From: Johannes Weiner <hannes@cmpxchg.org>
To: Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: kswapd endless loop for compaction
Date: Wed, 21 Nov 2012 17:01:26 -0500 [thread overview]
Message-ID: <20121121220126.GA2301@cmpxchg.org> (raw)
In-Reply-To: <20121120190440.GA24381@cmpxchg.org>
Just to be clear, this is not fixed by Dave's patch to NR_FREE_PAGES
accounting.
I can still get 3.7-rc5 + Dave's fix to drop into an endless loop in
kswapd within a couple of minutes on my test box.
As described below, the bug comes from contradicting conditions in
balance_pgdat(), not an accounting problem.
On Tue, Nov 20, 2012 at 02:04:41PM -0500, Johannes Weiner wrote:
> Hi guys,
>
> while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
> a busy spin state without doing reclaim. printk-style debugging told
> me that this happens when the distance between a zone's high watermark
> and its low watermark is less than two huge pages (DMA zone).
>
> 1. The first loop in balance_pgdat() over the zones finds all zones to
> be above their high watermark and only does goto out (all_zones_ok).
>
> 2. pgdat_balanced() at the out: label also just checks the high
> watermark, so the node is considered balanced and the order is not
> reduced.
>
> 3. In the `if (order)' block after it, compaction_suitable() checks if
> the zone's low watermark + twice the huge page size is okay, which
> it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
> it go back to loop_again:.
>
> This will go on until somebody else allocates and breaches the high
> watermark and then hopefully goes on to reclaim the zone above low
> watermark + 2 * THP.
>
> I'm not really sure what the correct solution is. Should we modify
> the zone_watermark_ok() checks in balance_pgdat() to take into account
> the higher watermark requirements for reclaim on behalf of compaction?
> Change the check in compaction_suitable() / not use it directly?
>
> Thanks,
> Johannes
>
> --
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next prev parent reply other threads:[~2012-11-22 23:03 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-20 19:04 kswapd endless loop for compaction Johannes Weiner
2012-11-21 22:01 ` Johannes Weiner [this message]
2012-11-22 14:40 ` Jaegeuk Hanse
2012-11-23 8:50 ` Jaegeuk Hanse
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