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

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Message-ID: <1305237882.2575.100.camel@mulgrave.site>
Date:	Thu, 12 May 2011 17:04:41 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	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>,
	Johannes Weiner <hannes@...xchg.org>,
	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, 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 ...

By the way, I stripped off all the patches, so this is a plain old
2.6.38.6 kernel with the default FC15 config.

James

---

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0665520..1bdea7d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2255,6 +2255,8 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
 	if (remaining)
 		return true;

+	return false;
+
 	/* Check the watermark levels */
 	for (i = 0; i < pgdat->nr_zones; i++) {
 		struct zone *zone = pgdat->node_zones + i;

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