linux-kernel - Re: [Question] Should direct reclaim time be bounded?

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Date:   Wed, 10 Jul 2019 16:36:58 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Hillf Danton <hdanton@...a.com>, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [Question] Should direct reclaim time be bounded?

On 7/10/19 12:44 PM, Michal Hocko wrote:
> On Wed 10-07-19 11:42:40, Mike Kravetz wrote:
> [...]
>> As Michal suggested, I'm going to do some testing to see what impact
>> dropping the __GFP_RETRY_MAYFAIL flag for these huge page allocations
>> will have on the number of pages allocated.
> 
> Just to clarify. I didn't mean to drop __GFP_RETRY_MAYFAIL from the
> allocation request. I meant to drop the special casing of the flag in
> should_continue_reclaim. I really have hard time to argue for this
> special casing TBH. The flag is meant to retry harder but that shouldn't
> be reduced to a single reclaim attempt because that alone doesn't really
> help much with the high order allocation. It is more about compaction to
> be retried harder.

Thanks Michal.  That is indeed what you suggested earlier.  I remembered
incorrectly.  Sorry.

Removing the special casing for __GFP_RETRY_MAYFAIL in should_continue_reclaim
implies that it will return false if nothing was reclaimed (nr_reclaimed == 0)
in the previous pass.

When I make such a modification and test, I see long stalls as a result
of should_compact_retry returning true too often.  On a system I am currently
testing, should_compact_retry has returned true 36000000 times.  My guess
is that this may stall forever.  Vlastmil previously asked about this behavior,
so I am capturing the reason.  Like before [1], should_compact_retry is
returning true mostly because compaction_withdrawn() returns COMPACT_DEFERRED.

Total 36000000
      35437500	COMPACT_DEFERRED
        562500  COMPACT_PARTIAL_SKIPPED

[1] https://lkml.org/lkml/2019/6/5/643
-- 
Mike Kravetz