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Date: Wed, 25 Jun 2014 10:57:36 +0200 From: Vlastimil Babka <vbabka@...e.cz> To: Naoya Horiguchi <n-horiguchi@...jp.nec.com> CC: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, David Rientjes <rientjes@...gle.com>, Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>, Joonsoo Kim <iamjoonsoo.kim@....com>, Michal Nazarewicz <mina86@...a86.com>, Christoph Lameter <cl@...ux.com>, Rik van Riel <riel@...hat.com>, Zhang Yanfei <zhangyanfei@...fujitsu.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v3 12/13] mm, compaction: try to capture the just-created high-order freepage On 06/25/2014 03:57 AM, Naoya Horiguchi wrote: > 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@...e.cz> >> Cc: Minchan Kim <minchan@...nel.org> >> Cc: Mel Gorman <mgorman@...e.de> >> Cc: Joonsoo Kim <iamjoonsoo.kim@....com> >> Cc: Michal Nazarewicz <mina86@...a86.com> >> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com> >> Cc: Christoph Lameter <cl@...ux.com> >> Cc: Rik van Riel <riel@...hat.com> >> Cc: David Rientjes <rientjes@...gle.com> >> --- > ... >> @@ -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? Good catch, thanks! It should also use ALIGN properly as the non-locked test above. > Thanks, > Naoya Horiguchi > >> continue; >> } >> } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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