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@suse.cz>
Fixes: a211c6550efc ("mm: page_alloc: defrag_mode kswapd/kcompactd watermarks")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
mm/vmscan.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index cc422ad830d6..3783e45bfc92 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6747,8 +6747,14 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx)
/*
* In defrag_mode, watermarks must be met in whole
* blocks to avoid polluting allocator fallbacks.
+ *
+ * However, kswapd usually cannot accomplish this on
+ * its own and needs kcompactd support. Once it's
+ * reclaimed a compaction gap, and kswapd_shrink_node
+ * has dropped order, simply ensure there are enough
+ * base pages for compaction, wake kcompactd & sleep.
*/
- if (defrag_mode)
+ if (defrag_mode && order)
item = NR_FREE_PAGES_BLOCKS;
else
item = NR_FREE_PAGES;
--
2.49.0On 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@suse.cz>
> Fixes: a211c6550efc ("mm: page_alloc: defrag_mode kswapd/kcompactd watermarks")
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Thanks!
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