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Date: Sun, 25 Nov 2012 18:31:01 -0500 From: Rik van Riel <riel@...hat.com> To: Johannes Weiner <hannes@...xchg.org> CC: Johannes Hirte <johannes.hirte@....tu-ilmenau.de>, akpm@...ux-foundation.org, mgorman@...e.de, Valdis.Kletnieks@...edu, jirislaby@...il.com, jslaby@...e.cz, zkabelac@...hat.com, mm-commits@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org Subject: Re: [PATCH] mm,vmscan: free pages if compaction_suitable tells us to 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@....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. -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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