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

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Date:	Wed, 30 Sep 2015 17:12:00 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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>
Subject: Re: [PATCH v2 2/9] mm/compaction: introduce compaction depleted
 state on zone

Hello, Vlastimil.

First of all, thanks for review!

On Fri, Sep 25, 2015 at 10:04:49AM +0200, Vlastimil Babka wrote:
> 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?

Okay. I will fix it.

Thanks.
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