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

Andrew Morton
2014-05-07T21:20:36Z

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?