linux-kernel - Re: [PATCH v2 2/9] mm/compaction: introduce compaction depleted state on zone

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Date:	Fri, 25 Sep 2015 10:04:49 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <js1304@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 2/9] mm/compaction: introduce compaction depleted state
 on zone

On 08/24/2015 04:19 AM, Joonsoo Kim wrote:
> Further compaction attempt is deferred when some of compaction attempts
> already fails. But, after some number of trial are skipped, compaction
> restarts work to check whether compaction is now possible or not. It
> scans whole range of zone to determine this possibility and if compaction
> possibility doesn't recover, this whole range scan is quite big overhead.
> As a first step to reduce this overhead, this patch implement compaction
> depleted state on zone.
> 
> The way to determine depletion of compaction possility is checking number
> of success on previous compaction attempt. If number of successful
> compaction is below than specified threshold, we guess that compaction
> will not successful next time so mark the zone as compaction depleted.
> In this patch, threshold is choosed by 1 to imitate current compaction
> deferring algorithm. In the following patch, compaction algorithm will be
> changed and this threshold is also adjusted to that change.
> 
> In this patch, only state definition is implemented. There is no action
> for this new state so no functional change. But, following patch will
> add some handling for this new state.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>  include/linux/mmzone.h |  3 +++
>  mm/compaction.c        | 44 +++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 44 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 754c259..700e9b5 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -517,6 +517,8 @@ struct zone {
>  	unsigned int		compact_considered;
>  	unsigned int		compact_defer_shift;
>  	int			compact_order_failed;
> +	unsigned long		compact_success;
> +	unsigned long		compact_depletion_depth;
>  #endif
>  
>  #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> @@ -543,6 +545,7 @@ enum zone_flags {
>  					 * many pages under writeback
>  					 */
>  	ZONE_FAIR_DEPLETED,		/* fair zone policy batch depleted */
> +	ZONE_COMPACTION_DEPLETED,	/* compaction possiblity depleted */
>  };
>  
>  static inline unsigned long zone_end_pfn(const struct zone *zone)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c2d3d6a..de96e9d 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -129,6 +129,23 @@ static struct page *pageblock_pfn_to_page(unsigned long start_pfn,
>  
>  /* Do not skip compaction more than 64 times */
>  #define COMPACT_MAX_DEFER_SHIFT 6
> +#define COMPACT_MIN_DEPLETE_THRESHOLD 1UL
> +
> +static bool compaction_depleted(struct zone *zone)
> +{
> +	unsigned long threshold;
> +	unsigned long success = zone->compact_success;
> +
> +	/*
> +	 * Now, to imitate current compaction deferring approach,
> +	 * choose threshold to 1. It will be changed in the future.
> +	 */
> +	threshold = COMPACT_MIN_DEPLETE_THRESHOLD;
> +	if (success >= threshold)
> +		return false;
> +
> +	return true;
> +}
>  
>  /*
>   * Compaction is deferred when compaction fails to result in a page
> @@ -223,6 +240,16 @@ static void __reset_isolation_suitable(struct zone *zone)
>  	zone->compact_cached_free_pfn = end_pfn;
>  	zone->compact_blockskip_flush = false;
>  
> +	if (compaction_depleted(zone)) {
> +		if (test_bit(ZONE_COMPACTION_DEPLETED, &zone->flags))
> +			zone->compact_depletion_depth++;
> +		else {
> +			set_bit(ZONE_COMPACTION_DEPLETED, &zone->flags);
> +			zone->compact_depletion_depth = 0;
> +		}
> +	}
> +	zone->compact_success = 0;

It's possible that the following comment is made moot by further patches, but:

I assume doing this in __reset_isolation_suitable() is to react on the
compaction_restarting() state. But __reset_isolation_suitable() is called also
from manually invoked compaction, and from kswapd. What if compaction has
succeeded S times, but threshold is T, S < T and T is larger than 1 (by a later
patch). Then kswapd or manual compaction will reset S to zero, without giving it
chance to reach T, even when compaction would succeed?
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