Re: [PATCH v6 11/13] mm, compaction: skip buddy pages by their order in the migrate scanner

[Vlastimil Babka <vbabka@suse.cz>]

On 08/22/2014 12:11 AM, Andrew Morton wrote:
> On Mon,  4 Aug 2014 10:55:22 +0200 Vlastimil Babka <vbabka@suse.cz> 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. The reduction is >60% with __GFP_NO_KSWAPD
>> allocations, along with success rates better by few percent.
>> 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.
>
> What is this "later patch"?  Or is the changelog stale?

Yes it is stale, that later patch was postponed due to apparent bad 
effect on fragmentation. I guess we can drop the last paragraph from 
this commit log.

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