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Date:	Fri, 11 Jul 2014 11:38:04 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Minchan Kim <minchan@...nel.org>
CC:	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>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 05/13] mm, compaction: report compaction as contended
 only due to lock contention

On 07/11/2014 10:28 AM, Vlastimil Babka wrote:
> On 06/23/2014 03:39 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.
>
> Well that behavior is pretty new, introduced recently by David's patches
> and now he himself is saying that it makes page fault THP allocations
> very unsuccessful and the need_resched() is not a good termination
> criterion. But OK, I can restrict this to khugepaged...
>
>> 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.
>   >
>> 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.
>
> Yeah that could be useful hint when we eventually move away from
> need_resched() to a fixed/tunable work per compaction attempt. But I'll
> rather not introduce it in the series.
>
>> 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?
>
> Doesn't seem to help. I'll drop it.

Actually the patch was wrong as the need_resched() check occured under 
if (mode != MIGRATE_ASYNC) branch. But need_resched() contention is only 
considered in MIGRATE_ASYNC mode. Oops.

>> 2. How long could we increase latency for temporal allocation
>>      for HUGEPAGE_ALWAYS system?
>
> Don't have the data.
>

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