Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

[Michal Hocko]

On Mon 16-05-16 09:31:44, Vlastimil Babka wrote:
> On 05/13/2016 04:15 PM, Michal Hocko wrote:
> > On Tue 10-05-16 09:36:02, Vlastimil Babka wrote:
> > > 
> > > - should_compact_retry() is only called when should_reclaim_retry() returns
> > >    false. This means that compaction priority cannot get increased as long
> > >    as reclaim makes sufficient progress. Theoretically, reclaim should stop
> > >    retrying for high-order allocations as long as the high-order page doesn't
> > >    exist but due to races, this may result in spurious retries when the
> > >    high-order page momentarily does exist.
> > 
> > This is intentional behavior and I would like to preserve it if it is
> > possible. For higher order pages should_reclaim_retry retries as long
> > as there are some eligible high order pages present which are just hidden
> > by the watermark check. So this is mostly to get us over watermarks to
> > start carrying about fragmentation. If we race there then nothing really
> > terrible should happen and we should eventually converge to a terminal
> > state.
> > 
> > Does this make sense to you?
> 
> Yeah it should work, my only worry was that this may get subtly wrong (as
> experience shows us) and due to e.g. slightly different watermark checks
> and/or a corner-case zone such as ZONE_DMA, should_reclaim_retry() would
> keep returning true, even if reclaim couldn't/wouldn't help anything. Then
> compaction would be needlessly kept at ineffective priority.

watermark check for ZONE_DMA should always fail because it fails even
when is completely free to the lowmem reserves. I had a subtle bug in
the original code to check highzone_idx rather than classzone_idx but
that should the fix has been posted recently:
http://lkml.kernel.org/r/1463051677-29418-2-git-send-email-mhocko@xxxxxxxxxx

> Also my understanding of the initial compaction priorities is to lower the
> latency if fragmentation is just light and there's enough memory. Once we
> start struggling, I don't see much point in not switching to the full
> compaction priority quickly.

That is true but why to compact when there are high order pages and they
are just hidden by the watermark check.
-- 
Michal Hocko
SUSE Labs