From 9fcd6d2e052eef525e94a9ae58dbe7ed4df4f5a7 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 8 Sep 2015 15:02:49 -0700 Subject: [PATCH] mm, compaction: skip compound pages by order in free scanner The compaction free scanner is looking for PageBuddy() pages and skipping all others. For large compound pages such as THP or hugetlbfs, we can save a lot of iterations if we skip them at once using their compound_order(). This is generally unsafe and we can read a bogus value of order due to a race, but if we are careful, the only danger is skipping too much. When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs pages, the vmstat compact_free_scanned count decreased by at least 15%. Signed-off-by: Vlastimil Babka Cc: Minchan Kim Cc: Mel Gorman Acked-by: Joonsoo Kim Acked-by: Michal Nazarewicz Cc: Naoya Horiguchi Cc: Christoph Lameter Cc: Rik van Riel Cc: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/mm/compaction.c b/mm/compaction.c index 1ccb015ab1eb..8f64d3533990 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -437,6 +437,24 @@ static unsigned long isolate_freepages_block(struct compact_control *cc, if (!valid_page) valid_page = page; + + /* + * For compound pages such as THP and hugetlbfs, we can save + * potentially a lot of iterations if we skip them at once. + * The check is racy, but we can consider only valid values + * and the only danger is skipping too much. + */ + if (PageCompound(page)) { + unsigned int comp_order = compound_order(page); + + if (likely(comp_order < MAX_ORDER)) { + blockpfn += (1UL << comp_order) - 1; + cursor += (1UL << comp_order) - 1; + } + + goto isolate_fail; + } + if (!PageBuddy(page)) goto isolate_fail; @@ -496,6 +514,13 @@ isolate_fail: } + /* + * There is a tiny chance that we have read bogus compound_order(), + * so be careful to not go outside of the pageblock. + */ + if (unlikely(blockpfn > end_pfn)) + blockpfn = end_pfn; + trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn, nr_scanned, total_isolated); -- 2.20.1