Re: [PATCH v3 09/13] mm, compaction: skip buddy pages by their order in the migrate scanner

[Minchan Kim]

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@xxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

-- 
Kind regards,
Minchan Kim
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