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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 24BFBC76186 for ; Thu, 25 Jul 2019 01:12:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EA6CF21855 for ; Thu, 25 Jul 2019 01:12:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728786AbfGYBMo (ORCPT ); Wed, 24 Jul 2019 21:12:44 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:7508 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726115AbfGYBMo (ORCPT ); Wed, 24 Jul 2019 21:12:44 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x6P18K3J113058 for ; Wed, 24 Jul 2019 21:12:42 -0400 Received: from e12.ny.us.ibm.com (e12.ny.us.ibm.com [129.33.205.202]) by mx0b-001b2d01.pphosted.com with ESMTP id 2txxrxqna5-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 24 Jul 2019 21:12:42 -0400 Received: from localhost by e12.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 25 Jul 2019 02:12:39 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x6P1CcmU16253938 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 25 Jul 2019 01:12:38 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 536CDB2068; Thu, 25 Jul 2019 01:12:38 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 31145B2066; Thu, 25 Jul 2019 01:12:38 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.189.166]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 25 Jul 2019 01:12:38 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 5437A16C34F8; Wed, 24 Jul 2019 18:12:43 -0700 (PDT) Date: Wed, 24 Jul 2019 18:12:43 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker 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? Reply-To: paulmck@linux.ibm.com References: <20190724115331.GA29059@linux.ibm.com> <20190724132257.GA1029@lenoir> <20190724135219.GY14271@linux.ibm.com> <20190724143012.GB1029@lenoir> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190724143012.GB1029@lenoir> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19072501-0060-0000-0000-00000365276F X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011490; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000287; SDB=6.01237008; UDB=6.00652024; IPR=6.01018375; MB=3.00027878; MTD=3.00000008; XFM=3.00000015; UTC=2019-07-25 01:12:41 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19072501-0061-0000-0000-00004A47F81F Message-Id: <20190725011243.GZ14271@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-25_01:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1907250010 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 04:30:13PM +0200, Frederic Weisbecker wrote: > 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); Does this get cleared automatically if preempted, or should I clear this is one of RCU's scheduler hooks? (My guess is the latter, but I figured I should ask.) Thanx, Paul