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

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Message-Id: <d6be479a-a61c-a79e-1249-c7190123073c@linux.vnet.ibm.com>
Date:   Wed, 24 May 2017 18:41:34 -0500
From:   Michael Bringmann <mwb@...ux.vnet.ibm.com>
To:     Reza Arbab <arbab@...ux.vnet.ibm.com>
Cc:     Balbir Singh <bsingharora@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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.ib>,
        Shailendra Singh <shailendras@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

I will get a log based on the latest 4.12 kernel to show what happens
one way or the other, with this patch removed.

On 05/24/2017 09:36 AM, Reza Arbab wrote:
> On Tue, May 23, 2017 at 05:44:23PM -0500, Michael Bringmann wrote:
>> On 05/23/2017 04:49 PM, Reza Arbab wrote:
>>> On Tue, May 23, 2017 at 03:05:08PM -0500, Michael Bringmann wrote:
>>>> On 05/23/2017 10:52 AM, Reza Arbab wrote:
>>>>> On Tue, May 23, 2017 at 10:15:44AM -0500, Michael Bringmann wrote:
>>>>>> +static void setup_nodes(void)
>>>>>> +{
>>>>>> +    int i, l = 32 /* MAX_NUMNODES */;
>>>>>> +
>>>>>> +    for (i = 0; i < l; i++) {
>>>>>> +        if (!node_possible(i)) {
>>>>>> +            setup_node_data(i, 0, 0);
>>>>>> +            node_set(i, node_possible_map);
>>>>>> +        }
>>>>>> +    }
>>>>>> +}
>>>>>
>>>>> This seems to be a workaround for 3af229f2071f ("powerpc/numa: Reset node_possible_map to only node_online_map").
>>>>
>>>> They may be related, but that commit is not a replacement.  The above patch ensures that
>>>> there are enough of the nodes initialized at startup to allow for memory hot-add into a
>>>> node that was not used at boot.  (See 'setup_node_data' function in 'numa.c'.)  That and
>>>> recording that the node was initialized.
>>>
>>> Is it really necessary to preinitialize these empty nodes using setup_node_data()? When you do memory hotadd into a node that was not used at boot, the node data already gets set up by
>>>
>>> add_memory
>>>  add_memory_resource
>>>    hotadd_new_pgdat
>>>      arch_alloc_nodedata <-- allocs the pg_data_t
>>>      ...
>>>      free_area_init_node <-- sets NODE_DATA(nid)->node_id, etc.
>>>
>>> Removing setup_node_data() from that loop leaves only the call to node_set(). If 3af229f2071f (which reduces node_possible_map) was reverted, you wouldn't need to do that either.
>>
>> With or without 3af229f2071f, we would still need to add something, somewhere to add new
>> bits to the 'node_possible_map'.  That is not being done.
> 
> Without 3af229f2071f, those bits would already BE set in node_possible_map. You wouldn't have to do anything.
> 

-- 
Michael W. Bringmann
Linux Technology Center
IBM Corporation
Tie-Line  363-5196
External: (512) 286-5196
Cell:       (512) 466-0650
mwb@...ux.vnet.ibm.com