On Fri, 19 Jul 2013 16:55:23 -0400 Johannes Weiner [off-list ref] wrote:

When the page allocator fails to get a page from all zones in its
given zonelist, it wakes up the per-node kswapds for all zones that
are at their low watermark.

However, with a system under load and the free page counters being
per-cpu approximations, the observed counter value in a zone can
fluctuate enough that the allocation fails but the kswapd wakeup is
also skipped while the zone is still really close to the low
watermark.

When one node misses a wakeup like this, it won't be aged before all
the other node's zones are down to their low watermarks again.  And
skipping a full aging cycle is an obvious fairness problem.

Kswapd runs until the high watermarks are restored, so it should also
be woken when the high watermarks are not met.  This ages nodes more
equally and creates a safety margin for the page counter fluctuation.

Well yes, but what guarantee is there that the per-cpu counter error
problem is reliably fixed?  AFAICT this patch "fixes" it because the
gap between the low and high watermarks happens to be larger than the
per-cpu counter fluctuation, yes?  If so, there are surely all sorts of
situations where it will break again.

To fix this reliably, we should be looking at constraining counter
batch sizes or performing a counter summation to get the more accurate
estimate?

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