linux-kernel - Re: kswapd craziness in 3.7

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Date:	Tue, 27 Nov 2012 12:58:18 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	George Spelvin <linux@...izon.com>,
	Johannes Hirte <johannes.hirte@....tu-ilmenau.de>,
	Tomas Racek <tracek@...hat.com>, Jan Kara <jack@...e.cz>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Josh Boyer <jwboyer@...il.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Jiri Slaby <jslaby@...e.cz>,
	Thorsten Leemhuis <fedora@...mhuis.info>,
	Zdenek Kabelac <zkabelac@...hat.com>,
	Bruno Wolff III <bruno@...ff.to>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kswapd craziness in 3.7

Note that in the meantime, I've also applied (through Andrew) the
patch that reverts commit c654345924f7 (see commit 82b212f40059
'Revert "mm: remove __GFP_NO_KSWAPD"').

I wonder if that revert may be bogus, and a result of this same issue.
Maybe that revert should be reverted, and replaced with your patch?

Mel? Zdenek? What's the status here?

                 Linus

On Tue, Nov 27, 2012 at 12:48 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> Hi everyone,
>
> I hope I included everybody that participated in the various threads
> on kswapd getting stuck / exhibiting high CPU usage.  We were looking
> at at least three root causes as far as I can see, so it's not really
> clear who observed which problem.  Please correct me if the
> reported-by, tested-by, bisected-by tags are incomplete.
>
> One problem was, as it seems, overly aggressive reclaim due to scaling
> up reclaim goals based on compaction failures.  This one was reverted
> in 9671009 mm: revert "mm: vmscan: scale number of pages reclaimed by
> reclaim/compaction based on failures".
>
> Another one was an accounting problem where a freed higher order page
> was underreported, and so kswapd had trouble restoring watermarks.
> This one was fixed in ef6c5be fix incorrect NR_FREE_PAGES accounting
> (appears like memory leak).
>
> The third one is a problem with small zones, like the DMA zone, where
> the high watermark is lower than the low watermark plus compaction gap
> (2 * allocation size).  The zonelist reclaim in kswapd would do
> nothing because all high watermarks are met, but the compaction logic
> would find its own requirements unmet and loop over the zones again.
> Indefinitely, until some third party would free enough memory to help
> meet the higher compaction watermark.  The problematic code has been
> there since the 3.4 merge window for non-THP higher order allocations
> but has been more prominent since the 3.7 merge window, where kswapd
> is also woken up for the much more common THP allocations.
>
> The following patch should fix the third issue by making both reclaim
> and compaction code in kswapd use the same predicate to determine
> whether a zone is balanced or not.
>
> Hopefully, the sum of all three fixes should tame kswapd enough for
> 3.7.
>
> Johannes
>
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