Date:	Tue, 24 Jun 2014 08:35:07 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 05/13] mm, compaction: report compaction as contended
 only due to lock contention

Hello Zhang,

On Mon, Jun 23, 2014 at 04:55:55PM +0800, Zhang Yanfei wrote:
> 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 <rientjes@...gle.com>
> >> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> >> Cc: Minchan Kim <minchan@...nel.org>
> >> Cc: Mel Gorman <mgorman@...e.de>
> >> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> >> Cc: Michal Nazarewicz <mina86@...a86.com>
> >> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> >> Cc: Christoph Lameter <cl@...ux.com>
> >> Cc: Rik van Riel <riel@...hat.com>
> >> ---
> >>  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?

One example goes

Old:
page fault
huge page allocation
__alloc_pages_slowpath
__alloc_pages_direct_compact
compact_zone_order
        isolate_migratepages
        compact_checklock_irqsave
                need_resched is true
                cc->contended = true;
        return ISOLATE_ABORT
return COMPACT_PARTIAL with *contented = cc.contended;
COMPACTFAIL
if (contended_compaction && gfp_mask & __GFP_NO_KSWAPD)
        goto nopage;

New:

page fault
huge page allocation
__alloc_pages_slowpath
__alloc_pages_direct_compact
compact_zone_order
        isolate_migratepages
        compact_unlock_should_abort
                need_resched is true
                cc->contended = COMPACT_CONTENDED_SCHED;
                return true;
        return ISOLATE_ABORT
return COMPACT_PARTIAL with *contended = cc.contended == COMPACT_CONTENDED_LOCK (1)
COMPACTFAIL
if (contended_compaction && gfp_mask & __GFP_NO_KSWAPD)
        no goto nopage because contended_compaction was false by (1)

__alloc_pages_direct_reclaim
if (should_alloc_retry)
else
        __alloc_pages_direct_compact again with ASYNC_MODE
                        

> 
> 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
> 
> --
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-- 
Kind regards,
Minchan Kim
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