Re: [RFC PATCH] drivers/base: use cpu->node_id for from_nid

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jul 30, 2015 at 08:35:51PM +0200, Michael Holzheu wrote:
> Hello Greg,
> 
> Is it possible to use "from_nid = cpu->node_id"?
> 
> Background:
> 
> I am currently working on (fake) NUMA support for s390. At startup, for
> "deconfigured" CPUs, we don't know to which nodes the CPUs belong. Therefore
> we always return node 0 for cpu_to_node().
> 
> For each present CPU the register_cpu() function is called which sets an
> initial NUMA node via cpu_to_node(), which is then first node 0 for
> "deconfigured" CPUs on s390.
> 
> After we "configure" a CPU we know to which node it belongs. Then when setting
> a CPU online, the following is done in cpu_subsys_online():
> 
>      from_nid = cpu_to_node(cpuid); -> we return node x
>      cpu_up(cpuid);
>      to_nid = cpu_to_node(cpuid);   -> we return node x
>      if (from_nid != to_nid)        -> x != x -> false
>            change_cpu_under_node(cpu, from_nid, to_nid);
> 
> The result is that each CPU that was deconfigured at boot time stays in
> node 0 because cpu_to_node() returns the same node before and after
> setting the CPU online.
> 
> Using "cpu->node_id" for "from_nid" instead of calling cpu_to_node()
> would help in our case.
> 
> Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
> ---
>  drivers/base/cpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index f160ea4..2dd889c 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -47,7 +47,7 @@ static int __ref cpu_subsys_online(struct device *dev)
>  	int from_nid, to_nid;
>  	int ret;
>  
> -	from_nid = cpu_to_node(cpuid);
> +	from_nid = cpu->node_id;
>  	if (from_nid == NUMA_NO_NODE)
>  		return -ENODEV;
>  

I really have no idea the answer to any of these questions, sorry...