Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0

[Mel Gorman]

On Fri, May 13, 2011 at 12:15:06AM +0200, Johannes Weiner wrote:
> On Thu, May 12, 2011 at 05:04:41PM -0500, James Bottomley wrote:
> > On Thu, 2011-05-12 at 15:04 -0500, James Bottomley wrote:
> > > Confirmed, I'm afraid ... I can trigger the problem with all three
> > > patches under PREEMPT.  It's not a hang this time, it's just kswapd
> > > taking 100% system time on 1 CPU and it won't calm down after I unload
> > > the system.
> > 
> > Just on a "if you don't know what's wrong poke about and see" basis, I
> > sliced out all the complex logic in sleeping_prematurely() and, as far
> > as I can tell, it cures the problem behaviour.  I've loaded up the
> > system, and taken the tar load generator through three runs without
> > producing a spinning kswapd (this is PREEMPT).  I'll try with a
> > non-PREEMPT kernel shortly.
> > 
> > What this seems to say is that there's a problem with the complex logic
> > in sleeping_prematurely().  I'm pretty sure hacking up
> > sleeping_prematurely() just to dump all the calculations is the wrong
> > thing to do, but perhaps someone can see what the right thing is ...
> 
> I think I see the problem: the boolean logic of sleeping_prematurely()
> is odd.  If it returns true, kswapd will keep running.  So if
> pgdat_balanced() returns true, kswapd should go to sleep.
> 
> This?
> 

You're right.

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2b701e0..092d773 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2261,7 +2261,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>  	 * must be balanced
>  	 */
>  	if (order)
> -		return pgdat_balanced(pgdat, balanced, classzone_idx);
> +		return !pgdat_balanced(pgdat, balanced, classzone_idx);
>  	else
>  		return !all_zones_ok;
>  }

-- 
Mel Gorman
SUSE Labs
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