAL task to run. > >> > >> Do you think it is ok to add a new sched_idle_balance? > > > >I don't see any reason why the scheduler should not pull a sched_idle > >task if there is an imbalance. That will happen anyway during the next > > >periodic load balance > OK. We should not pull the SCHED_IDLE tasks only in load_balance. > > > Do you think it make sense to do an extra load_balance when cpu is > about to run SCHED_IDLE task (switched from normal/RT)? I'm not sure to get your point here. Do you mean if a sched_idle task is picked to become the running task whereas there are runnable normal tasks ? This can happen if normal tasks are long running tasks. We should not in this case. The only case is when the running task, which is not a sched_idle task but a normal/rt/deadline one, goes to sleep and there are only sched_idle tasks enqueued. In this case and only in this case, we should trigger a load_balance to get a chance to pull a waiting normal task from another CPU. This means checking this state in pick_next_task_fair() and in balance_fair() > By doing this SCHED_NORMAL tasks waiting on other cpus would get > a chance to be pulled to this cpu and run, it is helpful to reduce the latency > of SCHED_NORMAL tasks. > > > >>> > >> > > >> >> > >> >> > > >> >> > >> >> >Also why not doing that for default case too ? i.e. balance_fair() ? > >> >> You are right, if you think this scenario makes sense, we will send a > >> >> refined patch soon :-) > >> >> > >> >> > > >> >> >> + sched_idle_cpu(rq->cpu)) > >> >> >> + goto idle; > >> >> >> + > >> >> >> again: > >> >> >> if (!sched_fair_runnable(rq)) > >> >> >> goto idle; > >> >> >> -- > >> >> >> 1.8.3.1 > >> >> >> > >> >> >>