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 1F809CA0EC6 for ; Mon, 11 Sep 2023 21:58:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354776AbjIKVzV (ORCPT ); Mon, 11 Sep 2023 17:55:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52890 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237976AbjIKN0k (ORCPT ); Mon, 11 Sep 2023 09:26:40 -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 ESMTP id 6F76B12A for ; Mon, 11 Sep 2023 06:25:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1694438747; 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=jWAqz+TSHcXXWRExk4GJdL39eZS3mXtkMY/iF+0aEtI=; b=V+Ve/+YM4dt1w6eRt4clDvCUl2RCboXz0N36Gj8zedm4WMDzybaoVP3o1qAKpza74SXxzR DS15L06w62RTVrt5R9ApgGMYGQTnJp4lz1GF42PjJhvZ1CyY3mbGdrwyV65PRvQGb/bp8S ojPFbEQJ0/+uLh11yjDp0gN2hst/mBU= Received: from mimecast-mx02.redhat.com (mx-ext.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-122-Vf3Fhd4BMrKrF1Oc_npyog-1; Mon, 11 Sep 2023 09:25:44 -0400 X-MC-Unique: Vf3Fhd4BMrKrF1Oc_npyog-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B64693C0C4A2; Mon, 11 Sep 2023 13:25:43 +0000 (UTC) Received: from lorien.usersys.redhat.com (unknown [10.39.192.93]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6829B40C2064; Mon, 11 Sep 2023 13:25:39 +0000 (UTC) Date: Mon, 11 Sep 2023 09:25:35 -0400 From: Phil Auld To: Hao Jia Cc: mingo@redhat.com, peterz@infradead.org, mingo@kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [External] Re: [PATCH 0/2] Fix nohz_full vs rt bandwidth Message-ID: <20230911132535.GA24480@lorien.usersys.redhat.com> References: <20230821094927.51079-1-jiahao.os@bytedance.com> <1823b6fd-037f-38dc-2d33-0879d77768c3@bytedance.com> <20230907141703.GA441901@lorien.usersys.redhat.com> <20230908124501.GA471894@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 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Hao, On Mon, Sep 11, 2023 at 11:39:02AM +0800 Hao Jia wrote: > On 2023/9/8 Phil Auld wrote: > > On Fri, Sep 08, 2023 at 10:57:26AM +0800 Hao Jia wrote: > > > On 2023/9/7 Phil Auld wrote: > > > > Hi Hao, ... > > > > > > > > Are you actually hitting this in the real world? > > > > > > > > We, for example, no longer enable RT_GROUP_SCHED so this is a non-issue > > > > for our use cases. I'd recommend considering that. (Does it even > > > > work with cgroup2?) > > > > > > > > > > Yes, it has always been there. Regardless of whether RT_GROUP_SCHED is > > > enabled or not, rt bandwidth is always enabled. If RT_GROUP_SCHED is not > > > enabled, all rt tasks in the system are a group, and rt_runtime is 950000, > > > and rt_period is 1000000.So rt bandwidth is always enabled by default. > > > > Sure, there is that. But I think Daniel is actively trying to remove it. > > > > Thank you for your reply. Maybe I'm missing something. Can you give me some > links to discussions about it? > Sure, try this one: https://lore.kernel.org/lkml/cover.1693510979.git.bristot@kernel.org/ > > Also I'm not sure you answered my question. Are you actually hitting this > > in the real world? I'd be tempted to think this is a mis-configuration or > > mis-use of RT. Plus you can disable that throttling and use stalld to catch > > cases where the rt task goes out of control. > > > > > Are you actually hitting this in the real world? > > I tested on my machine using default settings (rt_runtime is 950000, and > rt_period is 1000000.). The rt task is supposed to be throttled after > running for 0.95 seconds, but due to the influence of NO_HZ_FULL, it may be > throttled after running for about 1.4 seconds. This will only cause the > rt_bandwidth throttle to be delayed, but no warning will be triggered. Yes, you can hit this in testing. I'm asking if it's causing your real-world applicaton issues or is this just a theoretical problem you can contrive a test for? Are you actually hitting this when running your workload? >From what you are showing (a test setup) I'm guessing no. > > > > Plus you can disable that throttling and use stalld to catch cases where > the rt task goes out of control. > > IIRC, if we disable rt_bandwidth. The rt task is always running, which may > cause cfs task starvation and hung_task warnning. This may be the reason why > rt_bandwidth is enabled by default (rt_runtime is 950000, and rt_period is > 1000000). That's what stalld is for. Some rt applications don't like giving up 5% of the cpu time when they don't really need to. Cheers, Phil --