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 767B9EB64D7 for ; Thu, 29 Jun 2023 00:54:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230476AbjF2Ayq (ORCPT ); Wed, 28 Jun 2023 20:54:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34378 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229749AbjF2Ayk (ORCPT ); Wed, 28 Jun 2023 20:54:40 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 86DA12117 for ; Wed, 28 Jun 2023 17:53:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1688000030; 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=xzZHhIzDDuackwUVlvpBeapF4XcO6szZYosiE5wGDwM=; b=ffA65hFWlXy8jb4hAAdwuZ3fTEOd/oy3mGOA0Ab/3ZAdfSTuksZyTWEwhynWKgRz3E8rDC PEdU5bySkjK9YSVyQHSO8IK0fsRUzz3ri82EqDafZywt0VY5fTasGrNYZdAediOiUyvMmZ 16ddEUah0Ke4mxHrA152lxs/UYA4NOw= 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-161-1tWZ0C-HNQ6LNvr3pFuuIA-1; Wed, 28 Jun 2023 20:53:46 -0400 X-MC-Unique: 1tWZ0C-HNQ6LNvr3pFuuIA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8BCDB2814250; Thu, 29 Jun 2023 00:53:45 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.22.17.16]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EEC02F5CE4; Thu, 29 Jun 2023 00:53:44 +0000 (UTC) Date: Wed, 28 Jun 2023 20:53:42 -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 Subject: Re: [PATCH v2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use Message-ID: <20230629005342.GB8069@lorien.usersys.redhat.com> References: <20230627191201.344110-1-pauld@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 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 28, 2023 at 02:42:16PM -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. Currentlyi, 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. > > > > Add sched_feat HZ_BW (off by default) to control this 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 > > --- > > > > v2: Ben pointed out that the bit could get cleared in the dequeue path > > if we migrate a newly enqueued task without preempting curr. Added a > > check for that edge case to sched_can_stop_tick. Removed the call to > > sched_can_stop_tick from sched_fair_update_stop_tick since it was > > redundant. > > > > kernel/sched/core.c | 12 +++++++++++ > > kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++++++++++++ > > kernel/sched/features.h | 2 ++ > > 3 files changed, 59 insertions(+) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index a68d1276bab0..646f60bfc7e7 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1194,6 +1194,8 @@ static void nohz_csd_func(void *info) > > #endif /* CONFIG_NO_HZ_COMMON */ > > > > #ifdef CONFIG_NO_HZ_FULL > > +extern bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq); > > + > > bool sched_can_stop_tick(struct rq *rq) > > { > > int fifo_nr_running; > > @@ -1229,6 +1231,16 @@ 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. > > + */ > > + if (sched_feat(HZ_BW) && rq->nr_running == 1 && rq->curr > > + && rq->curr->sched_class == &fair_sched_class && task_on_rq_queued(rq->curr)) { > > + if (sched_cfs_bandwidth_active(task_cfs_rq(rq->curr))) > > Actually, something I should have noticed earlier is that this should > probably be hierarchical, right? You need to check every ancestor > cfs_rq, not just the immediate parent. And at that point it probably > makes sense to have sched_cfs_bandwidth_active take a task_struct. > Are you saying a child cfs_rq with a parent that has runtime_enabled could itself not have runtime_enabled? I may be missing something but I don't see how that works. account_cfs_rq_runtime() for example just looks at the immediate cfs_rq of curr and bails if it does not have runtime_enabled. How could that task get throttled if it exceeds some parent's limit? Confused :) Cheers, Phil --