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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 C93DAC7618B for ; Wed, 24 Jul 2019 14:30:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A050822ADB for ; Wed, 24 Jul 2019 14:30:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563978618; bh=rIcPLNA+uDJDlrIUJ0agl7S6TInb6IGvxF9wZ4XeCak=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=M1UfRjHPLjVFTllUIQ0p5q+I70fkKIP2eHhzKB9iFpsEYaWjDy6MhfrbqO6dUMWXs fQ9n7kcQUWq006fI3EqM1AF7H/H/98TSUHemM0h6Vf0g6eMkGFh6MRnfixsuBhhbrW j+re8FzPnP5C3J5pmIlHKktXoxMrOEKie2lVsxD4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728081AbfGXOaR (ORCPT ); Wed, 24 Jul 2019 10:30:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:58904 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727243AbfGXOaR (ORCPT ); Wed, 24 Jul 2019 10:30:17 -0400 Received: from localhost (lfbn-ncy-1-174-150.w83-194.abo.wanadoo.fr [83.194.254.150]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B1131217F4; Wed, 24 Jul 2019 14:30:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563978616; bh=rIcPLNA+uDJDlrIUJ0agl7S6TInb6IGvxF9wZ4XeCak=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JsWkt6Hxlk0pDJcWDr/0H2LinxuM0ErbKnt4XMSSV4DtD4yrYej2z5KTId4Fy+mxz r4DOpW4GiKfixBQtK+dlYRV4kfEoyYBJNwA9g4HggVqR3JoLImGHHk8y07T2mc5mAC bXDKmMhx2THOQXYgs0lYeyaOrfNv7ML8ZB61tAzE= Date: Wed, 24 Jul 2019 16:30:13 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: fweisbec@gmail.com, tglx@linutronix.de, mingo@kernel.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, peterz@infradead.org Subject: Re: How to turn scheduler tick on for current nohz_full CPU? Message-ID: <20190724143012.GB1029@lenoir> References: <20190724115331.GA29059@linux.ibm.com> <20190724132257.GA1029@lenoir> <20190724135219.GY14271@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190724135219.GY14271@linux.ibm.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 24, 2019 at 06:52:19AM -0700, Paul E. McKenney wrote: > On Wed, Jul 24, 2019 at 03:22:59PM +0200, Frederic Weisbecker wrote: > > On Wed, Jul 24, 2019 at 04:53:31AM -0700, Paul E. McKenney wrote: > > > Hello! > > > > > > One of the callback-invocation forward-progress issues turns out to > > > be nohz_full CPUs not turning their scheduling-clock interrupt back on > > > when running in kernel mode. Given that callback floods can cause RCU's > > > callback-invocation loop to run for some time, it would be good for this > > > loop to re-enable this interrupt. Of course, this problem applies to > > > pretty much any kernel code that might loop for an extended time period, > > > not just RCU. > > > > > > I took a quick look at kernel/time/tick-sched.c and the closest thing > > > I found was tick_nohz_full_kick_cpu(), except that (1) it isn't clear > > > that this does much when invoked on the current CPU and (2) it doesn't > > > help in rcutorture TREE04. In contrast, disabling NO_HZ_FULL and using > > > RCU_NOCB_CPU instead works quite well. > > > > > > So what should I be calling instead of tick_nohz_full_kick_cpu() to > > > re-enable the current CPU's scheduling-clock interrupt? > > > > Indeed, kernel code is assumed to be quick enough (between two extended grace > > periods) to avoid running the tick for RCU. But some long lasting kernel code > > may require to tick temporarily. > > > > You can use tick_nohz_dep_set_cpu(cpu, TICK_DEP_MASK_RCU) with the > > following: > > > > diff --git a/include/linux/tick.h b/include/linux/tick.h > > index f92a10b5e112..3f476e2a4bf7 100644 > > --- a/include/linux/tick.h > > +++ b/include/linux/tick.h > > @@ -108,7 +108,8 @@ enum tick_dep_bits { > > TICK_DEP_BIT_POSIX_TIMER = 0, > > TICK_DEP_BIT_PERF_EVENTS = 1, > > TICK_DEP_BIT_SCHED = 2, > > - TICK_DEP_BIT_CLOCK_UNSTABLE = 3 > > + TICK_DEP_BIT_CLOCK_UNSTABLE = 3, > > + TICK_DEP_BIT_RCU = 4 > > }; > > > > #define TICK_DEP_MASK_NONE 0 > > @@ -116,6 +117,7 @@ enum tick_dep_bits { > > #define TICK_DEP_MASK_PERF_EVENTS (1 << TICK_DEP_BIT_PERF_EVENTS) > > #define TICK_DEP_MASK_SCHED (1 << TICK_DEP_BIT_SCHED) > > #define TICK_DEP_MASK_CLOCK_UNSTABLE (1 << TICK_DEP_BIT_CLOCK_UNSTABLE) > > +#define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU) > > > > #ifdef CONFIG_NO_HZ_COMMON > > extern bool tick_nohz_enabled; > > I will give this a try, thank you! (Testing will take a few days.) Sure! For the background: expect a self-IPI to fire which then restart the tick on IRQ exit. Once you're later done with the work, don't forget to remove the tick dependency: tick_nohz_dep_clear_cpu(cpu, TICK_DEP_MASK_RCU); Thanks.