Messages in this thread | | | Date | Sun, 25 Nov 2012 18:31:01 -0500 | From | Rik van Riel <> | Subject | Re: [PATCH] mm,vmscan: free pages if compaction_suitable tells us to |
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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@fem.tu-ilmenau.de> 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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