linux-kernel - Re: [PATCH 3/3] mm: slub: Default slub_max

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Date:	Fri, 13 May 2011 00:15:06 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Colin King <colin.king@...onical.com>,
	Raghavendra D Prabhu <raghu.prabhu13@...il.com>,
	Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>,
	Rik van Riel <riel@...hat.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0

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?

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;
 }
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