linux-kernel - Re: [PATCH v5 07/14] mm, compaction: khugepaged should not give up due to need

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Message-ID: <20140729082714.GD1610@js1304-P5Q-DELUXE>
Date:	Tue, 29 Jul 2014 17:27:14 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
	Minchan Kim <minchan@...nel.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH v5 07/14] mm, compaction: khugepaged should not give up
 due to need_resched()

On Tue, Jul 29, 2014 at 12:31:13AM -0700, David Rientjes wrote:
> On Tue, 29 Jul 2014, Joonsoo Kim wrote:
> 
> > I have a silly question here.
> > Why need_resched() is criteria to stop async compaction?
> > need_resched() is flagged up when time slice runs out or other reasons.
> > It means that we should stop async compaction at arbitrary timing
> > because process can be on compaction code at arbitrary moment. I think
> > that it isn't reasonable and it doesn't ensure anything. Instead of
> > this approach, how about doing compaction on certain amounts of pageblock
> > for async compaction?
> > 
> 
> Not a silly question at all, I had the same feeling in 
> https://lkml.org/lkml/2014/5/21/730 and proposed it to be a tunable that 
> indicates how much work we are willing to do for thp in the pagefault 
> path.  It suffers from the fact that past failure to isolate and/or 

Oh... you already suggested the same idea.

> migrate memory to free an entire pageblock doesn't indicate that the next 
> pageblock will fail as well, but there has to be cutoff at some point or 
> async compaction becomes unnecessarily expensive.  We can always rely on 
> khugepaged later to do the collapse, assuming we're not faulting memory 
> and then immediately pinning it.
> 
> I think there's two ways to go about it:
> 
>  - allow a single thp fault to be expensive and then rely on deferred
>    compaction to avoid subsequent calls in the near future, or
> 
>  - try to make all thp faults be as least expensive as possible so that
>    the cumulative effect of faulting large amounts of memory doesn't end
>    up with lengthy stalls.

Hmm, if thp faults want to pay cost as least as possible, how about
making thp faults skip async/sync compaction at all?

Thanks.
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