Re: [PATCH] memcg: oom: ignore oom warnings from memory.max

From: Shakeel Butt <shakeelb@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Greg Thelen <gthelen@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>, Cgroups <cgroups@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] memcg: oom: ignore oom warnings from memory.max
Date: Mon, 4 May 2020 12:23:51 -0700	[thread overview]
Message-ID: <CALvZod79hWns9366B+8ZK2Roz8c+vkdA80HqFNMep56_pumdRQ@mail.gmail.com> (raw)
In-Reply-To: <20200504160613.GU22838@dhcp22.suse.cz>

On Mon, May 4, 2020 at 9:06 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 04-05-20 08:35:57, Shakeel Butt wrote:
> > On Mon, May 4, 2020 at 8:00 AM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Mon 04-05-20 07:53:01, Shakeel Butt wrote:
> [...]
> > > > I am trying to see if "no eligible task" is really an issue and should
> > > > be warned for the "other use cases". The only real use-case I can
> > > > think of are resource managers adjusting the limit dynamically. I
> > > > don't see "no eligible task" a concerning reason for such use-case.
> > >
> > > It is very much a concerning reason to notify about like any other OOM
> > > situation due to hard limit breach. In this case it is worse in some
> > > sense because the limit cannot be trimmed down because there is no
> > > directly reclaimable memory at all. Such an oom situation is
> > > effectivelly conserved.
> > > --
> >
> > Let me make a more precise statement and tell me if you agree. The "no
> > eligible task" is concerning for the charging path but not for the
> > writer of memory.max. The writer can read the usage and
> > cgroup.[procs|events] to figure out the situation if needed.
>
> I really hate to repeat myself but this is no different from a regular
> oom situation.

Conceptually yes there is no difference but there is no *divine
restriction* to not make a difference if there is a real world
use-case which would benefit from it.

> Admin sets the hard limit and the kernel tries to act
> upon that.
>
> You cannot make any assumption about what admin wanted or didn't want
> to see.

Actually we always make assumptions on how the feature we implement
will be used and as new use-cases come the assumptions evolve.

> We simply trigger the oom killer on memory.max and this is a
> documented behavior. No eligible task or no task at all is a simply a
> corner case

For "sweep before tear down" use-case this is not a corner case.

> when the kernel cannot act and mentions that along with the
> oom report so that whoever consumes that information can debug or act on
> that fact.
>
> Silencing the oom report is simply removing a potentially useful
> aid to debug further a potential problem.

*Potentially* useful for debugging versus actually beneficial for
"sweep before tear down" use-case. Also I am not saying to make "no
dumps for memory.max when no eligible tasks" a set in stone rule. We
can always reevaluate when such information will actually be useful.

Johannes/Andrew, what's your opinion?