linux-kernel - Re: kswapd craziness in 3.7

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Date:	Wed, 19 Dec 2012 23:24:00 +0100
From:	Zlatko Calusic <zlatko.calusic@...on.hr>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: kswapd craziness in 3.7

On 11.12.2012 01:19, Zlatko Calusic wrote:
>> On 10.12.2012 20:13, Linus Torvalds wrote:
>>>
>>> It's worth giving this as much testing as is at all possible, but at
>>> the same time I really don't think I can delay 3.7 any more without
>>> messing up the holiday season too much. So unless something obvious
>>> pops up, I will do the release tonight. So testing will be minimal -
>>> but it's not like we haven't gone back-and-forth on this several times
>>> already, and we revert to *mostly* the same old state as 3.6 anyway,
>>> so it should be fairly safe.
>>>
>
> So, here's what I found. In short: close, but no cigar!
>
> Kswapd is certainly no more CPU pig, and memory seems to be utilized
> properly (the kernel still likes to keep 400MB free, somebody else can
> confirm if that's to be expected on a 4GB THP-enabled machine). So it
> looks very decent, and much better than anything I run in last 10 days,
> barring !THP kernel.
>
> What remains a mystery is that kswapd occassionaly still likes to get
> stuck in a D state, only now it recovers faster than before (sometimes
> in a matter of seconds, but sometimes it takes a few minutes). Now, I
> admit it's a small, maybe even cosmetic issue. But, it could also be a
> warning sign of a bigger problem that will reveal itself on a more
> loaded machine.
>

Ha, I nailed it!

The cigar aka the explanation together with a patch will follow shortly 
in a separate topic.

It's a genuine bug that has been with us for a long long time.
-- 
Zlatko
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