Re: [PATCH v2 12/18] mm, compaction: more reliably increase direct compaction priority

From	Michal Hocko <mhocko@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 12/18] mm, compaction: more reliably increase direct compaction priority
Date	2016-06-01 16:00 +0200
Message-ID	<rFeQG-Fc-19@gated-at.bofh.it> (permalink)
References	<rERAJ-38J-1@gated-at.bofh.it> <rERKq-3dI-21@gated-at.bofh.it>
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On Tue 31-05-16 15:08:12, 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. 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.
> 
> We can remove these corner cases by increasing compaction priority regardless
> of compaction_failed(). Examining further the compaction result can be
> postponed 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,
> because hen 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 being below 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
> possible due to low compaction priority so we can ignore them thanks to the
> patch. Since there are no other callers of compaction_withdrawn(), remove it.

I agree with the change in general. I think that keeping compaction_withdrawn
even with a single check is better because it abstracts the fact from a
specific constant.

Now that I think about that some more I guess you also want to update
compaction_retries inside should_compact_retry as well, or at least
update it only when we have reached the lowest priority. What do you
think?
 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Other than that this makes sense
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/compaction.h | 46 ----------------------------------------------
>  mm/page_alloc.c            | 37 ++++++++++++++++++++++---------------
>  2 files changed, 22 insertions(+), 61 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 29dc7c05bd3b..4bef69a83f8f 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -105,47 +105,6 @@ static inline bool compaction_failed(enum compact_result result)
>  	return false;
>  }
>  
> -/*
> - * Compaction  has backed off for some reason. It might be throttling or
> - * lock contention. Retrying is still worthwhile.
> - */
> -static inline bool compaction_withdrawn(enum compact_result result)
> -{
> -	/*
> -	 * Compaction backed off due to watermark checks for order-0
> -	 * so the regular reclaim has to try harder and reclaim something.
> -	 */
> -	if (result == COMPACT_SKIPPED)
> -		return true;
> -
> -	/*
> -	 * If compaction is deferred for high-order allocations, it is
> -	 * because sync compaction recently failed. If this is the case
> -	 * and the caller requested a THP allocation, we do not want
> -	 * to heavily disrupt the system, so we fail the allocation
> -	 * instead of entering direct reclaim.
> -	 */
> -	if (result == COMPACT_DEFERRED)
> -		return true;
> -
> -	/*
> -	 * If compaction in async mode encounters contention or blocks higher
> -	 * priority task we back off early rather than cause stalls.
> -	 */
> -	if (result == COMPACT_CONTENDED)
> -		return true;
> -
> -	/*
> -	 * Page scanners have met but we haven't scanned full zones so this
> -	 * is a back off in fact.
> -	 */
> -	if (result == COMPACT_PARTIAL_SKIPPED)
> -		return true;
> -
> -	return false;
> -}
> -
> -
>  bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
>  					int alloc_flags);
>  
> @@ -183,11 +142,6 @@ static inline bool compaction_failed(enum compact_result result)
>  	return false;
>  }
>  
> -static inline bool compaction_withdrawn(enum compact_result result)
> -{
> -	return true;
> -}
> -
>  static inline int kcompactd_run(int nid)
>  {
>  	return 0;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 27923af8e534..dee486936ccf 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3235,28 +3235,35 @@ 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 > MIN_COMPACT_PRIORITY) {
> -			(*compact_priority)--;
> -			return true;
> -		}
> -		return false;
> +	if (compact_result == COMPACT_SKIPPED)
> +		return compaction_zonelist_suitable(ac, order, alloc_flags);
> +
> +	/*
> +	 * 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 (*compact_priority > MIN_COMPACT_PRIORITY) {
> +		(*compact_priority)--;
> +		return true;
>  	}
>  
>  	/*
> -	 * 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 considers all the zones as unfixably fragmented and we
> +	 * are on the highest priority, which means it can't be due to
> +	 * heuristics and it doesn't really make much sense to retry.
>  	 */
> -	if (compaction_withdrawn(compact_result))
> -		return compaction_zonelist_suitable(ac, order, alloc_flags);
> +	if (compaction_failed(compact_result))
> +		return false;
>  
>  	/*
> +	 * 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
> -- 
> 2.8.3
> 
> --
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-- 
Michal Hocko
SUSE Labs

Thread

[PATCH v2 00/18] make direct compaction more deterministic Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
  [PATCH v2 02/18] mm, page_alloc: set alloc_flags only once in slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
  [PATCH v2 15/18] mm, compaction: use proper alloc_flags in __compaction_suitable() Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
  [PATCH v2 14/18] mm, compaction: create compact_gap wrapper Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
    Re: [PATCH v2 14/18] mm, compaction: create compact_gap wrapper Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:10 +0200
  [PATCH v2 17/18] mm, vmscan: make compaction_ready() more accurate and readable Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:10 +0200
    Re: [PATCH v2 17/18] mm, vmscan: make compaction_ready() more  accurate and readable Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:20 +0200
  [PATCH v2 04/18] mm, page_alloc: restructure direct compaction handling in slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 03/18] mm, page_alloc: don't retry initial attempt in slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 03/18] mm, page_alloc: don't retry initial attempt in  slowpath Michal Hocko <mhocko@kernel.org> - 2016-06-01 15:30 +0200
      Re: [PATCH v2 03/18] mm, page_alloc: don't retry initial attempt in  slowpath Vlastimil Babka <vbabka@suse.cz> - 2016-06-01 17:00 +0200
  [PATCH v2 05/18] mm, page_alloc: make THP-specific decisions more generic Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 13/18] mm, compaction: use correct watermark when checking allocation success Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 13/18] mm, compaction: use correct watermark when  checking allocation success Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:00 +0200
  [PATCH v2 06/18] mm, thp: remove __GFP_NORETRY from khugepaged and madvised allocations Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 06/18] mm, thp: remove __GFP_NORETRY from khugepaged  and madvised allocations Michal Hocko <mhocko@kernel.org> - 2016-06-01 15:40 +0200
  [PATCH v2 12/18] mm, compaction: more reliably increase direct compaction priority Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 12/18] mm, compaction: more reliably increase direct  compaction priority Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:00 +0200
  [PATCH v2 11/18] mm, compaction: add the ultimate direct compaction priority Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 10/18] mm, compaction: cleanup unused functions Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 10/18] mm, compaction: cleanup unused functions Michal Hocko <mhocko@kernel.org> - 2016-06-01 15:50 +0200
  [PATCH v2 01/18] mm, compaction: don't isolate PageWriteback pages in MIGRATE_SYNC_LIGHT mode Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 09/18] mm, compaction: make whole_zone flag ignore cached scanner positions Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in should_continue_reclaim() Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in  should_continue_reclaim() Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:30 +0200
      Re: [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in  should_continue_reclaim() Vlastimil Babka <vbabka@suse.cz> - 2016-06-01 17:20 +0200
        Re: [PATCH v2 18/18] mm, vmscan: use proper classzone_idx in  should_continue_reclaim() Michal Hocko <mhocko@kernel.org> - 2016-06-01 17:50 +0200
  [PATCH v2 08/18] mm, compaction: simplify contended compaction handling Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
  [PATCH v2 16/18] mm, compaction: require only min watermarks for non-costly orders Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200
    Re: [PATCH v2 16/18] mm, compaction: require only min watermarks for  non-costly orders Michal Hocko <mhocko@kernel.org> - 2016-06-01 16:10 +0200
  [PATCH v2 07/18] mm, compaction: introduce direct compaction priority Vlastimil Babka <vbabka@suse.cz> - 2016-05-31 15:20 +0200

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