From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx185.postini.com [74.125.245.185]) by kanga.kvack.org (Postfix) with SMTP id 29F026B005A for ; Fri, 7 Sep 2012 00:12:21 -0400 (EDT) Received: by dadi14 with SMTP id i14so1653785dad.14 for ; Thu, 06 Sep 2012 21:12:20 -0700 (PDT) Date: Fri, 7 Sep 2012 12:12:12 +0800 From: Shaohua Li Subject: Re: [patch 1/2]compaction: check migrated page number Message-ID: <20120907041212.GA31391@kernel.org> References: <20120906104404.GA12718@kernel.org> <20120906121725.GQ11266@suse.de> <20120906125526.GA1025@kernel.org> <20120906132551.GS11266@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120906132551.GS11266@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: linux-mm@kvack.org, akpm@linux-foundation.org, aarcange@redhat.com On Thu, Sep 06, 2012 at 02:25:51PM +0100, Mel Gorman wrote: > On Thu, Sep 06, 2012 at 08:55:26PM +0800, Shaohua Li wrote: > > On Thu, Sep 06, 2012 at 01:17:25PM +0100, Mel Gorman wrote: > > > On Thu, Sep 06, 2012 at 06:44:04PM +0800, Shaohua Li wrote: > > > > > > > > isolate_migratepages_range() might isolate none pages, for example, when > > > > zone->lru_lock is contended and compaction is async. In this case, we should > > > > abort compaction, otherwise, compact_zone will run a useless loop and make > > > > zone->lru_lock is even contended. > > > > > > > > > > It might also isolate no pages because the range was 100% allocated and > > > there were no free pages to isolate. This is perfectly normal and I suspect > > > this patch effectively disables compaction. What problem did you observe > > > that this patch is aimed at? > > > > I'm running a random swapin/out workload. When memory is fragmented enough, I > > saw 100% cpu usage. perf shows zone->lru_lock is heavily contended in > > isolate_migratepages_range. I'm using slub(I didn't see the problem with slab), > > the allocation is for radix_tree_node slab, which needs 4 pages. > > Ok, the fragmentaiton is due to high-order unmovable kernel allocations from > SLUB which will have diminishing returns over time. One option to address > this is to check if it's a high-order kernel allocation that can fail and > not compact in that case. SLUB will fall back to using order-0 instead. I tried actually, and it doesn't help. The problem is compact_zone keeps running isolate_migratepages_range, which does nothing except doing a lock/unlock. > > Even If I just > > apply the second patch, the system is still in 100% cpu usage. The > > spin_is_contended check can't cure the problem completely. > > Are you sure it's really contention in that case and not just a lot of > time is spent in compaction trying to satisfy the radix_tree_node > allocation requests? certainly it's the contention. > > Trace shows > > compact_zone will run a useless loop and each loop contend the lru_lock. With > > this patch, the cpu usage becomes normal (about 20% utilization). > > I suspect the reason why this patch has an effect is because compaction is > no longer running. It finds a 100% full pageblock quickly and then aborts and > that is not the right fix. Can you try something like this instead please? That debug patch doesn't help. My system just hang. I thought your worry is valid, we shouldn't abort if 100% full pageblock is found. How about this one? With it, the cpu usage is normal in my workload. Occassionally I saw cpu usage reaches high (up to 80%), but recovered immediately. Without the patch, the cpu usage keeps in 100%. Thanks, Shaohua Subject: compaction: check migrated page number isolate_migratepages_range() might isolate none pages, for example, when zone->lru_lock is contended and compaction is async. In this case, we should abort compaction, otherwise, compact_zone will run a useless loop and make zone->lru_lock is even contended. Signed-off-by: Shaohua Li --- mm/compaction.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) Index: linux/mm/compaction.c =================================================================== --- linux.orig/mm/compaction.c 2012-09-06 18:37:52.636413761 +0800 +++ linux/mm/compaction.c 2012-09-07 10:51:16.734081959 +0800 @@ -618,7 +618,7 @@ typedef enum { static isolate_migrate_t isolate_migratepages(struct zone *zone, struct compact_control *cc) { - unsigned long low_pfn, end_pfn; + unsigned long low_pfn, end_pfn, old_low_pfn; /* Do not scan outside zone boundaries */ low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn); @@ -633,8 +633,9 @@ static isolate_migrate_t isolate_migrate } /* Perform the isolation */ + old_low_pfn = low_pfn; low_pfn = isolate_migratepages_range(zone, cc, low_pfn, end_pfn); - if (!low_pfn) + if (!low_pfn || old_low_pfn == low_pfn) return ISOLATE_ABORT; cc->migrate_pfn = low_pfn; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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