Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758465AbcK3Rep (ORCPT ); Wed, 30 Nov 2016 12:34:45 -0500 Received: from mail-wm0-f66.google.com ([74.125.82.66]:35400 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932949AbcK3Reh (ORCPT ); Wed, 30 Nov 2016 12:34:37 -0500 Date: Wed, 30 Nov 2016 18:34:34 +0100 From: Michal Hocko To: "Paul E. McKenney" Cc: Peter Zijlstra , 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` Message-ID: <20161130173433.GL18432@dhcp22.suse.cz> References: <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> <20161130172355.GA3924@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161130172355.GA3924@linux.vnet.ibm.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1959 Lines: 41 On Wed 30-11-16 09:23:55, Paul E. McKenney wrote: > 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. > > > > > > 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... > > Just in case there is any doubt on this point, any patch of mine adding > cond_resched_rcu_qs() functionality to cond_resched() cannot go upstream > without Peter's Acked-by. Yeah, that is clear to me. I just wanted to clarify the "why are we going through this again" part ;) > Or did you have some other solution in mind? Not really. The fact that cond_resched() cannot silence RCU stall detector under some circumstances is sad. I believe we shouldn't have two different APIs to control scheduling and RCU latencies because that just asks for whack a mole games and some level of confusion... -- Michal Hocko SUSE Labs