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

[Vlastimil Babka]

On 5/15/23 13:33, Mel Gorman wrote:
> 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@xxxxxxxxxxxxxxxxxxx>

My concerns about patch 3/4 don't affect this part so

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  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;
>  			}
>  		}