Re: kswapd craziness in 3.7

[Linus Torvalds <torvalds@linux-foundation.org>]

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
>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>