Re: [PATCH v2 1/2] mm/compaction: do not count migratepages when unnecessary

[David Rientjes]

On Wed, 7 May 2014, Vlastimil Babka wrote:

> During compaction, update_nr_listpages() has been used to count remaining
> non-migrated and free pages after a call to migrage_pages(). The freepages
> counting has become unneccessary, and it turns out that migratepages counting
> is also unnecessary in most cases.
> 
> The only situation when it's needed to count cc->migratepages is when
> migrate_pages() returns with a negative error code. Otherwise, the non-negative
> return value is the number of pages that were not migrated, which is exactly
> the count of remaining pages in the cc->migratepages list.
> 
> Furthermore, any non-zero count is only interesting for the tracepoint of
> mm_compaction_migratepages events, because after that all remaining unmigrated
> pages are put back and their count is set to 0.
> 
> This patch therefore removes update_nr_listpages() completely, and changes the
> tracepoint definition so that the manual counting is done only when the
> tracepoint is enabled, and only when migrate_pages() returns a negative error
> code.
> 
> Furthermore, migrate_pages() and the tracepoints won't be called when there's
> nothing to migrate. This potentially avoids some wasted cycles and reduces the
> volume of uninteresting mm_compaction_migratepages events where "nr_migrated=0
> nr_failed=0". In the stress-highalloc mmtest, this was about 75% of the events.
> The mm_compaction_isolate_migratepages event is better for determining that
> nothing was isolated for migration, and this one was just duplicating the info.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

I like this, before our two patches update_nr_listpages() was expensive 
when called for each pageblock and being able to remove it is certainly a 
step in the right direction to make compaction as fast as possible.
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