Re: [PATCH 04/14] mm, compaction: distinguish COMPACT_DEFERRED from COMPACT_SKIPPED

[Hillf Danton]

> 
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> try_to_compact_pages can currently return COMPACT_SKIPPED even when the
> compaction is defered for some zone just because zone DMA is skipped
> in 99% of cases due to watermark checks. This makes COMPACT_DEFERRED
> basically unusable for the page allocator as a feedback mechanism.
> 
> Make sure we distinguish those two states properly and switch their
> ordering in the enum. This would mean that the COMPACT_SKIPPED will be
> returned only when all eligible zones are skipped.
> 
> As a result COMPACT_DEFERRED handling for THP in __alloc_pages_slowpath
> will be more precise and we would bail out rather than reclaim.
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---

Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  include/linux/compaction.h        | 7 +++++--
>  include/trace/events/compaction.h | 2 +-
>  mm/compaction.c                   | 8 +++++---
>  3 files changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 4458fd94170f..7e177d111c39 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -4,13 +4,16 @@
>  /* Return values for compact_zone() and try_to_compact_pages() */
>  /* When adding new states, please adjust include/trace/events/compaction.h */
>  enum compact_result {
> -	/* compaction didn't start as it was deferred due to past failures */
> -	COMPACT_DEFERRED,
>  	/*
>  	 * compaction didn't start as it was not possible or direct reclaim
>  	 * was more suitable
>  	 */
>  	COMPACT_SKIPPED,
> +	/* compaction didn't start as it was deferred due to past failures */
> +	COMPACT_DEFERRED,
> +	/* compaction not active last round */
> +	COMPACT_INACTIVE = COMPACT_DEFERRED,
> +
>  	/* compaction should continue to another pageblock */
>  	COMPACT_CONTINUE,
>  	/*
> diff --git a/include/trace/events/compaction.h b/include/trace/events/compaction.h
> index e215bf68f521..6ba16c86d7db 100644
> --- a/include/trace/events/compaction.h
> +++ b/include/trace/events/compaction.h
> @@ -10,8 +10,8 @@
>  #include <trace/events/mmflags.h>
> 
>  #define COMPACTION_STATUS					\
> -	EM( COMPACT_DEFERRED,		"deferred")		\
>  	EM( COMPACT_SKIPPED,		"skipped")		\
> +	EM( COMPACT_DEFERRED,		"deferred")		\
>  	EM( COMPACT_CONTINUE,		"continue")		\
>  	EM( COMPACT_PARTIAL,		"partial")		\
>  	EM( COMPACT_COMPLETE,		"complete")		\
> diff --git a/mm/compaction.c b/mm/compaction.c
> index b06de27b7f72..13709e33a2fc 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1637,7 +1637,7 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
>  	int may_perform_io = gfp_mask & __GFP_IO;
>  	struct zoneref *z;
>  	struct zone *zone;
> -	enum compact_result rc = COMPACT_DEFERRED;
> +	enum compact_result rc = COMPACT_SKIPPED;
>  	int all_zones_contended = COMPACT_CONTENDED_LOCK; /* init for &= op */
> 
>  	*contended = COMPACT_CONTENDED_NONE;
> @@ -1654,8 +1654,10 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
>  		enum compact_result status;
>  		int zone_contended;
> 
> -		if (compaction_deferred(zone, order))
> +		if (compaction_deferred(zone, order)) {
> +			rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
>  			continue;
> +		}
> 
>  		status = compact_zone_order(zone, order, gfp_mask, mode,
>  				&zone_contended, alloc_flags,
> @@ -1726,7 +1728,7 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
>  	 * If at least one zone wasn't deferred or skipped, we report if all
>  	 * zones that were tried were lock contended.
>  	 */
> -	if (rc > COMPACT_SKIPPED && all_zones_contended)
> +	if (rc > COMPACT_INACTIVE && all_zones_contended)
>  		*contended = COMPACT_CONTENDED_LOCK;
> 
>  	return rc;
> --
> 2.8.0.rc3