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

[Andrew Morton]

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