Re: [Xen-devel] [PATCH v2 2/2] xen: fill topology info for all present cpus

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 09/04/2018 09:49 AM, Jan Beulich wrote:
> > > > On 04.09.18 at 09:38, <jgross@xxxxxxxx> wrote:
> > On 03/09/18 15:41, Jan Beulich wrote:
> > > > > > On 31.08.18 at 17:22, <jgross@xxxxxxxx> wrote:
> > > > The topology information obtainable via XEN_SYSCTL_cputopoinfo is
> > > > filled rather weird: the size of the array is derived from the highest
> > > > online cpu number, so in case there are trailing offline cpus they
> > > > will not be included.
> > > >
> > > > On a dual core system with 4 threads booted with smt=0 without this
> > > > patch xl info -n will print:
> > > >
> > > > cpu_topology           :
> > > > cpu:    core    socket     node
> > > >    0:       0        0        0
> > > >    1:       0        0        0
> > > >    2:       1        0        0
> > > >
> > > > while with this patch the output is:
> > > >
> > > > cpu_topology           :
> > > > cpu:    core    socket     node
> > > >    0:       0        0        0
> > > >    1:       0        0        0
> > > >    2:       1        0        0
> > > >    3:       1        0        0
> > > >
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > for x86. I'd like this to have an ARM ack as well, though, for
> > > there possibly being differences in how cpu_present_map is
> > > handled in their case. In particular I notice that the only access
> > > to the variable is in smp_prepare_cpus().
> > How relevant is that information for arm at all?
> >
> > cpu_to_core(i) et al always return 0. So topology information is not
> > really helpful.
Well, this is common code and at some point we will read the CPU 
topology to improve scheduling. This has not been done so far because 
there are other important part to focus first.
> Stefano, Julien?
I think the patch makes sense for Arm too:

Acked-by: Julien Grall <julien.grall@xxxxxxx>

At the moment, cpu_present_map is a copy of cpu_online_map. I would 
expect this to change when hotplug will be supported.
Cheers,

--
Julien Grall

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