From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f71.google.com (mail-pg0-f71.google.com [74.125.83.71]) by kanga.kvack.org (Postfix) with ESMTP id 620F36B0260 for ; Wed, 30 Nov 2016 12:50:24 -0500 (EST) Received: by mail-pg0-f71.google.com with SMTP id x23so37416899pgx.6 for ; Wed, 30 Nov 2016 09:50:24 -0800 (PST) Received: from bombadil.infradead.org (bombadil.infradead.org. [2001:1868:205::9]) by mx.google.com with ESMTPS id a28si65289514pgn.295.2016.11.30.09.50.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Nov 2016 09:50:23 -0800 (PST) Date: Wed, 30 Nov 2016 18:50:16 +0100 From: Peter Zijlstra Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Message-ID: <20161130175015.GR3092@twins.programming.kicks-ass.net> References: <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> <20161130170557.GK18432@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161130170557.GK18432@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: "Paul E. McKenney" , Donald Buczek , Paul Menzel , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett On Wed, Nov 30, 2016 at 06:05:57PM +0100, Michal Hocko wrote: > On Wed 30-11-16 17:38:20, 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? > > > > Finding out what benchmark that was and running it against this patch > > would make sense. See commit: 4a81e8328d37 ("rcu: Reduce overhead of cond_resched() checks for RCU") Someone actually wrote down what the problem was. > > Also, I seem to have missed, why are we going through this again? > > Well, the point I've brought that up is because having basically two > APIs for cond_resched is more than confusing. Basically all longer in > kernel loops do cond_resched() but it seems that this will not help the > silence RCU lockup detector in rare cases where nothing really wants to > schedule. I am really not sure whether we want to sprinkle > cond_resched_rcu_qs at random places just to silence RCU detector... Right.. now, this is obviously all PREEMPT=n code, which therefore also implies this is rcu-sched. Paul, now doesn't rcu-sched, when the grace-period has been long in coming, try and force it? And doesn't that forcing include prodding CPUs with resched_cpu() ? I'm thinking not, because if it did, that would make cond_resched() actually schedule, which would then call into rcu_note_context_switch() which would then make RCU progress, no? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org