Re: [RFC 2/2] mm: alloc/free depth based PCP high auto-tuning

[Mel Gorman]

On Fri, Jul 21, 2023 at 03:28:43PM +0800, Huang, Ying wrote:
> Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Wed, Jul 19, 2023 at 01:59:00PM +0800, Huang, Ying wrote:
> >> > The big remaaining corner case to watch out for is where the sum
> >> > of the boosted pcp->high exceeds the low watermark.  If that should ever
> >> > happen then potentially a premature OOM happens because the watermarks
> >> > are fine so no reclaim is active but no pages are available. It may even
> >> > be the case that the sum of pcp->high should not exceed *min* as that
> >> > corner case means that processes may prematurely enter direct reclaim
> >> > (not as bad as OOM but still bad).
> >> 
> >> Sorry, I don't understand this.  When pages are moved from buddy to PCP,
> >> zone NR_FREE_PAGES will be decreased in rmqueue_bulk().  That is, pages
> >> in PCP will be counted as used instead of free.  And, in
> >> zone_watermark_ok*() and zone_watermark_fast(), zone NR_FREE_PAGES is
> >> used to check watermark.  So, if my understanding were correct, if the
> >> number of pages in PCP is larger than low/min watermark, we can still
> >> trigger reclaim.  Whether is my understanding correct?
> >> 
> >
> > You're right, I didn't check the timing of the accounting and all that
> > occurred to me was "the timing of when watermarks trigger kswapd or
> > direct reclaim may change as a result of PCP adaptive resizing". Even
> > though I got the timing wrong, the shape of the problem just changes.
> > I suspect that excessively large PCP high relative to the watermarks may
> > mean that reclaim happens prematurely if too many pages are pinned by PCP
> > pages as the zone free pages approaches the watermark.
> 
> Yes.  I think so too.  In addition to reclaim, falling back to remote
> NUMA node may happen prematurely too.
> 

Yes, with the added bonus that this is relatively easy to detect from
the NUMA miss stats. I say "relative" because in a lot of cases, it'll be
difficult to distinguish from the noise. Hence, it's better to be explicit in
the change log that the potential problem is known and has been considered.
That way, if bisect points the finger at adaptive resizing, there will be
some notes on how to investigate the bug.

> > While disabling the adaptive resizing during reclaim will limit the
> > worst of the problem, it may still be the case that kswapd is woken
> > early simply because there are enough CPUs pinning pages in PCP
> > lists. Similarly, depending on the size of pcp->high and the gap
> > between the watermarks, it's possible for direct reclaim to happen
> > prematurely. I could still be wrong because I'm not thinking the
> > problem through fully, examining the code or thinking about the
> > implementation. It's simply worth keeping in mind the impact elevated
> > PCP high values has on the timing of watermarks failing. If it's
> > complex enough, it may be necessary to have a separate patch dealing
> > with the impact of elevated pcp->high on watermarks.
> 
> Sure.  I will keep this in mind.  We may need to check zone watermark
> when tuning pcp->high and free some pages from PCP before falling back
> to other node or reclaiming.
> 

That would certainly be one option, a cap on adaptive resizing as memory
gets lower. It's not perfect but ideally the worst-case behaviour would be
that PCP adaptive sizing returns to existing behaviour when memory usage
is persistently high and near watermarks within a zone.

-- 
Mel Gorman
SUSE Labs