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

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Date:   Fri, 28 May 2021 11:52:53 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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 11:49, 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