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 <vba...@suse.cz> > Cc: Minchan Kim <minc...@kernel.org> > Cc: Mel Gorman <mgor...@suse.de> > Cc: Joonsoo Kim <iamjoonsoo....@lge.com> > Cc: Michal Nazarewicz <min...@mina86.com> > Cc: Naoya Horiguchi <n-horigu...@ah.jp.nec.com> > Cc: Christoph Lameter <c...@linux.com> > Cc: Rik van Riel <r...@redhat.com> > Cc: David Rientjes <rient...@google.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? Thanks, Naoya Horiguchi > continue; > } > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/