Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753962AbaFWI4L (ORCPT ); Mon, 23 Jun 2014 04:56:11 -0400 Received: from cn.fujitsu.com ([59.151.112.132]:24785 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1752870AbaFWI4I (ORCPT ); Mon, 23 Jun 2014 04:56:08 -0400 X-IronPort-AV: E=Sophos;i="5.00,759,1396972800"; d="scan'208";a="32278176" Message-ID: <53A7EB9B.5000406@cn.fujitsu.com> Date: Mon, 23 Jun 2014 16:55:55 +0800 From: Zhang Yanfei User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20131030 Thunderbird/17.0.10 MIME-Version: 1.0 To: Minchan Kim CC: Vlastimil Babka , , Andrew Morton , David Rientjes , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel , Subject: Re: [PATCH v3 05/13] mm, compaction: report compaction as contended only due to lock contention References: <1403279383-5862-1-git-send-email-vbabka@suse.cz> <1403279383-5862-6-git-send-email-vbabka@suse.cz> <20140623013903.GA12413@bbox> In-Reply-To: <20140623013903.GA12413@bbox> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.167.225.89] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Minchan, On 06/23/2014 09:39 AM, Minchan Kim wrote: > Hello Vlastimil, > > On Fri, Jun 20, 2014 at 05:49:35PM +0200, Vlastimil Babka wrote: >> Async compaction aborts when it detects zone lock contention or need_resched() >> is true. David Rientjes has reported that in practice, most direct async >> compactions for THP allocation abort due to need_resched(). This means that a >> second direct compaction is never attempted, which might be OK for a page >> fault, but khugepaged is intended to attempt a sync compaction in such case and >> in these cases it won't. >> >> This patch replaces "bool contended" in compact_control with an enum that >> distinguieshes between aborting due to need_resched() and aborting due to lock >> contention. This allows propagating the abort through all compaction functions >> as before, but declaring the direct compaction as contended only when lock >> contention has been detected. >> >> A second problem is that try_to_compact_pages() did not act upon the reported >> contention (both need_resched() or lock contention) and could proceed with >> another zone from the zonelist. When need_resched() is true, that means >> initializing another zone compaction, only to check again need_resched() in >> isolate_migratepages() and aborting. For zone lock contention, the unintended >> consequence is that the contended status reported back to the allocator >> is decided from the last zone where compaction was attempted, which is rather >> arbitrary. >> >> This patch fixes the problem in the following way: >> - need_resched() being true after async compaction returned from a zone means >> that further zones should not be tried. We do a cond_resched() so that we >> do not hog the CPU, and abort. "contended" is reported as false, since we >> did not fail due to lock contention. >> - aborting zone compaction due to lock contention means we can still try >> another zone, since it has different locks. We report back "contended" as >> true only if *all* zones where compaction was attempted, it aborted due to >> lock contention. >> >> As a result of these fixes, khugepaged will proceed with second sync compaction >> as intended, when the preceding async compaction aborted due to need_resched(). >> Page fault compactions aborting due to need_resched() will spare some cycles >> previously wasted by initializing another zone compaction only to abort again. >> Lock contention will be reported only when compaction in all zones aborted due >> to lock contention, and therefore it's not a good idea to try again after >> reclaim. >> >> Reported-by: David Rientjes >> 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 >> --- >> mm/compaction.c | 48 +++++++++++++++++++++++++++++++++++++++--------- >> mm/internal.h | 15 +++++++++++---- >> 2 files changed, 50 insertions(+), 13 deletions(-) >> >> diff --git a/mm/compaction.c b/mm/compaction.c >> index ebe30c9..e8cfac9 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -180,9 +180,14 @@ static void update_pageblock_skip(struct compact_control *cc, >> } >> #endif /* CONFIG_COMPACTION */ >> >> -static inline bool should_release_lock(spinlock_t *lock) >> +enum compact_contended should_release_lock(spinlock_t *lock) >> { >> - return need_resched() || spin_is_contended(lock); >> + if (spin_is_contended(lock)) >> + return COMPACT_CONTENDED_LOCK; >> + else if (need_resched()) >> + return COMPACT_CONTENDED_SCHED; >> + else >> + return COMPACT_CONTENDED_NONE; > > If you want to raise priority of lock contention than need_resched > intentionally, please write it down on comment. > >> } >> >> /* >> @@ -197,7 +202,9 @@ static inline bool should_release_lock(spinlock_t *lock) >> static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags, >> bool locked, struct compact_control *cc) >> { >> - if (should_release_lock(lock)) { >> + enum compact_contended contended = should_release_lock(lock); >> + >> + if (contended) { >> if (locked) { >> spin_unlock_irqrestore(lock, *flags); >> locked = false; >> @@ -205,7 +212,7 @@ static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags, >> >> /* async aborts if taking too long or contended */ >> if (cc->mode == MIGRATE_ASYNC) { >> - cc->contended = true; >> + cc->contended = contended; >> return false; >> } > > >> >> @@ -231,7 +238,7 @@ static inline bool compact_should_abort(struct