On 11/25/2012 05:44 PM, Johannes Weiner wrote:
On Sun, Nov 25, 2012 at 01:29:50PM -0500, Rik van Riel wrote:
On Sun, 25 Nov 2012 17:57:28 +0100
Johannes Hirte <johannes.hirte@xxxxxxxxxxxxxxxxx> wrote:
With kernel 3.7-rc6 I've still problems with kswapd0 on my laptop
And this is most of the time. I've only observed this behavior on the
laptop. Other systems don't show this.
This suggests it may have something to do with small memory zones,
where we end up with the "funny" situation that the high watermark
(+ balance gap) for a particular zone is less than the low watermark
+ 2<<order pages, which is the number of free pages required to keep
compaction_suitable happy.
Could you try this patch?
It's not quite enough because it's not reaching the conditions you
changed, see analysis in https://lkml.org/lkml/2012/11/20/567
You are right, I forgot the preliminary loop in balance_pgdat().
But even fixing it up (by adding the compaction_suitable() test in
this preliminary scan over the zones and setting end_zone accordingly)
is not enough because no actual reclaim happens at priority 12 in a
small zone. So the number of free pages is not actually changing and
the compaction_suitable() checks keep the loop going.
Indeed, it is a hairy situation. I tried to come up with a simple
patch, but apparently that is not enough...
The problem is fairly easy to reproduce, by the way. Just boot with
mem=800M to have a relatively small lowmem reserve in the DMA zone.
Fill it up with page cache, then allocate transparent huge pages.
With your patch and my fix to the preliminary zone loop, there won't
be any hung task warnings anymore because kswapd actually calls
shrink_slab() and there is a rescheduling point in there, but it still
loops forever.
It also seems a bit aggressive to try to balance a small zone like DMA
for a huge page when it's not a GFP_DMA allocation, but none of these
checks actually take the classzone into account. Do we have any
agreement over what this whole thing is supposed to be doing?
It is supposed to free memory, in order to:
1) allow allocations to succeed, and
2) balance memory pressure between zones
I think the compaction_suitable check in the final loop
over the zones is backwards.
We need to loop back to the start if compaction_suitable
returns COMPACT_SKIPPED for _every_ zone in the pgdat.
Does that sound reasonable?
I'll whip up a patch.
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