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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2011-05-23 at 13:03 -0700, Andrew Morton wrote:
> 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?

Yes, they do.  They just don't hang on my sandybridge system in the same
way than non-PREEMPT kernels do.  I'm still sure it's got something to
do with rescheduling kswapd onto a different CPU ...

> 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?

Still in the dark about this one, too.

James


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