linux-kernel - Re: OOM detection regressions since 4.7

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Message-ID: <20160822111344.GJ13596@dhcp22.suse.cz>
Date:   Mon, 22 Aug 2016 13:13:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, greg@...e.cz,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arkadiusz Miskiewicz <a.miskiewicz@...il.com>,
        Ralf-Peter Rohbeck <Ralf-Peter.Rohbeck@...ntum.com>,
        Jiri Slaby <jslaby@...e.com>, Olaf Hering <olaf@...fle.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <js1304@...il.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: OOM detection regressions since 4.7

On Mon 22-08-16 13:01:13, Markus Trippelsdorf wrote:
> On 2016.08.22 at 12:56 +0200, Michal Hocko wrote:
> > On Mon 22-08-16 12:16:14, Markus Trippelsdorf wrote:
> > > On 2016.08.22 at 11:32 +0200, Michal Hocko wrote:
> > > > [1] http://lkml.kernel.org/r/20160731051121.GB307@x4
> > > 
> > > For the report [1] above:
> > > 
> > > markus@x4 linux % cat .config | grep CONFIG_COMPACTION
> > > # CONFIG_COMPACTION is not set
> > 
> > Hmm, without compaction and a heavy fragmentation then I am afraid we
> > cannot really do much. What is the reason to disable compaction in the
> > first place?
> 
> I don't recall. Must have been some issue in the past. I will re-enable
> the option.

Well, without the compaction there is no source of high order pages at
all. You can only reclaim and hope that some of the reclaimed pages will
find its buddy on the list and form the higher order page. This can take
for ever. We used to have the lumpy reclaim and that could help but this
is long gone.

I do not think we can really sanely optimize for high-order heavy loads
without COMPACTION sanely. At least not without reintroducing lumpy
reclaim or something similar. To be honest I am even not sure which
configurations should disable compaction - except for really highly
controlled !mmu or other one purpose systems.

-- 
Michal Hocko
SUSE Labs