Re: [PATCH 06/10] mm, compaction: skip buddy pages by their order in the migrate scanner

[David Rientjes]

On Mon, 9 Jun 2014, 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.
> 
> Preliminary results with stress-highalloc from mmtests show a 10% reduction in
> number of pages scanned by migration scanner. This change is also important to
> later allow detecting when a cc->order block of pages cannot be compacted, and
> the scanner should skip to the next block instead of wasting time.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> V2: fix low_pfn > end_pfn check; comments
>     kept page_order_unsafe() approach for now
> 

Please see http://marc.info/?l=linux-mm&m=140235272808846, I'd love to be 
proved wrong.
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