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

David Hildenbrand
2021-05-28T09:08:32Z

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().

--
Thanks,

David / dhildenb