linux-kernel - Re: [patch v3 6/6] mm, compaction: terminate async compaction when rescheduling

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Date:	Wed, 7 May 2014 14:20:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch v3 6/6] mm, compaction: terminate async compaction when
 rescheduling

On Tue, 6 May 2014 19:22:52 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> Async compaction terminates prematurely when need_resched(), see
> compact_checklock_irqsave().  This can never trigger, however, if the 
> cond_resched() in isolate_migratepages_range() always takes care of the 
> scheduling.
> 
> If the cond_resched() actually triggers, then terminate this pageblock scan for 
> async compaction as well.
> 
> ..
>
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -500,8 +500,13 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  			return 0;
>  	}
>  
> +	if (cond_resched()) {
> +		/* Async terminates prematurely on need_resched() */
> +		if (cc->mode == MIGRATE_ASYNC)
> +			return 0;
> +	}

Comment comments the obvious.  What is less obvious is *why* we do this.


Someone please remind my why sync and async compaction use different
scanning cursors?

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