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

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Message-ID: <e73d39cc-df2f-5000-0212-6073955fb291@redhat.com>
Date:   Fri, 28 May 2021 11:08:01 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, "Tang, Feng" <feng.tang@...el.com>
Subject: Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active
 CPUs

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