linux-kernel - Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

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Message-Id: <20170526143147.z4lmtrs7vowucbkf@arbab-laptop.localdomain>
Date:   Fri, 26 May 2017 09:31:47 -0500
From:   Reza Arbab <arbab@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Balbir Singh <bsingharora@...il.com>,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Bharata B Rao <bharata@...ux.vnet.ibm.com>,
        Shailendra Singh <shailendras@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev@...ts.ozlabs.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

On Fri, May 26, 2017 at 01:46:58PM +1000, Michael Ellerman wrote:
>Reza Arbab <arbab@...ux.vnet.ibm.com> writes:
>
>> On Thu, May 25, 2017 at 04:19:53PM +1000, Michael Ellerman wrote:
>>>The commit message for 3af229f2071f says:
>>>
>>>    In practice, we never see a system with 256 NUMA nodes, and in fact, we
>>>    do not support node hotplug on power in the first place, so the nodes
>>>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>    that are online when we come up are the nodes that will be present for
>>>    the lifetime of this kernel.
>>>
>>>Is that no longer true?
>>
>> I don't know what the reasoning behind that statement was at the time,
>> but as far as I can tell, the only thing missing for node hotplug now is
>> Balbir's patchset [1]. He fixes the resource issue which motivated
>> 3af229f2071f and reverts it.
>>
>> With that set, I can instantiate a new numa node just by doing
>> add_memory(nid, ...) where nid doesn't currently exist.
>
>But does that actually happen on any real system?

I don't know if anything currently tries to do this. My interest in 
having this working is so that in the future, our coherent gpu memory 
could be added as a distinct node by the device driver.

-- 
Reza Arbab