Re: [PATCH v3 12/13] mm, compaction: try to capture the just-created high-order freepage

[Naoya Horiguchi]

On Fri, Jun 20, 2014 at 05:49:42PM +0200, Vlastimil Babka wrote:
> Compaction uses watermark checking to determine if it succeeded in creating
> a high-order free page. My testing has shown that this is quite racy and it
> can happen that watermark checking in compaction succeeds, and moments later
> the watermark checking in page allocation fails, even though the number of
> free pages has increased meanwhile.
> 
> It should be more reliable if direct compaction captured the high-order free
> page as soon as it detects it, and pass it back to allocation. This would
> also reduce the window for somebody else to allocate the free page.
> 
> Capture has been implemented before by 1fb3f8ca0e92 ("mm: compaction: capture
> a suitable high-order page immediately when it is made available"), but later
> reverted by 8fb74b9f ("mm: compaction: partially revert capture of suitable
> high-order page") due to a bug.
> 
> This patch differs from the previous attempt in two aspects:
> 
> 1) The previous patch scanned free lists to capture the page. In this patch,
>    only the cc->order aligned block that the migration scanner just finished
>    is considered, but only if pages were actually isolated for migration in
>    that block. Tracking cc->order aligned blocks also has benefits for the
>    following patch that skips blocks where non-migratable pages were found.
> 
> 2) The operations done in buffered_rmqueue() and get_page_from_freelist() are
>    closely followed so that page capture mimics normal page allocation as much
>    as possible. This includes operations such as prep_new_page() and
>    page->pfmemalloc setting (that was missing in the previous attempt), zone
>    statistics are updated etc. Due to subtleties with IRQ disabling and
>    enabling this cannot be simply factored out from the normal allocation
>    functions without affecting the fastpath.
> 
> This patch has tripled compaction success rates (as recorded in vmstat) in
> stress-highalloc mmtests benchmark, although allocation success rates increased
> only by a few percent. Closer inspection shows that due to the racy watermark
> checking and lack of lru_add_drain(), the allocations that resulted in direct
> compactions were often failing, but later allocations succeeeded in the fast
> path. So the benefit of the patch to allocation success rates may be limited,
> but it improves the fairness in the sense that whoever spent the time
> compacting has a higher change of benefitting from it, and also can stop
> compacting sooner, as page availability is detected immediately. With better
> success detection, the contribution of compaction to high-order allocation
> success success rates is also no longer understated by the vmstats.
> 
> 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>
> ---
...
> @@ -669,6 +708,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  				continue;
>  			if (PageTransHuge(page)) {
>  				low_pfn += (1 << compound_order(page)) - 1;
> +				next_capture_pfn = low_pfn + 1;

Don't we need if (next_capture_pfn) here?

Thanks,
Naoya Horiguchi

>  				continue;
>  			}
>  		}
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