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

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Date:   Tue, 23 May 2017 17:44:23 -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



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.

> 
>> I didn't see where any part of commit 3af229f2071f would touch the 'node_possible_map'
>> which is needed by 'numa.c' and 'workqueue.c'.  The nodemask created and updated by
>> 'mem_cgroup_may_update_nodemask()' does not appear to be the same mask.
> 
> Are you sure you're looking at 3af229f2071f? It only adds one line of code; the reduction of node_possible_map.
> 

-- 
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