Re: [Xen-devel] [PATCH v7 01/14] x86: add socket_to_cpumask

[Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>]

On Tue, May 19, 2015 at 07:52:04AM +0100, Jan Beulich wrote:
> >>> On 19.05.15 at 08:47, <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> > On Tue, May 19, 2015 at 07:28:49AM +0100, Jan Beulich wrote:
> >> >>> On 19.05.15 at 08:12, <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> >> > On Mon, May 18, 2015 at 02:21:40PM +0100, Jan Beulich wrote:
> >> >> >>> On 08.05.15 at 10:56, <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> >> >> > @@ -112,6 +115,8 @@ static int __devinit MP_processor_info_x(struct 
> > mpc_config_processor *m,
> >> >> >  {
> >> >> >       int ver, apicid, cpu = 0;
> >> >> >       
> >> >> > +     total_cpus++;
> >> >> > +
> >> >> >       if (!(m->mpc_cpuflag & CPU_ENABLED)) {
> >> >> >               if (!hotplug)
> >> >> >                       ++disabled_cpus;
> >> >> 
> >> >> Is there a reason you can't use disabled_cpus and avoid adding yet
> >> >> another variable?
> >> > 
> >> > The problem is not with disabled_cpus but with num_processors, which
> >> > does not keep the original detected cpus in current code.
> >> > Hence 'total_cpus = disabled_cpus + num_processors' may not be correct
> >> > in some cases.
> >> 
> >> Please be more specific about when this is a problem (I do note that
> >> I'm aware that the equation will not always hold, but during my
> >> inspection while reviewing your change I didn't see that this would
> >> ever become problematic).
> > 
> > What I really need is the original cpu count enumerated from MADT. If
> > not introduce total_cpus then the only way getting it AFAICS is
> > 'disabled_cpus + num_processors'.
> > 
> > The problem is that MP_processor_info_x() have some earlier returns
> > before increasing num_processors. In those cases, the cpu detected will
> > neither counted to disabled_cpus nor num_processors, which means
> > 'disabled_cpus + num_processors' is potentially small than what I need.
> 
> As said - I understand this. But you still fail to explain under what
> (realistic, i.e. other than someone bogusly setting NR_CPUS=1)
> conditions this ends up being a problem.

As we calculate nr_sockets with:

nr_sockets = total_cpus / _cpus_per_socket__

If the calculated total_cpus is smaller than the actual cpu count on the
hardware, then the nr_sockets is also potentially smaller than the
actual socket count on the hardware. This is not the expectation.

Chao

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