Subject: Re: [PATCH V2 2/2] powerpc:numa Do not allocate bootmem memory for non existing nodes

From: Raghavendra K T <[email-masked]>
Date: Tue, 22 Sep 2015 16:18:18 +0530

On 09/22/2015 10:59 AM, Michael Ellerman wrote:
> On Tue, 2015-09-15 at 07:38 +0530, Raghavendra K T wrote:
>>
>> ... nothing
>
> Sure this patch looks obvious, but please give me a changelog that proves
> you've thought about it thoroughly.
>
> For example is it OK to use for_each_node() at this point in boot? Is there any
> historical reason why we did it with a hard coded loop? If so what has changed.
> What systems have you tested on? etc. etc.
>
> cheers

Changelog:

With the setup_nr_nodes(), we have already initialized
node_possible_map. So it is safe to use for_each_node here.

There are many places in the kernel that use hardcoded 'for' loop with
nr_node_ids, because all other architectures have numa nodes populated
serially. That should be reason we had maintained same for powerpc.

But since on power we have sparse numa node ids possible, we
unnecessarily allocate memory for non existent numa nodes.

For e.g., on a system with 0,1,16,17 as numa nodes nr_node_ids=18
and we allocate memory for nodes 2-14.

The patch is boot tested on a 4 node tuleta [ confirming with printks ].
that it works as expected.

>
>> Signed-off-by: Raghavendra K T <[email-masked]>
>> ---
>>   arch/powerpc/mm/numa.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>> index 8b9502a..8d8a541 100644
>> --- a/arch/powerpc/mm/numa.c
>> +++ b/arch/powerpc/mm/numa.c
>> @@ -80,7 +80,7 @@ static void __init setup_node_to_cpumask_map(void)
>>   		setup_nr_node_ids();
>>
>>   	/* allocate the map */
>> -	for (node = 0; node < nr_node_ids; node++)
>> +	for_each_node(node)
>>   		alloc_bootmem_cpumask_var(&node_to_cpumask_map[node]);
>>
>>   	/* cpumask_of_node() will now work */
>
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