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 B5965EB64DC for ; Tue, 11 Jul 2023 22:23:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232016AbjGKWXU (ORCPT ); Tue, 11 Jul 2023 18:23:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60842 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229512AbjGKWXQ (ORCPT ); Tue, 11 Jul 2023 18:23:16 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1295310F1 for ; Tue, 11 Jul 2023 15:22:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1689114149; 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: in-reply-to:in-reply-to:references:references; bh=PQ2+rDjcjMB3Qu7cGr9+SsEPr1twMRHj94h2nff+OLA=; b=NwwZaIFPu5qZmBi61QjgxGHB3REhXoDY/YfW53yK2Nz0WDDRVemqOAdqFMWThpGSlPzH92 3yCBv4SIvdSa3f4Z8M8lIBkDo43itsWEK/rEs8gstnhkEdwpEfoqpP3R32oudcVrlfAArl +obU3787Xj2aSvpwMr6c4k1HZYU1ScU= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-222-h2BfI1zsPrWm3d_MtO_K-w-1; Tue, 11 Jul 2023 18:22:24 -0400 X-MC-Unique: h2BfI1zsPrWm3d_MtO_K-w-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 32F173C100A1; Tue, 11 Jul 2023 22:22:24 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.22.17.109]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7508B200AD6E; Tue, 11 Jul 2023 22:22:23 +0000 (UTC) Date: Tue, 11 Jul 2023 18:22:21 -0400 From: Phil Auld To: Benjamin Segall Cc: linux-kernel@vger.kernel.org, Juri Lelli , Ingo Molnar , Daniel Bristot de Oliveira , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , Steven Rostedt , Mel Gorman , Frederic Weisbecker , Tejun Heo Subject: Re: [PATCH v5 2/2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use Message-ID: <20230711222221.GD150804@lorien.usersys.redhat.com> References: <20230707195748.2918490-1-pauld@redhat.com> <20230707195748.2918490-3-pauld@redhat.com> <20230711131024.GA150804@lorien.usersys.redhat.com> <20230711141210.GC150804@lorien.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 11, 2023 at 03:07:12PM -0700 Benjamin Segall wrote: > Phil Auld writes: > > > On Tue, Jul 11, 2023 at 09:10:24AM -0400 Phil Auld wrote: > >> On Mon, Jul 10, 2023 at 04:54:58PM -0700 Benjamin Segall wrote: > >> > Phil Auld writes: > >> > > >> > > CFS bandwidth limits and NOHZ full don't play well together. Tasks > >> > > can easily run well past their quotas before a remote tick does > >> > > accounting. This leads to long, multi-period stalls before such > >> > > tasks can run again. Currently, when presented with these conflicting > >> > > requirements the scheduler is favoring nohz_full and letting the tick > >> > > be stopped. However, nohz tick stopping is already best-effort, there > >> > > are a number of conditions that can prevent it, whereas cfs runtime > >> > > bandwidth is expected to be enforced. > >> > > > >> > > Make the scheduler favor bandwidth over stopping the tick by setting > >> > > TICK_DEP_BIT_SCHED when the only running task is a cfs task with > >> > > runtime limit enabled. We use cfs_b->hierarchical_quota to > >> > > determine if the task requires the tick. > >> > > > >> > > Add check in pick_next_task_fair() as well since that is where > >> > > we have a handle on the task that is actually going to be running. > >> > > > >> > > Add check in sched_can_stop_tick() to cover some edge cases such > >> > > as nr_running going from 2->1 and the 1 remains the running task. > >> > > > >> > > Add sched_feat HZ_BW (off by default) to control the tick_stop > >> > > behavior. > >> > > > >> > > Signed-off-by: Phil Auld > >> > > Cc: Ingo Molnar > >> > > Cc: Peter Zijlstra > >> > > Cc: Vincent Guittot > >> > > Cc: Juri Lelli > >> > > Cc: Dietmar Eggemann > >> > > Cc: Valentin Schneider > >> > > Cc: Ben Segall > >> > > Cc: Frederic Weisbecker > >> > > --- > >> > > kernel/sched/core.c | 12 ++++++++++ > >> > > kernel/sched/fair.c | 49 +++++++++++++++++++++++++++++++++++++++++ > >> > > kernel/sched/features.h | 2 ++ > >> > > kernel/sched/sched.h | 1 + > >> > > 4 files changed, 64 insertions(+) > >> > > > >> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > >> > > index 1b214e10c25d..4b8534abdf4f 100644 > >> > > --- a/kernel/sched/core.c > >> > > +++ b/kernel/sched/core.c > >> > > @@ -1229,6 +1229,18 @@ bool sched_can_stop_tick(struct rq *rq) > >> > > if (rq->nr_running > 1) > >> > > return false; > >> > > > >> > > + /* > >> > > + * If there is one task and it has CFS runtime bandwidth constraints > >> > > + * and it's on the cpu now we don't want to stop the tick. > >> > > + * This check prevents clearing the bit if a newly enqueued task here is > >> > > + * dequeued by migrating while the constrained task continues to run. > >> > > + * E.g. going from 2->1 without going through pick_next_task(). > >> > > + */ > >> > > + if (sched_feat(HZ_BW) && rq->nr_running == 1 && task_on_rq_queued(rq->curr)) { > >> > > + if (cfs_task_bw_constrained(rq->curr)) > >> > > + return false; > >> > > + } > >> > > + > >> > > >> > I think we still need the fair_sched_class check with the bit being on > >> > cfs_rq/tg rather than task. > >> > > >> > >> Is there a way a non-fair_sched_class task will be in a cfs_rq with > >> cfs_rq->runtime_enabled and/or cfs_b->hierarchical_quota set to non > >> RUNTIME_INF? I suppose if they are stale and it's had its class changed? > >> > >> That makes the condition pretty ugly but I can add that back if needed. > >> > > > > Sigh, yeah. I took that out when I had the bit in the task. I'll put it > > back in... > > > > Yeah, cfs_rq (and rt_rq) are set unconditionally, and a cgroup can have > a mix of fair and RT tasks (whether or not that's a good idea from a > sysadmin perspective). > Thanks. I think I'll try the condition as a single-use static inline function. The generated code seems the same but it is a bit nicer to read. Cheers, Phil --