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

[Huang, Ying]

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.

> 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.

--
Best Regards,
Huang, Ying