Date:	Tue, 31 May 2016 14:07:12 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 12/13] mm, compaction: more reliably increase direct
 compaction priority

On 05/31/2016 08:37 AM, Joonsoo Kim wrote:
> On Tue, May 10, 2016 at 09:36:02AM +0200, Vlastimil Babka wrote:
>> During reclaim/compaction loop, compaction priority can be increased by the
>> should_compact_retry() function, but the current code is not optimal for
>> several reasons:
>>
>> - priority is only increased when compaction_failed() is true, which means
>>   that compaction has scanned the whole zone. This may not happen even after
>>   multiple attempts with the lower priority due to parallel activity, so we
>>   might needlessly struggle on the lower priority.
>>
>> - should_compact_retry() is only called when should_reclaim_retry() returns
>>   false. This means that compaction priority cannot get increased as long
>>   as reclaim makes sufficient progress. Theoretically, reclaim should stop
>>   retrying for high-order allocations as long as the high-order page doesn't
>>   exist but due to races, this may result in spurious retries when the
>>   high-order page momentarily does exist.
>>
>> We can remove these corner cases by making sure that should_compact_retry() is
>> always called, and increases compaction priority if possible. Examining further
>> the compaction result can be done only after reaching the highest priority.
>> This is a simple solution and we don't need to worry about reaching the highest
>> priority "too soon" here - when should_compact_retry() is called it means that
>> the system is already struggling and the allocation is supposed to either try
>> as hard as possible, or it cannot fail at all. There's not much point staying
>> at lower priorities with heuristics that may result in only partial compaction.
>>
>> The only exception here is the COMPACT_SKIPPED result, which means that
>> compaction could not run at all due to failing order-0 watermarks. In that
>> case, don't increase compaction priority, and check if compaction could proceed
>> when everything reclaimable was reclaimed. Before this patch, this was tied to
>> compaction_withdrawn(), but the other results considered there are in fact only
>> due to low compaction priority so we can ignore them thanks to the patch.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> ---
>>  mm/page_alloc.c | 46 +++++++++++++++++++++++-----------------------
>>  1 file changed, 23 insertions(+), 23 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index aa9c39a7f40a..623027fb8121 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3248,28 +3248,27 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>>  		return false;
>>
>>  	/*
>> -	 * compaction considers all the zone as desperately out of memory
>> -	 * so it doesn't really make much sense to retry except when the
>> -	 * failure could be caused by insufficient priority
>> +	 * Compaction backed off due to watermark checks for order-0
>> +	 * so the regular reclaim has to try harder and reclaim something
>> +	 * Retry only if it looks like reclaim might have a chance.
>>  	 */
>> -	if (compaction_failed(compact_result)) {
>> -		if (*compact_priority > 0) {
>> -			(*compact_priority)--;
>> -			return true;
>> -		}
>> -		return false;
>> -	}
>> +	if (compact_result == COMPACT_SKIPPED)
>> +		return compaction_zonelist_suitable(ac, order, alloc_flags);
>>
>>  	/*
>> -	 * make sure the compaction wasn't deferred or didn't bail out early
>> -	 * due to locks contention before we declare that we should give up.
>> -	 * But do not retry if the given zonelist is not suitable for
>> -	 * compaction.
>> +	 * Compaction could have withdrawn early or skip some zones or
>> +	 * pageblocks. We were asked to retry, which means the allocation
>> +	 * should try really hard, so increase the priority if possible.
>>  	 */
>> -	if (compaction_withdrawn(compact_result))
>> -		return compaction_zonelist_suitable(ac, order, alloc_flags);
>> +	if (*compact_priority > 0) {
>> +		(*compact_priority)--;
>> +		return true;
>> +	}
>>
>>  	/*
>> +	 * The remaining possibility is that compaction made progress and
>> +	 * created a high-order page, but it was allocated by somebody else.
>> +	 * To prevent thrashing, limit the number of retries in such case.
>>  	 * !costly requests are much more important than __GFP_REPEAT
>>  	 * costly ones because they are de facto nofail and invoke OOM
>>  	 * killer to move on while costly can fail and users are ready
>> @@ -3527,6 +3526,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  						struct alloc_context *ac)
>>  {
>>  	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
>> +	bool should_retry;
>>  	struct page *page = NULL;
>>  	unsigned int alloc_flags;
>>  	unsigned long did_some_progress;
>> @@ -3695,22 +3695,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  	else
>>  		no_progress_loops++;
>>
>> -	if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
>> -				 did_some_progress > 0, no_progress_loops))
>> -		goto retry;
>> -
>> +	should_retry = should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
>> +				 did_some_progress > 0, no_progress_loops);
>>  	/*
>>  	 * It doesn't make any sense to retry for the compaction if the order-0
>>  	 * reclaim is not able to make any progress because the current
>>  	 * implementation of the compaction depends on the sufficient amount
>>  	 * of free memory (see __compaction_suitable)
>>  	 */
>> -	if (did_some_progress > 0 &&
>> -			should_compact_retry(ac, order, alloc_flags,
>> +	if (did_some_progress > 0)
>> +		should_retry |= should_compact_retry(ac, order, alloc_flags,
>>  				compact_result, &compact_priority,
>> -				compaction_retries))
>> +				compaction_retries);
>> +	if (should_retry)
>>  		goto retry;
>
> Hmm... it looks odd that we check should_compact_retry() when
> did_some_progress > 0. If system is full of anonymous memory and we
> don't have swap, we can't reclaim anything but we can compact.

Right, thanks.

> And, your patchset make me think that it's better to separate retry
> loop for order-0 allocation and high-order allocation completely.

I don't know, the loops is already large enough. Basically duplicating 
it sounds like a lot of bloat. Hiding the order-specific decisions in 
helpers sounds better.

> Current code is a mix of these two types of criteria and is hard to
> follow. Your patchset make it simpler but we can do better if
> separating them completely. Any thought?
>
> Thanks.
>

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