Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 27 Nov 2012 12:58:18 -0800 | Subject | Re: kswapd craziness in 3.7 |
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Note that in the meantime, I've also applied (through Andrew) the patch that reverts commit c654345924f7 (see commit 82b212f40059 'Revert "mm: remove __GFP_NO_KSWAPD"').
I wonder if that revert may be bogus, and a result of this same issue. Maybe that revert should be reverted, and replaced with your patch?
Mel? Zdenek? What's the status here?
Linus
On Tue, Nov 27, 2012 at 12:48 PM, Johannes Weiner <hannes@cmpxchg.org> wrote: > Hi everyone, > > I hope I included everybody that participated in the various threads > on kswapd getting stuck / exhibiting high CPU usage. We were looking > at at least three root causes as far as I can see, so it's not really > clear who observed which problem. Please correct me if the > reported-by, tested-by, bisected-by tags are incomplete. > > One problem was, as it seems, overly aggressive reclaim due to scaling > up reclaim goals based on compaction failures. This one was reverted > in 9671009 mm: revert "mm: vmscan: scale number of pages reclaimed by > reclaim/compaction based on failures". > > Another one was an accounting problem where a freed higher order page > was underreported, and so kswapd had trouble restoring watermarks. > This one was fixed in ef6c5be fix incorrect NR_FREE_PAGES accounting > (appears like memory leak). > > The third one is a problem with small zones, like the DMA zone, where > the high watermark is lower than the low watermark plus compaction gap > (2 * allocation size). The zonelist reclaim in kswapd would do > nothing because all high watermarks are met, but the compaction logic > would find its own requirements unmet and loop over the zones again. > Indefinitely, until some third party would free enough memory to help > meet the higher compaction watermark. The problematic code has been > there since the 3.4 merge window for non-THP higher order allocations > but has been more prominent since the 3.7 merge window, where kswapd > is also woken up for the much more common THP allocations. > > The following patch should fix the third issue by making both reclaim > and compaction code in kswapd use the same predicate to determine > whether a zone is balanced or not. > > Hopefully, the sum of all three fixes should tame kswapd enough for > 3.7. > > Johannes >
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