Re: [PATCH v2] mm/compaction : fix the wrong return value for isolate_migratepages()

[Mel Gorman <mgorman@suse.de>]

On Sat, Dec 31, 2011 at 05:09:45PM +0800, Huang Shijie wrote:
> When we do not get any migrate page, we should return ISOLATE_NONE.
> 

Why?

Returning ISOLATE_SUCCESS means that we fall through. This means busy
work in migrate_pages(), updating list accounting and the list. It's
wasteful but is it functionally incorrect? What problem did you observe?

If this is simply a performance issue then minimally COMPACTBLOCKS
still needs to be updated, we still want to see the tracepoint etc. To
preserve that, I would suggest as an alternative to leave it returning
ISOLATE_SUCCESS but move


               err = migrate_pages(&cc->migratepages, compaction_alloc,
                                (unsigned long)cc, false,
                                cc->sync ? MIGRATE_SYNC_LIGHT : MIGRATE_ASYNC);
                update_nr_listpages(cc);

inside a if (nr_migrate) check to avoid some overhead.

-- 
Mel Gorman
SUSE Labs

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