linux-kernel - Re: [PATCH v2] mm/oom_kill: show oom eligibility when displaying the current memory state of all tasks

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Message-ID: <YMigO5N55QhnrB87@dhcp22.suse.cz>
Date:   Tue, 15 Jun 2021 14:42:35 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org, vbabka@...e.cz,
        penguin-kernel@...ove.sakura.ne.jp, llong@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/oom_kill: show oom eligibility when displaying the
 current memory state of all tasks

On Tue 15-06-21 12:51:47, Aaron Tomlin wrote:
> On Mon 2021-06-14 08:44 +0200, Michal Hocko wrote:
> > Well, I have to say that I have a bit hard time understand the problem
> > statement here. First of all you are very likely basing your observation
> > on an old kernel which is missing a fix which should make the situation
> > impossible IIRC. You should be focusing on a justification why the new
> > information is helpful for the current tree.
> 
> Michal,
> 
> Not exactly.
> 
> See oom_reap_task(). Let's assume an OOM event occurred within the context
> of a memcg and 'memory.oom.group' was not set. If I understand correctly,
> once all attempts to OOM reap the specified task were "unsuccessful" then
> MMF_OOM_SKIP is applied; and, the assumption is it will be terminated
> shorty due to the pending fatal signal (see __oom_kill_process()) i.e. a
> SIGKILL is sent to the "victim" before the OOM reaper is notified. Now
> assuming the above task did not exited yet, another task, in the same
> memcg, could also trigger an OOM event.  Therefore, when showing potential
> OOM victims the task above with MMF_OOM_SKIP set, will indeed be displayed.
> 
> I understanding the point on OOM_SCORE_ADJ_MIN. This can be easily
> identified and is clear to the viewer. However, the same cannot be stated
> for MMF_OOM_SKIP.

This is all true but it is not really clear why that is really a
problem. Kernel log already contains information about reaped processes
as they are reported to the log. I fully acknowledge that this is rather
spartan but on the other hand from years of experience reading oom
reports I have to say the dump_tasks is the least interesting part of
the report (while being the most verbose one).

All that being said, I am not really opposing extending the information
although I am a bit worried about leaking too much internal state to the
log. What I am asking for here is a justification why this addition is a
general improvement and how it helps uderstanding oom reports further.
So please focus on that part.
-- 
Michal Hocko
SUSE Labs