Re: [PATCH v3 05/13] mm, compaction: report compaction as contended only due to lock contention

[Zhang Yanfei]

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@xxxxxxxxxx>
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Minchan Kim <minchan@xxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxx>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
>> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>> Cc: Christoph Lameter <cl@xxxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> ---
>>  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
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