From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx162.postini.com [74.125.245.162]) by kanga.kvack.org (Postfix) with SMTP id AD3D76B0044 for ; Thu, 9 Aug 2012 04:45:16 -0400 (EDT) Date: Thu, 9 Aug 2012 17:46:53 +0900 From: Minchan Kim Subject: Re: [PATCH 5/5] mm: have order > 0 compaction start near a pageblock with free pages Message-ID: <20120809084653.GB21033@bbox> References: <1344452924-24438-1-git-send-email-mgorman@suse.de> <1344452924-24438-6-git-send-email-mgorman@suse.de> <20120809001212.GB17835@bbox> <20120809082328.GC12690@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120809082328.GC12690@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Linux-MM , Rik van Riel , Jim Schutt , LKML On Thu, Aug 09, 2012 at 09:23:28AM +0100, Mel Gorman wrote: > On Thu, Aug 09, 2012 at 09:12:12AM +0900, Minchan Kim wrote: > > > > > > > > > Second, it updates compact_cached_free_pfn in a more limited set of > > > circumstances. > > > > > > If a scanner has wrapped, it updates compact_cached_free_pfn to the end > > > of the zone. When a wrapped scanner isolates a page, it updates > > > compact_cached_free_pfn to point to the highest pageblock it > > > can isolate pages from. > > > > Okay until here. > > > > Great. > > > > > > > If a scanner has not wrapped when it has finished isolated pages it > > > checks if compact_cached_free_pfn is pointing to the end of the > > > zone. If so, the value is updated to point to the highest > > > pageblock that pages were isolated from. This value will not > > > be updated again until a free page scanner wraps and resets > > > compact_cached_free_pfn. > > > > I tried to understand your intention of this part but unfortunately failed. > > By this part, the problem you mentioned could happen again? > > > > Potentially yes, I did say it still races in the changelog. > > > C > > Process A M S F > > |---------------------------------------| > > Process B M FS > > > > C is zone->compact_cached_free_pfn > > S is cc->start_pfree_pfn > > M is cc->migrate_pfn > > F is cc->free_pfn > > > > In this diagram, Process A has just reached its migrate scanner, wrapped > > around and updated compact_cached_free_pfn to end of the zone accordingly. > > > > Yes. Now that it has wrapped it updates the compact_cached_free_pfn > every loop of isolate_freepages here. > > if (isolated) { > high_pfn = max(high_pfn, pfn); > > /* > * If the free scanner has wrapped, update > * compact_cached_free_pfn to point to the highest > * pageblock with free pages. This reduces excessive > * scanning of full pageblocks near the end of the > * zone > */ > if (cc->order > 0 && cc->wrapped) > zone->compact_cached_free_pfn = high_pfn; > } > > > > > Simultaneously, Process B finishes isolating in a block and peek > > compact_cached_free_pfn position and know it's end of the zone so > > update compact_cached_free_pfn to highest pageblock that pages were > > isolated from. > > > > Yes, they race at this point. One of two things happen here and I agree > that this is racy > > 1. Process A does another iteration of its loop and sets it back > 2. Process A does not do another iteration of the loop, the cached_pfn > is further along that it should. The next compacting process will > wrap early and reset cached_pfn again but continue to scan the zone. > > Either option is relatively harmless because in both cases the zone gets > scanned. In patch 4 it was possible that large portions of the zone were > frequently missed. > > > Process A updates compact_cached_free_pfn to the highest pageblock which > > was set by process B because process A has wrapped. It ends up big jump > > without any scanning in process A. > > > > It recovers quickly and is nowhere near as severe as what patch 4 > suffers from. Agreed. Thanks, Mel. -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org