linux-kernel - Re: [PATCH 1/5] mm, vmscan: Do not account skipped pages as scanned

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Date:	Thu, 28 Jul 2016 10:38:22 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] mm, vmscan: Do not account skipped pages as scanned

On Mon, Jul 25, 2016 at 10:20:14AM +0100, Mel Gorman wrote:
> On Mon, Jul 25, 2016 at 05:04:56PM +0900, Minchan Kim wrote:
> > > @@ -1429,6 +1429,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> > >  			continue;
> > >  		}
> > >  
> > > +		/* Pages skipped do not contribute to scan */
> > > +		scan++;
> > > +
> > 
> > As I mentioned in previous version, under irq-disabled-spin-lock, such
> > unbounded operation would make the latency spike worse if there are
> > lot of pages we should skip.
> > 
> > Don't we take care it?
> 
> It's not unbounded, it's bound by the size of the LRU list and it's not
> going to be enough to trigger a warning. While the lock hold time may be
> undesirable, unlocking it every SWAP_CLUSTER_MAX pages may increase overall
> contention. There also is the question of whether skipped pages should be
> temporarily putback before unlocking the LRU to avoid isolated pages being
> unavailable for too long. It also cannot easily just return early without
> prematurely triggering OOM due to a lack of progress. I didn't feel the
> complexity was justified.

I measured the lock holding time and it took max 96ms during 360M
scanning with hackbench. It was very easy to reproduce with node-lru
because it should skip too many pages.

Given that my box is much faster than usual mobile CPU, it would
take more time in embedded system. I think irq disable during 96ms would
be worth to be fixed.

Anyway, I'm done by that which I measured time by hand so it's up to you
that whether you want to fix or leave as it is until someone reports it with
more real workload.

> 
> -- 
> Mel Gorman
> SUSE Labs
> 
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