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

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

On Tue, May 19, 2015 at 08:31:53AM +0100, Jan Beulich wrote:
> >>> On 19.05.15 at 09:10, <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
> > 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.
> 
> Sure - but you still don't say what is going to go wrong. Remember,
> when I asked you to change to the total count I gave an explicit
> example: Use of "nosmp" would have yielded a zero nr_sockets in
> the earlier code. Yet with the sum of num_processors and
> disabled_cpus this can't happen anymore afaict.

"nosmp" only has side effect on max_cpus and nr_cpu_ids, but they are
never used at all when calculating nr_sockets. So I can't see any reason
why with "num_processors + disabled_cpus" the nr_sockets would not be
zero, I think this is a bug that I should fix in nosmp case.

> Hence I'm looking
> forward to you detailing the conditions under which you would see
> an issue without introducing total_cpus.

As said before, with "num_processors + disabled_cpus" I may get a
smaller nr_sockets than the machine actually has. This is my exact
problem: I may miss enumerating some CAT-enabled sockets. While the
assumption is that I will follow your suggestion to make nr_socket >=
the socket count that the machine actually has.

Chao

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