linux-kernel - Re: [PATCH] oom_kill: oom_score_adj broken for processes with small memory usage

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Message-ID: <20210716122547.GI3431@minyard.net>
Date:   Fri, 16 Jul 2021 07:25:47 -0500
From:   Corey Minyard <cminyard@...sta.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     minyard@....org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom_kill: oom_score_adj broken for processes with small
 memory usage

On Fri, Jul 16, 2021 at 07:19:24AM +0200, Michal Hocko wrote:
> On Thu 01-07-21 07:54:30, minyard@....org wrote:
> > From: Corey Minyard <cminyard@...sta.com>
> > 
> > If you have a process with less than 1000 totalpages, the calculation:
> > 
> >   adj = (long)p->signal->oom_score_adj;
> >   ...
> >   adj *= totalpages / 1000;
> > 
> > will always result in adj being zero no matter what oom_score_adj is,
> > which could result in the wrong process being picked for killing.
> > 
> > Fix by adding 1000 to totalpages before dividing.
> 
> Yes, this is a known limitation of the oom_score_adj and its scale.
> Is this a practical problem to be solved though? I mean 0-1000 pages is
> not really that much different from imprecision at a larger scale where
> tasks are effectively considered equal.

Known limitation?  Is this documented?  I couldn't find anything that
said "oom_score_adj doesn't work at all with programs with <1000 pages
besides setting the value to -1000".

> 
> I have to say I do not really like the proposed workaround. It doesn't
> really solve the problem yet it adds another special case.

The problem is that if you have a small program, there is no way to
set it's priority besides completely disablling the OOM killer for
it.

I don't understand the special case comment.  How is this adding a
special case?  This patch removes a special case.  Small programs
working different than big programs is a special case.  Making them all
work the same is removing an element of surprise from someone expecting
things to work as documented.

-corey