compact_control *cc) >> /* async compaction aborts if contended */ >> if (need_resched()) { >> if (cc->mode == MIGRATE_ASYNC) { >> - cc->contended = true; >> + cc->contended = COMPACT_CONTENDED_SCHED; >> return true; >> } >> >> @@ -1101,7 +1108,8 @@ static unsigned long compact_zone_order(struct zone *zone, int order, >> VM_BUG_ON(!list_empty(&cc.freepages)); >> VM_BUG_ON(!list_empty(&cc.migratepages)); >> >> - *contended = cc.contended; >> + /* We only signal lock contention back to the allocator */ >> + *contended = cc.contended == COMPACT_CONTENDED_LOCK; > > Please write down *WHY* as well as your intention we can know by looking at code. > >> return ret; >> } >> >> @@ -1132,6 +1140,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist, >> struct zone *zone; >> int rc = COMPACT_SKIPPED; >> int alloc_flags = 0; >> + bool all_zones_contended = true; >> >> /* Check if the GFP flags allow compaction */ >> if (!order || !may_enter_fs || !may_perform_io) >> @@ -1146,6 +1155,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist, >> for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx, >> nodemask) { >> int status; >> + bool zone_contended; >> >> if (compaction_deferred(zone, order)) >> continue; >> @@ -1153,8 +1163,9 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist, >> *deferred = false; >> >> status = compact_zone_order(zone, order, gfp_mask, mode, >> - contended); >> + &zone_contended); >> rc = max(status, rc); >> + all_zones_contended &= zone_contended; >> >> /* If a normal allocation would succeed, stop compacting */ >> if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0, >> @@ -1168,12 +1179,31 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist, >> * succeeding after all, it will be reset. >> */ >> defer_compaction(zone, order); >> + /* >> + * If we stopped compacting due to need_resched(), do >> + * not try further zones and yield the CPU. >> + */ > > For what? It would make your claim more clear. > >> + if (need_resched()) { > > compact_zone_order returns true state of contended only if it was lock contention > so it couldn't return true state of contended by need_resched so you made > need_resched check in here. It's fragile to me because it could be not a result > from ahead compact_zone_order call. More clear thing is compact_zone_order > should return zone_contended as enum, not bool and in here, you could check it. > > It means you could return enum in compact_zone_order and make the result bool > in try_to_compact_pages. > >> + /* >> + * We might not have tried all the zones, so >> + * be conservative and assume they are not >> + * all lock contended. >> + */ >> + all_zones_contended = false; >> + cond_resched(); >> + break; >> + } >> } >> } >> >> - /* If at least one zone wasn't deferred, we count a compaction stall */ >> - if (!*deferred) >> + /* >> + * If at least one zone wasn't deferred, we count a compaction stall >> + * and we report if all zones that were tried were contended. >> + */ >> + if (!*deferred) { >> count_compact_event(COMPACTSTALL); >> + *contended = all_zones_contended; > > Why don't you initialize contended as *false* in function's intro? > >> + } >> >> return rc; >> } >> diff --git a/mm/internal.h b/mm/internal.h >> index a1b651b..2c187d2 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -117,6 +117,13 @@ extern int user_min_free_kbytes; >> >> #if defined CONFIG_COMPACTION || defined CONFIG_CMA >> >> +/* Used to signal whether compaction detected need_sched() or lock contention */ >> +enum compact_contended { >> + COMPACT_CONTENDED_NONE = 0, /* no contention detected */ >> + COMPACT_CONTENDED_SCHED, /* need_sched() was true */ >> + COMPACT_CONTENDED_LOCK, /* zone lock or lru_lock was contended */ >> +}; >> + >> /* >> * in mm/compaction.c >> */ >> @@ -144,10 +151,10 @@ struct compact_control { >> int order; /* order a direct compactor needs */ >> int migratetype; /* MOVABLE, RECLAIMABLE etc */ >> struct zone *zone; >> - bool contended; /* True if a lock was contended, or >> - * need_resched() true during async >> - * compaction >> - */ >> + enum compact_contended contended; /* Signal need_sched() or lock >> + * contention detected during >> + * compaction >> + */ >> }; >> >> unsigned long >> -- > > Anyway, most big concern is that you are changing current behavior as > I said earlier. > > Old behavior in THP page fault when it consumes own timeslot was just > abort and fallback 4K page but with your patch, new behavior is > take a rest when it founds need_resched and goes to another round with > async, not sync compaction. I'm not sure we need another round with > async compaction at the cost of increasing latency rather than fallback > 4 page. I don't see the new behavior works like what you said. If need_resched is true, it calls cond_resched() and after a rest it just breaks the loop. Why there is another round with async compact? Thanks. > > It might be okay if the VMA has MADV_HUGEPAGE which is good hint to > indicate non-temporal VMA so latency would be trade-off but it's not > for temporal big memory allocation in HUGEPAGE_ALWAYS system. > > If you really want to go this, could you show us numbers? > > 1. How many could we can be successful in direct compaction by this patch? > 2. How long could we increase latency for temporal allocation > for HUGEPAGE_ALWAYS system? > -- Thanks. Zhang Yanfei -- 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/