linux-kernel - Re: [PATCH] mm/page_alloc: try oom if reclaim is unable to make forward progress

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Message-ID: <20210326112254.jy5jkiwtgj3pqkt2@ava.usersys.com>
Date:   Fri, 26 Mar 2021 11:22:54 +0000
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm/page_alloc: try oom if reclaim is unable to make
 forward progress

Hi Michal,

On Fri 2021-03-26 09:16 +0100, Michal Hocko wrote:
> The oom killer is never triggered for costly allocation request.

Yes - I agree. Looking at __alloc_pages_may_oom() I can see for a costly
order allocation request the OOM killer is explicitly not used.
If I understand correctly, the patch I proposed was for the following
scenarios:

  1.    The costly order allocation request to fail when
        "some" progress is made (i.e. order-0) and the last
        compaction request was "skipped"

  2.    A non-costly order allocation request that is
        unable to make any progress and failed over the
        maximum reclaim retry count in a row and the last
        known compaction result was skipped to consider
        using the OOM killer for assistance

> Both reclaim and compaction maintain their own retries counters as they
> are targeting a different operation. Although the compaction really
> depends on the reclaim to do some progress.

Yes. Looking at should_compact_retry() if the last known compaction result
was skipped i.e. suggesting there was not enough order-0 pages to support
compaction, so assistance is needed from reclaim
(see __compaction_suitable()).

I noticed that the value of compaction_retries, compact_result and
compact_priority was 0, COMPACT_SKIPPED and 1 i.e. COMPACT_PRIO_SYNC_LIGHT,
respectively.

> OK, this sound unexpected as it indicates that the reclaim is able to
> make a forward progress but compaction doesn't want to give up and keeps
> retrying. Are you able to reproduce this or could you find out which
> specific condition keeps compaction retrying? I would expect that it is
> one of the 3 conditions before the max_retries is checked.

Unfortunately, I have been told it is not entirely reproducible.
I suspect it is the following in should_compact_retry() - as I indicated
above the last known value stored in compaction_retries was 0:

        if (order > PAGE_ALLOC_COSTLY_ORDER)
                max_retries /= 4;
        if (*compaction_retries <= max_retries) {
                ret = true;
                goto out;
        }

Kind regards,

-- 
Aaron Tomlin