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Subject: Re: [PATCH] mm, memory_hotplug: update pcp lists everytime onlining a
 memory block
To:     Charan Teja Kalla <charante@codeaurora.org>,
        akpm@linux-foundation.org, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, vinmenon@codeaurora.org
References: <1596372896-15336-1-git-send-email-charante@codeaurora.org>
 <e15bc2ca-6bc5-158c-41df-af13aba75e54@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
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Date:   Mon, 3 Aug 2020 16:00:08 +0200
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On 03.08.20 15:28, Charan Teja Kalla wrote:
> Thanks David for the comments.
> 
> On 8/3/2020 1:35 PM, David Hildenbrand wrote:
>> On 02.08.20 14:54, Charan Teja Reddy wrote:
>>> When onlining a first memory block in a zone, pcp lists are not updated
>>> thus pcp struct will have the default setting of ->high = 0,->batch = 1.
>>> This means till the second memory block in a zone(if it have) is onlined
>>> the pcp lists of this zone will not contain any pages because pcp's
>>> ->count is always greater than ->high thus free_pcppages_bulk() is
>>> called to free batch size(=1) pages every time system wants to add a
>>> page to the pcp list through free_unref_page(). To put this in a word,
>>> system is not using benefits offered by the pcp lists when there is a
>>> single onlineable memory block in a zone. Correct this by always
>>> updating the pcp lists when memory block is onlined.
>>
>> I guess such setups are rare ... but I can imagine it being the case
>> with virtio-mem in the future ... not 100% if this performance
>> optimization is really relevant in practice ... how did you identify this?
> 
> Even the Snapdragon hardware that I had tested on contain multiple
> onlineable memory blocks. But we have the use case in which we online
> single memory block and once it is filled then online the next block. In
> the step where single block is onlined, we observed the below pageset

Out of interest, why? Is it to optimize energy consumption?

-- 
Thanks,

David / dhildenb