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

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Date:   Thu, 11 Jul 2019 09:12:45 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
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 Wed 10-07-19 16:36:58, Mike Kravetz wrote:
> 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.

This smells like a problem to me. But somebody more familiar with
compaction should comment.

> 
> Total 36000000
>       35437500	COMPACT_DEFERRED
>         562500  COMPACT_PARTIAL_SKIPPED
> 
> 
> [1] https://lkml.org/lkml/2019/6/5/643
> -- 
> Mike Kravetz

-- 
Michal Hocko
SUSE Labs