Messages in this thread | | | Date | Mon, 23 Jun 2014 12:05:41 +0900 | From | Minchan Kim <> | Subject | Re: [PATCH v3 09/13] mm, compaction: skip buddy pages by their order in the migrate scanner |
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On Fri, Jun 20, 2014 at 05:49:39PM +0200, Vlastimil Babka wrote: > The migration scanner skips PageBuddy pages, but does not consider their order > as checking page_order() is generally unsafe without holding the zone->lock, > and acquiring the lock just for the check wouldn't be a good tradeoff. > > Still, this could avoid some iterations over the rest of the buddy page, and > if we are careful, the race window between PageBuddy() check and page_order() > is small, and the worst thing that can happen is that we skip too much and miss > some isolation candidates. This is not that bad, as compaction can already fail > for many other reasons like parallel allocations, and those have much larger > race window. > > This patch therefore makes the migration scanner obtain the buddy page order > and use it to skip the whole buddy page, if the order appears to be in the > valid range. > > It's important that the page_order() is read only once, so that the value used > in the checks and in the pfn calculation is the same. But in theory the > compiler can replace the local variable by multiple inlines of page_order(). > Therefore, the patch introduces page_order_unsafe() that uses ACCESS_ONCE to > prevent this. > > Testing with stress-highalloc from mmtests shows a 15% reduction in number of > pages scanned by migration scanner. This change is also a prerequisite for a > later patch which is detecting when a cc->order block of pages contains > non-buddy pages that cannot be isolated, and the scanner should thus skip to > the next block immediately. > > Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Minchan Kim <minchan@kernel.org>
-- Kind regards, Minchan Kim
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