linux-ext4 - Re: [PATCH 3/3] mm: slub: Default slub_max

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Date:	Fri, 13 May 2011 07:58:17 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	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 Fri, May 13, 2011 at 7:15 AM, Johannes Weiner <hannes@...xchg.org> 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?

Yes. Good catch.

>
> 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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-- 
Kind regards,
Minchan Kim
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