Re: [PATCH 2/2] mm: vmscan: Correctly check if reclaimer should schedule during shrink_slab

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 23 May 2011 10:53:55 +0100
Mel Gorman <mgorman@suse.de> wrote:

> It has been reported on some laptops that kswapd is consuming large
> amounts of CPU and not being scheduled when SLUB is enabled during
> large amounts of file copying. It is expected that this is due to
> kswapd missing every cond_resched() point because;
> 
> shrink_page_list() calls cond_resched() 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().
> 
> 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 it 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 calling cond_resched().
> 
> shrink_slab only calls cond_resched() if we are reclaiming slab
> 	pages. If there are a large number of direct reclaimers, the
> 	shrinker_rwsem can be contended and prevent kswapd calling
> 	cond_resched().
> 
> This patch modifies the shrink_slab() case. If the semaphore is
> contended, the caller will still check cond_resched(). After each
> successful call into a shrinker, the check for cond_resched() remains
> in case one shrinker is particularly slow.

So CONFIG_PREEMPT=y kernels don't exhibit this problem?

I'm still unconvinced that we know what's going on here.  What's kswapd
*doing* with all those cycles?  And if kswapd is now scheduling away,
who is doing that work instead?  Direct reclaim?

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