linux-kernel - Re: [PATCH] Revert "Revert "mm/compaction: fix set skip in fast_find

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Message-ID: <20230426153331.dfqagb47i4xo3ouv@techsingularity.net>
Date:   Wed, 26 Apr 2023 16:33:31 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "Revert "mm/compaction: fix set skip in
 fast_find_migrateblock""

On Wed, Apr 26, 2023 at 05:10:14PM +0200, Vlastimil Babka wrote:
> On 4/26/23 17:03, Baolin Wang wrote:
> > This reverts commit 95e7a450b8190673675836bfef236262ceff084a.
> > 
> > When I tested thpscale with v6.3 kernel, I found the compaction efficiency
> > had a great regression compared to v6.2-rc1 kernel. See below numbers:
> >                                     v6.2-rc             v6.3
> > Percentage huge-3        81.35 (   0.00%)       32.97 ( -59.47%)
> > Percentage huge-5        89.92 (   0.00%)       41.70 ( -53.63%)
> > Percentage huge-7        92.41 (   0.00%)       34.08 ( -63.12%)
> > Percentage huge-12       90.29 (   0.00%)       41.10 ( -54.49%)
> > Percentage huge-18       82.38 (   0.00%)       41.24 ( -49.95%)
> > Percentage huge-24       80.34 (   0.00%)       35.99 ( -55.20%)
> > Percentage huge-30       88.90 (   0.00%)       44.20 ( -50.28%)
> > Percentage huge-32       90.69 (   0.00%)       79.57 ( -12.25%)
> > 
> > Ops Compaction stalls                 113790.00      207099.00
> > Ops Compaction success                 33983.00      19488.00
> > Ops Compaction failures                79807.00      187611.00
> > Ops Compaction efficiency                 29.86          9.41
> > 
> > After some investigation, I found the commit 95e7a450b819
> > ("Revert mm/compaction: fix set skip in fast_find_migrateblock") caused
> > the regression. This commit revert the commit 7efc3b726103 ("mm/compaction:
> > fix set skip in fast_find_migrateblock") to fix a CPU stalling issue, which
> > is caused by compaction stucked in repeating fast_find_migrateblock().
> > 
> > And now the compaction stalling issue is addressed by commit cfccd2e63e7e
> > ("mm, compaction: finish pageblocks on complete migration failure"). So
> 
> IIRC at that time I was pointing out some scenarios that could make the
> problem appear even after that commit, and we wanted to revisit that
> when Mel is back.
> 

Yes, I've prototyped the fix against 6.3-rc7 and the revert is at the
end but the revert on its own has the potential for causing problems. The
series needs to be rebased, retested and posted. What I last tested
should show up shortly at

https://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git/ mm-follupfastmigrate-v1r1

-- 
Mel Gorman
SUSE Labs