linux-kernel - Re: [PATCH] mm, memory_hotplug: update pcp lists everytime onlining a memory block

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Message-ID: <ecf75fb9-10e6-5959-7e74-f1219b231305@redhat.com>
Date:   Mon, 3 Aug 2020 16:00:08 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Charan Teja Kalla <charante@...eaurora.org>,
        akpm@...ux-foundation.org, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, vinmenon@...eaurora.org
Subject: Re: [PATCH] mm, memory_hotplug: update pcp lists everytime onlining a
 memory block

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