Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs

[David Hildenbrand]

On 28.05.21 11:49, Mel Gorman wrote:
> On Fri, May 28, 2021 at 11:08:01AM +0200, David Hildenbrand wrote:
> > On 28.05.21 11:03, David Hildenbrand wrote:
> > > On 28.05.21 10:55, Mel Gorman wrote:
> > > > On Thu, May 27, 2021 at 12:36:21PM -0700, Dave Hansen wrote:
> > > > > Hi Mel,
> > > > >
> > > > > Feng Tang tossed these on a "Cascade Lake" system with 96 threads and
> > > > > ~512G of persistent memory and 128G of DRAM.  The PMEM is in "volatile
> > > > > use" mode and being managed via the buddy just like the normal RAM.
> > > > >
> > > > > The PMEM zones are big ones:
> > > > >
> > > > >            present  65011712 = 248 G
> > > > >            high       134595 = 525 M
> > > > >
> > > > > The PMEM nodes, of course, don't have any CPUs in them.
> > > > >
> > > > > With your series, the pcp->high value per-cpu is 69584 pages or about
> > > > > 270MB per CPU.  Scaled up by the 96 CPU threads, that's ~26GB of
> > > > > worst-case memory in the pcps per zone, or roughly 10% of the size of
> > > > > the zone.
> > >
> > > When I read about having such big amounts of free memory theoretically
> > > stuck in PCP lists, I guess we really want to start draining the PCP in
> > > alloc_contig_range(), just as we do with memory hotunplug when offlining.
> >
> > Correction: we already drain the pcp, we just don't temporarily disable it,
> > so a race as described in offline_pages() could apply:
> >
> > "Disable pcplists so that page isolation cannot race with freeing
> >   in a way that pages from isolated pageblock are left on pcplists."
> >
> > Guess we'd then want to move the draining before start_isolate_page_range()
> > in alloc_contig_range().
>
> Or instead of draining, validate the PFN range in alloc_contig_range
> is within the same zone and if so, call zone_pcp_disable() before
> start_isolate_page_range and enable after __alloc_contig_migrate_range.

We require the caller to only pass a range within a single zone, so that 
should be fine.

The only ugly thing about zone_pcp_disable() is 
mutex_lock(&pcp_batch_high_lock) which would serialize all 
alloc_contig_range() and even with offline_pages().

--
Thanks,

David / dhildenb