From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758212AbcK3RDB (ORCPT ); Wed, 30 Nov 2016 12:03:01 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:60665 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1758117AbcK3RCz (ORCPT ); Wed, 30 Nov 2016 12:02:55 -0500 Date: Wed, 30 Nov 2016 09:02:49 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Michal Hocko , Donald Buczek , Paul Menzel , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Reply-To: paulmck@linux.vnet.ibm.com References: <20161124101525.GB20668@dhcp22.suse.cz> <583AA50A.9010608@molgen.mpg.de> <20161128110449.GK14788@dhcp22.suse.cz> <109d5128-f3a4-4b6e-db17-7a1fcb953500@molgen.mpg.de> <29196f89-c35e-f79d-8e4d-2bf73fe930df@molgen.mpg.de> <20161130110944.GD18432@dhcp22.suse.cz> <20161130115320.GO3924@linux.vnet.ibm.com> <20161130131910.GF18432@dhcp22.suse.cz> <20161130142955.GS3924@linux.vnet.ibm.com> <20161130163820.GQ3092@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161130163820.GQ3092@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16113017-0024-0000-0000-00001527CE87 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006168; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000193; SDB=6.00787434; UDB=6.00380899; IPR=6.00565114; BA=6.00004933; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00013493; XFM=3.00000011; UTC=2016-11-30 17:02:51 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16113017-0025-0000-0000-000046921029 Message-Id: <20161130170249.GZ3924@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-11-30_12:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1609300000 definitions=main-1611300273 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 30, 2016 at 05:38:20PM +0100, Peter Zijlstra wrote: > On Wed, Nov 30, 2016 at 06:29:55AM -0800, Paul E. McKenney wrote: > > We can, and you are correct that cond_resched() does not unconditionally > > supply RCU quiescent states, and never has. Last time I tried to add > > cond_resched_rcu_qs() semantics to cond_resched(), I got told "no", > > but perhaps it is time to try again. > > Well, you got told: "ARRGH my benchmark goes all regress", or something > along those lines. Didn't we recently dig out those commits for some > reason or other? Were "those commits" the benchmark or putting cond_resched_rcu_qs() functionality into cond_resched()? Either way, no idea. > Finding out what benchmark that was and running it against this patch > would make sense. Agreed, especially given that I believe cond_resched_rcu_qs() is lighter weight than it used to be. No idea what benchmarks they were, though. > Also, I seem to have missed, why are we going through this again? People are running workloads that force long-running loops in the kernel, which get them RCU CPU stall warning messages. My reaction has been to insert cond_resched_rcu_qs() as needed, and Michal wondered why cond_resched() couldn't just handle both scheduling latency and RCU quiescent states. I remembered trying it, but not what the issue was. So I posted the patch assuming that I would eventually either find out what the issue was or that the issue no longer applied. ;-) Thanx, Paul