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 6E619C41513 for ; Tue, 1 Aug 2023 11:18:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231944AbjHALSL (ORCPT ); Tue, 1 Aug 2023 07:18:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50246 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233815AbjHALR3 (ORCPT ); Tue, 1 Aug 2023 07:17:29 -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 C2E384693 for ; Tue, 1 Aug 2023 04:13:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1690888428; 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=kA4xN8X2MfzWKxTQBNCnJbEbWGPsqRHAYEWq7tejRS0=; b=eWchMFatNeSVcKp4llUP5FanSeWBcTf4n7Sf0Js/q2dTl2XgqEHOk0pD8lbCJ3doDM+QCz wwfmEddKPt6yricpwTD7CNORHNcJu0OOcNJhP9e2RKM9y2tXeW6fpZZhxBvP+NvLzrxJkk IQC6hhxmTgTV1+2J1Z3EJdgsgvVpaAI= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-623-79K1YrdOMW2wF6i12uudjw-1; Tue, 01 Aug 2023 07:13:45 -0400 X-MC-Unique: 79K1YrdOMW2wF6i12uudjw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id C096D81D9EC; Tue, 1 Aug 2023 11:13:44 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.22.8.125]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 85F3B2166B26; Tue, 1 Aug 2023 11:13:43 +0000 (UTC) Date: Tue, 1 Aug 2023 07:13:42 -0400 From: Phil Auld To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Juri Lelli , Ingo Molnar , Daniel Bristot de Oliveira , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , Ben Segall , Steven Rostedt , Mel Gorman , Frederic Weisbecker , Tejun Heo Subject: Re: [PATCH v6 2/2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use Message-ID: <20230801111342.GA268019@lorien.usersys.redhat.com> References: <20230712133357.381137-1-pauld@redhat.com> <20230712133357.381137-3-pauld@redhat.com> <20230731224934.GD51835@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230731224934.GD51835@hirez.programming.kicks-ass.net> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 01, 2023 at 12:49:34AM +0200 Peter Zijlstra wrote: > On Wed, Jul 12, 2023 at 09:33:57AM -0400, Phil Auld wrote: > > 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. > > These appear fine to me, except: > > > Add sched_feat HZ_BW (off by default) to control the tick_stop > > behavior. > > What was the thinking here? This means nobody will be using this -- why > would you want this default disabled? > That was just a hedge in case it caused issues. I'd probably have had to enable it in RHEL anyway. Using a feature was to make it inocuous when disabled. Would you prefer me to enable it or remove the sched_feat entirely? (or do you want to just switch that to true when you apply it?) Thanks, Phil --