Re: kswapd endless loop for compaction

[Johannes Weiner]

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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