linux-kernel - Re: [PATCH 2/2] mm: vmscan: fix kswapd exit condition in defrag

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Message-ID: <0277450c-c0f0-4aa2-9271-db45d417d827@suse.cz>
Date: Wed, 16 Apr 2025 16:54:34 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Brendan Jackman <jackmanb@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: vmscan: fix kswapd exit condition in defrag_mode

On 4/16/25 15:45, Johannes Weiner wrote:
> Vlastimil points out an issue with kswapd in defrag_mode not waking up
> kcompactd reliably.
> 
> Background: When kswapd is woken for any higher-order request, it
> initially checks those high-order watermarks to decide if work is
> necesary. However, it cannot (efficiently) meet the contiguity goal of
> such a request by itself. So once it has reclaimed a compaction gap,
> it adjusts the request down to check for free order-0 pages, then
> wakes kcompactd to coalesce them into larger blocks.
> 
> In defrag_mode, the initial watermark check needs to be analogously
> against free pageblocks. However, once kswapd drops the high-order to
> hand off contiguity work, it also needs to fall back to base page
> watermarks - otherwise it'll keep reclaiming until blocks are freed.
> 
> While it appears kcompactd is woken up frequently enough to do most of
> the compaction work, kswapd ends up overreclaiming by quite a bit:
> 
>                                                      DEFRAGMODE     DEFRAGMODE-thispatch
> Hugealloc Time mean                       79381.34 (    +0.00%)    88126.12 (   +11.02%)
> Hugealloc Time stddev                     85852.16 (    +0.00%)   135366.75 (   +57.67%)
> Kbuild Real time                            249.35 (    +0.00%)      226.71 (    -9.04%)
> Kbuild User time                           1249.16 (    +0.00%)     1249.37 (    +0.02%)
> Kbuild System time                          171.76 (    +0.00%)      166.93 (    -2.79%)
> THP fault alloc                           51666.87 (    +0.00%)    52685.60 (    +1.97%)
> THP fault fallback                        16970.00 (    +0.00%)    15951.87 (    -6.00%)
> Direct compact fail                         166.53 (    +0.00%)      178.93 (    +7.40%)
> Direct compact success                       17.13 (    +0.00%)        4.13 (   -71.69%)
> Compact daemon scanned migrate          3095413.33 (    +0.00%)  9231239.53 (  +198.22%)
> Compact daemon scanned free             2155966.53 (    +0.00%)  7053692.87 (  +227.17%)
> Compact direct scanned migrate           265642.47 (    +0.00%)    68388.33 (   -74.26%)
> Compact direct scanned free              130252.60 (    +0.00%)    55634.87 (   -57.29%)
> Compact total migrate scanned           3361055.80 (    +0.00%)  9299627.87 (  +176.69%)
> Compact total free scanned              2286219.13 (    +0.00%)  7109327.73 (  +210.96%)
> Alloc stall                                1890.80 (    +0.00%)     6297.60 (  +232.94%)
> Pages kswapd scanned                    9043558.80 (    +0.00%)  5952576.73 (   -34.18%)
> Pages kswapd reclaimed                  1891708.67 (    +0.00%)  1030645.00 (   -45.52%)
> Pages direct scanned                    1017090.60 (    +0.00%)  2688047.60 (  +164.29%)
> Pages direct reclaimed                    92682.60 (    +0.00%)   309770.53 (  +234.22%)
> Pages total scanned                    10060649.40 (    +0.00%)  8640624.33 (   -14.11%)
> Pages total reclaimed                   1984391.27 (    +0.00%)  1340415.53 (   -32.45%)
> Swap out                                 884585.73 (    +0.00%)   417781.93 (   -52.77%)
> Swap in                                  287106.27 (    +0.00%)    95589.73 (   -66.71%)
> File refaults                            551697.60 (    +0.00%)   426474.80 (   -22.70%)
> 
> Reported-by: Vlastimil Babka <vbabka@...e.cz>
> Fixes: a211c6550efc ("mm: page_alloc: defrag_mode kswapd/kcompactd watermarks")
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!