linux-ext4 - 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 10:57:20 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	minchan.kim@...il.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 09:26:09AM +0900, KOSAKI Motohiro wrote:
> >Lets see;
> >
> >shrink_page_list() only applies if inactive pages were isolated
> >	which in turn may not happen if all_unreclaimable is set in
> >	shrink_zones(). If for whatver reason, all_unreclaimable is
> >	set on all zones, we can miss calling cond_resched().
> >
> >shrink_slab only applies if we are reclaiming slab pages. If the first
> >	shrinker returns -1, we do not call cond_resched(). If that
> >	first shrinker is dcache and __GFP_FS is not set, direct
> >	reclaimers will not shrink at all. However, if there are
> >	enough of them running or if one of the other shrinkers
> >	is running for a very long time, kswapd could be starved
> >	acquiring the shrinker_rwsem and never reaching the
> >	cond_resched().
> 
> OK.
> 
> 
> >
> >balance_pgdat() only calls cond_resched if the zones are not
> >	balanced. For a high-order allocation that is balanced, it
> >	checks order-0 again. During that window, order-0 might have
> >	become unbalanced so it loops again for order-0 and returns
> >	that was reclaiming for order-0 to kswapd(). It can then find
> >	that a caller has rewoken kswapd for a high-order and re-enters
> >	balance_pgdat() without ever have called cond_resched().
> 
> Then, Shouldn't balance_pgdat() call cond_resched() unconditionally?
> The problem is NOT 100% cpu consumption. if kswapd will sleep, other
> processes need to reclaim old pages. The problem is, kswapd doesn't
> invoke context switch and other tasks hang-up.
> 

Which the shrink_slab patch does (either version). What's the gain from
sprinkling more cond_resched() around? If you think there is, submit
another pair of patches (include patch 1 from this series) but I'm not
seeing the advantage myself.

> 
> >While it appears unlikely, there are bad conditions which can result
> >in cond_resched() being avoided.
> >
> 
> 

-- 
Mel Gorman
SUSE Labs
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