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Date: Wed, 7 May 2014 14:20:33 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Greg Thelen <gthelen@google.com>, Hugh Dickins <hughd@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch v3 6/6] mm, compaction: terminate async compaction when
 rescheduling
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On Tue, 6 May 2014 19:22:52 -0700 (PDT) David Rientjes <rientjes@google.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?