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

[Michael Holzheu <holzheu@linux.vnet.ibm.com>]

On Wed, 5 Aug 2015 13:51:21 -0700
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> 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...

No problem, in the meantime I found another solution for my problem.

But thanks for trying :-)
Michael