Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753218AbbFPFmg (ORCPT ); Tue, 16 Jun 2015 01:42:36 -0400 Received: from LGEMRELSE6Q.lge.com ([156.147.1.121]:49027 "EHLO lgemrelse6q.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751392AbbFPFm1 (ORCPT ); Tue, 16 Jun 2015 01:42:27 -0400 X-Original-SENDERIP: 10.177.222.220 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Tue, 16 Jun 2015 14:44:36 +0900 From: Joonsoo Kim To: Vlastimil Babka Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Minchan Kim , Mel Gorman , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel , David Rientjes Subject: Re: [PATCH 4/6] mm, compaction: always skip compound pages by order in migrate scanner Message-ID: <20150616054436.GD12641@js1304-P5Q-DELUXE> References: <1433928754-966-1-git-send-email-vbabka@suse.cz> <1433928754-966-5-git-send-email-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1433928754-966-5-git-send-email-vbabka@suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4763 Lines: 113 On Wed, Jun 10, 2015 at 11:32:32AM +0200, Vlastimil Babka wrote: > The compaction migrate scanner tries to skip compound pages by their order, to > reduce number of iterations for pages it cannot isolate. The check is only done > if PageLRU() is true, which means it applies to THP pages, but not e.g. > hugetlbfs pages or any other non-LRU compound pages, which we have to iterate > by base pages. > > This limitation comes from the assumption that it's only safe to read > compound_order() when we have the zone's lru_lock and THP cannot be split under > us. But the only danger (after filtering out order values that are not below > MAX_ORDER, to prevent overflows) is that we skip too much or too little after > reading a bogus compound_order() due to a rare race. This is the same reasoning > as patch 99c0fd5e51c4 ("mm, compaction: skip buddy pages by their order in the > migrate scanner") introduced for unsafely reading PageBuddy() order. > > After this patch, all pages are tested for PageCompound() and we skip them by > compound_order(). The test is done after the test for balloon_page_movable() > as we don't want to assume if balloon pages (or other pages with own isolation > and migration implementation if a generic API gets implemented) are compound > or not. > > When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs > pages, the vmstat compact_migrate_scanned count decreased by 15%. > > Signed-off-by: Vlastimil Babka > Cc: Minchan Kim > Cc: Mel Gorman > Cc: Joonsoo Kim > Cc: Michal Nazarewicz > Cc: Naoya Horiguchi > Cc: Christoph Lameter > Cc: Rik van Riel > Cc: David Rientjes > --- > mm/compaction.c | 36 +++++++++++++++++------------------- > 1 file changed, 17 insertions(+), 19 deletions(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > index d334bb3..e37d361 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -680,6 +680,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > > /* Time to isolate some pages for migration */ > for (; low_pfn < end_pfn; low_pfn++) { > + bool is_lru; > + > /* > * Periodically drop the lock (if held) regardless of its > * contention, to give chance to IRQs. Abort async compaction > @@ -723,39 +725,35 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > * It's possible to migrate LRU pages and balloon pages > * Skip any other type of page > */ > - if (!PageLRU(page)) { > + is_lru = PageLRU(page); > + if (!is_lru) { > if (unlikely(balloon_page_movable(page))) { > if (balloon_page_isolate(page)) { > /* Successfully isolated */ > goto isolate_success; > } > } > - continue; > } > > /* > - * PageLRU is set. lru_lock normally excludes isolation > - * splitting and collapsing (collapsing has already happened > - * if PageLRU is set) but the lock is not necessarily taken > - * here and it is wasteful to take it just to check transhuge. > - * Check PageCompound without lock and skip the whole pageblock > - * if it's a transhuge page, as calling compound_order() > - * without preventing THP from splitting the page underneath us > - * may return surprising results. > - * If we happen to check a THP tail page, compound_order() > - * returns 0. It should be rare enough to not bother with > - * using compound_head() in that case. > + * Regardless of being on LRU, compound pages such as THP and > + * hugetlbfs are not to be compacted. We can potentially save > + * 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)) { > - int nr; > - if (locked) > - nr = 1 << compound_order(page); > - else > - nr = pageblock_nr_pages; > - low_pfn += nr - 1; > + unsigned int comp_order = compound_order(page); > + > + if (comp_order > 0 && comp_order < MAX_ORDER) > + low_pfn += (1UL << comp_order) - 1; > + > continue; > } How about moving this PageCompound() check up to the PageLRU check? Is there any relationship between balloon page and PageCompound()? It will remove is_lru and code would be more understandable. Otherwise, Acked-by: Joonsoo Kim Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/