[PATCH 4/4] Revert "Revert "mm/compaction: fix set skip in fast_find_migrateblock""

Mel Gorman
2023-05-15T11:38:00Z

This reverts commit 95e7a450b819 ("Revert "mm/compaction: fix set skip
in fast_find_migrateblock"").

Commit 7efc3b726103 ("mm/compaction: fix set skip in fast_find_migrateblock")
was reverted due to bug reports about khugepaged consuming large amounts
of CPU without making progress. The underlying bug was partially fixed
by commit cfccd2e63e7e ("mm, compaction: finish pageblocks on complete
migration failure") but it only mitigated the problem and Vlastimil Babka
pointing out the same issue could theoretically happen to kcompactd.

As pageblocks containing pages that fail to migrate should now be
forcibly rescanned to set the skip hint if skip hints are used,
fast_find_migrateblock() should no longer loop on a small subset
of pageblocks for prolonged periods of time. Revert the revert so
fast_find_migrateblock() is effective again.

Using the mmtests config workload-usemem-stress-numa-compact, the number
of unique ranges scanned was analysed for both kcompactd and !kcompactd
activity.

6.4.0-rc1-vanilla
kcompactd
7 range=(0x10d600~0x10d800)
7 range=(0x110c00~0x110e00)
7 range=(0x110e00~0x111000)
7 range=(0x111800~0x111a00)
7 range=(0x111a00~0x111c00)
!kcompactd
1 range=(0x113e00~0x114000)
1 range=(0x114000~0x114020)
1 range=(0x114400~0x114489)
1 range=(0x114489~0x1144aa)
1 range=(0x1144aa~0x114600)

6.4.0-rc1-mm-revertfastmigrate
kcompactd
17 range=(0x104200~0x104400)
17 range=(0x104400~0x104600)
17 range=(0x104600~0x104800)
17 range=(0x104800~0x104a00)
17 range=(0x104a00~0x104c00)
!kcompactd
1793 range=(0x15c200~0x15c400)
5436 range=(0x105800~0x105a00)
19826 range=(0x150a00~0x150c00)
19833 range=(0x150800~0x150a00)
19834 range=(0x11ce00~0x11d000)

6.4.0-rc1-mm-follupfastfind
kcompactd
22 range=(0x107200~0x107400)
23 range=(0x107400~0x107600)
23 range=(0x107600~0x107800)
23 range=(0x107c00~0x107e00)
23 range=(0x107e00~0x108000)
!kcompactd
3 range=(0x890240~0x890400)
5 range=(0x886e00~0x887000)
5 range=(0x88a400~0x88a600)
6 range=(0x88f800~0x88fa00)
9 range=(0x88a400~0x88a420)

Note that the vanilla kernel and the full series had some duplication of
ranges scanned but it was not severe and would be in line with compaction
resets when the skip hints are cleared. Just a revert of commit 7efc3b726103
("mm/compaction: fix set skip in fast_find_migrateblock") showed excessive
rescans of the same ranges so the series should not reintroduce bug 1206848.

Link: https://bugzilla.suse.com/show_bug.cgi?id=1206848
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/compaction.c | 1 -
1 file changed, 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index d7be990b1d60..91af6a8b7a98 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1859,7 +1859,6 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
pfn = cc->zone->zone_start_pfn;
cc->fast_search_fail = 0;
found_block = true;
- set_pageblock_skip(freepage);
break;
}
}
--
2.35.3