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 <[email protected]> Cc: Minchan Kim <[email protected]> Cc: Mel Gorman <[email protected]> Acked-by: Joonsoo Kim <[email protected]> Acked-by: Michal Nazarewicz <[email protected]> Cc: Naoya Horiguchi <[email protected]> Cc: Christoph Lameter <[email protected]> Cc: Rik van Riel <[email protected]> Cc: David Rientjes <[email protected]> --- mm/compaction.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/mm/compaction.c b/mm/compaction.c index 70b0776..b978693 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 @@ static unsigned long isolate_freepages_block(struct compact_control *cc, } + /* + * 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); -- 2.4.6 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
