From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753547AbbJ0JQK (ORCPT ); Tue, 27 Oct 2015 05:16:10 -0400 Received: from mail-wi0-f178.google.com ([209.85.212.178]:38019 "EHLO mail-wi0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752557AbbJ0JQG (ORCPT ); Tue, 27 Oct 2015 05:16:06 -0400 Date: Tue, 27 Oct 2015 10:16:03 +0100 From: Michal Hocko To: Tejun Heo Cc: Christoph Lameter , Tetsuo Handa , linux-mm@kvack.org, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, David Rientjes , oleg@redhat.com, kwalker@redhat.com, akpm@linux-foundation.org, hannes@cmpxchg.org, vdavydov@parallels.com, skozina@redhat.com, mgorman@suse.de, riel@redhat.com Subject: Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable() checks Message-ID: <20151027091603.GB9891@dhcp22.suse.cz> References: <20151022150623.GE26854@dhcp22.suse.cz> <20151022151528.GG30579@mtj.duckdns.org> <20151022153559.GF26854@dhcp22.suse.cz> <20151022153703.GA3899@mtj.duckdns.org> <20151022154922.GG26854@dhcp22.suse.cz> <20151022184226.GA19289@mtj.duckdns.org> <20151023083316.GB2410@dhcp22.suse.cz> <20151023103630.GA4170@mtj.duckdns.org> <20151023111145.GH2410@dhcp22.suse.cz> <20151023182109.GA14610@mtj.duckdns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151023182109.GA14610@mtj.duckdns.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 24-10-15 03:21:09, Tejun Heo wrote: > Hello, > > On Fri, Oct 23, 2015 at 01:11:45PM +0200, Michal Hocko wrote: > > > The problem here is not lack > > > of execution resource but concurrency management misunderstanding the > > > situation. > > > > And this sounds like a bug to me. > > I don't know. I can be argued either way, the other direction being a > kernel thread going RUNNING non-stop is buggy. Given how this has > been a complete non-issue for all the years, I'm not sure how useful > plugging this is. Well, I guess we haven't noticed because this is a pathological case. It also triggers OOM livelocks which were not reported in the past either. You do not reach this state normally unless you rely _want_ to kill your machine And vmstat is not the only instance. E.g. sysrq oom trigger is known to stay behind in similar cases. It should be changed to a dedicated WQ_MEM_RECLAIM wq and it would require runnable item guarantee as well. > > Don't we have some IO related paths which would suffer from the same > > problem. I haven't checked all the WQ_MEM_RECLAIM users but from the > > name I would expect they _do_ participate in the reclaim and so they > > should be able to make a progress. Now if your new IMMEDIATE flag will > > Seriously, nobody goes full-on RUNNING. Looping with cond_resched seems like general pattern in the kernel when there is no clear source to wait for. We have io_schedule when we know we should wait for IO (in case of congestion) but this is not necessarily the case - as you can see here. What should we wait for? A short nap without actually waiting on anything sounds like a dirty workaround to me. > > guarantee that then I would argue that it should be implicit for > > WQ_MEM_RECLAIM otherwise we always risk a similar situation. What would > > be a counter argument for doing that? > > Not serving any actual purpose and degrading execution behavior. I dunno, I am not familiar with WQ internals to see the risks but to me it sounds like WQ_MEM_RECLAIM gives an incorrect impression of safety wrt. memory pressure and as demonstrated it doesn't do that. Even if you consider cond_resched behavior of the page allocator as bug we should be able to handle this gracefully. -- Michal Hocko SUSE Labs