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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Thu, 9 Jul 2020 17:01:06 +0800
Message-ID: <CALOAHbDZLM4bfGi7EQDzE6FabBinnZJ7yF+uZADUgx2S_pfg1w@mail.gmail.com>
Subject: Re: [PATCH 2/2] doc, mm: clarify /proc/<pid>/oom_score value range
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Jul 9, 2020 at 4:18 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 09-07-20 15:41:11, Yafang Shao wrote:
> > On Thu, Jul 9, 2020 at 2:26 PM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > From: Michal Hocko <mhocko@suse.com>
> > >
> > > The exported value includes oom_score_adj so the range is no [0, 1000]
> > > as described in the previous section but rather [0, 2000]. Mention that
> > > fact explicitly.
> > >
> > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > > ---
> > >  Documentation/filesystems/proc.rst | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > > index 8e3b5dffcfa8..78a0dec323a3 100644
> > > --- a/Documentation/filesystems/proc.rst
> > > +++ b/Documentation/filesystems/proc.rst
> > > @@ -1673,6 +1673,9 @@ requires CAP_SYS_RESOURCE.
> > >  3.2 /proc/<pid>/oom_score - Display current oom-killer score
> > >  -------------------------------------------------------------
> > >
> > > +Please note that the exported value includes oom_score_adj so it is effectively
> > > +in range [0,2000].
> > > +
> >
> > [0, 2000] may be not a proper range, see my reply in another thread.[1]
> > As this value hasn't been documented before and nobody notices that, I
> > think there might be no user really care about it before.
> > So we should discuss the proper range if we really think the user will
> > care about this value.
>
> Even if we decide the range should change, I do not really assume this
> will happen, it is good to have the existing behavior clarified.
>

But the existing behavior is not defined in the kernel documentation
before, so I don't think that the user has a clear understanding of
the existing behavior.
The way to use the result of proc_oom_score is to compare which
processes will be killed first by the OOM killer, IOW, the user should
always use it to compare different processes. For example,

if proc_oom_score(process_a) > proc_oom_score(process_b)
then
     process_a will be killed before process_b
fi

And then  the user will "Use it together with
/proc/<pid>/oom_score_adj to tune which
 process should be killed in an out-of-memory situation."

That means what the user really cares about is the relative value, and
they will not care about the range or the absolute value.

-- 
Thanks
Yafang