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

[Mel Gorman]

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.

-- 
Mel Gorman
SUSE Labs