Re: [PATCH V4 06/15] mm, compaction: reduce zone checking frequency in the migration scanner

[Mel Gorman]

On Wed, Jul 16, 2014 at 03:48:14PM +0200, Vlastimil Babka wrote:
> The unification of the migrate and free scanner families of function has
> highlighted a difference in how the scanners ensure they only isolate pages
> of the intended zone. This is important for taking zone lock or lru lock of
> the correct zone. Due to nodes overlapping, it is however possible to
> encounter a different zone within the range of the zone being compacted.
> 
> The free scanner, since its inception by commit 748446bb6b ("mm: compaction:
> memory compaction core"), has been checking the zone of the first valid page
> in a pageblock, and skipping the whole pageblock if the zone does not match.
> 
> This checking was completely missing from the migration scanner at first, and
> later added by commit dc9086004b ("mm: compaction: check for overlapping
> nodes during isolation for migration") in a reaction to a bug report.
> But the zone comparison in migration scanner is done once per a single scanned
> page, which is more defensive and thus more costly than a check per pageblock.
> 
> This patch unifies the checking done in both scanners to once per pageblock,
> through a new pageblock_within_zone() function, which also includes pfn_valid()
> checks. It is more defensive than the current free scanner checks, as it checks
> both the first and last page of the pageblock, but less defensive by the
> migration scanner per-page checks. It assumes that node overlapping may result
> (on some architecture) in a boundary between two nodes falling into the middle
> of a pageblock, but that there cannot be a node0 node1 node0 interleaving
> within a single pageblock.
> 
> The result is more code being shared and a bit less per-page CPU cost in the
> migration scanner.
> 
> Reported-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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