linux-kernel - Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep

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Date:	Wed, 18 May 2011 10:58:59 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	James.Bottomley@...senpartnership.com, akpm@...ux-foundation.org,
	colin.king@...onical.com, raghu.prabhu13@...il.com, jack@...e.cz,
	chris.mason@...cle.com, cl@...ux.com, penberg@...nel.org,
	riel@...hat.com, hannes@...xchg.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long,
 allow it to sleep

On Wed, May 18, 2011 at 02:44:48PM +0900, Minchan Kim wrote:
> On Wed, May 18, 2011 at 10:05 AM, KOSAKI Motohiro
> <kosaki.motohiro@...fujitsu.com> wrote:
> >> It would be better to put cond_resched after balance_pgdat?
> >>
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index 292582c..61c45d0 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -2753,6 +2753,7 @@ static int kswapd(void *p)
> >>                 if (!ret) {
> >>                         trace_mm_vmscan_kswapd_wake(pgdat->node_id,
> >> order);
> >>                         order = balance_pgdat(pgdat,
> >> order,&classzone_idx);
> >> +                       cond_resched();
> >>                 }
> >>         }
> >>         return 0;
> >>
> >>>>> While it appears unlikely, there are bad conditions which can result
> >>>
> >>> in cond_resched() being avoided.
> >
> > Every reclaim priority decreasing or every shrink_zone() calling makes more
> > fine grained preemption. I think.
> 
> It could be.
> But in direct reclaim case, I have a concern about losing pages
> reclaimed to other tasks by preemption.
> 
> Hmm,, anyway, we also needs test.
> Hmm,, how long should we bother them(Colins and James)?
> First of all, Let's fix one just between us and ask test to them and
> send the last patch to akpm.
> 
> 1. shrink_slab
> 2. right after balance_pgdat
> 3. shrink_zone
> 4. reclaim priority decreasing routine.
> 
> Now, I vote 1) and 2).
> 

I've already submitted a pair of patches for option 1. I don't think
option 2 gains us anything. I think it's more likely we should worry
about all_unreclaimable being set when shrink_slab is returning 0 and we
are encountering so many dirty pages that pages_scanned is high enough.

-- 
Mel Gorman
SUSE Labs
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