Re: [patch] mm, oom: make a last minute check to prevent unnecessary memcg oom kills

From: Ami Fischman <fischman@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Robert Kolchmeyer <rkolchmeyer@google.com>,
	David Rientjes <rientjes@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] mm, oom: make a last minute check to prevent unnecessary memcg oom kills
Date: Wed, 18 Mar 2020 08:20:53 -0700	[thread overview]
Message-ID: <CAHuR8a-0R318DUK4n3tyug2n6D8+pKVUHbN0mwQcFbgy4gGTOg@mail.gmail.com> (raw)
In-Reply-To: <20200318095710.GG21362@dhcp22.suse.cz>

On Wed, Mar 18, 2020 at 2:57 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Tue 17-03-20 12:00:45, Ami Fischman wrote:
> > On Tue, Mar 17, 2020 at 11:26 AM Robert Kolchmeyer
> > <rkolchmeyer@google.com> wrote:
> > >
> > > On Tue, Mar 10, 2020 at 3:54 PM David Rientjes <rientjes@google.com> wrote:
> > > >
> > > > Robert, could you elaborate on the user-visible effects of this issue that
> > > > caused it to initially get reported?
> > >
> > > Ami (now cc'ed) knows more, but here is my understanding.
> >
> > Robert's description of the mechanics we observed is accurate.
> >
> > We discovered this regression in the oom-killer's behavior when
> > attempting to upgrade our system. The fraction of the system that
> > went unhealthy due to this issue was approximately equal to the
> > _sum_ of all other causes of unhealth, which are many and varied,
> > but each of which contribute only a small amount of
> > unhealth. This issue forced a rollback to the previous kernel
> > where we ~never see this behavior, returning our unhealth levels
> > to the previous background levels.
>
> Could you be more specific on the good vs. bad kernel versions? Because
> I do not remember any oom changes that would affect the
> time-to-check-time-to-kill race. The timing might be slightly different
> in each kernel version of course.

The original upgrade attempt included a large window of kernel
versions: 4.14.137 to 4.19.91.  In attempting to narrow down the
failure we found that in tests of 10 runs we went from 0/10
failures to 1/10 failure with the update from
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/74fab24be8994bb5bb8d1aa8828f50e16bb38346
(based on 4.19.60) to
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/6e0fef1b46bb91c196be56365d9af72e52bb4675
(also based on 4.19.60)
and then we went from 1/10 failures to 9/10 failures with the
upgrade to
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/a33dffa8e5c47b877e4daece938a81e3cc810b90
(which I believe is based on 4.19.72).

(this was all before we had the minimal repro yielding Robert's
61/100->0/100 stat in his previous email)