Re: [Xen-devel] [PATCH 2/2] xen: fill topology info for online cpus only

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.08.18 at 10:37, <wei.liu2@xxxxxxxxxx> wrote:
> On Thu, Aug 30, 2018 at 10:31:16AM +0200, Juergen Gross wrote:
>> On 30/08/18 10:16, Jan Beulich wrote:
>> >>>> On 29.08.18 at 20:23, <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, while the data is set to "invalid" for not present
>> >> cpus only.
>> >>
>> >> With smt=0 the information for parked threads is all zero, so it should
>> >> be best to return "invalid" information for offline cpus.
>> >>
>> >> On a dual core system 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
>> > 
>> > But there's nothing wrong here. The interesting part is what would be
>> > printed for CPU 3 (perhaps on a more than two cores system). After
>> > all topology is valid irrespective of whether a CPU is online - it all
>> > depends on whether the hypervisor still has the information available.
>> > It is for a reason that cpu_smpboot_free() invalidates certain fields
>> > only upon CPU removal:
>> > 
>> >     if ( remove )
>> >     {
>> >         c[cpu].phys_proc_id = XEN_INVALID_SOCKET_ID;
>> >         c[cpu].cpu_core_id = XEN_INVALID_CORE_ID;
>> >         c[cpu].compute_unit_id = INVALID_CUID;
>> > 
>> > On a 6-core system I see
>> > 
>> > cpu:    core    socket     node
>> >   0:       0        0        0
>> >   1:       0        0        0
>> >   2:       1        0        0
>> >   3:       1        0        0
>> >   4:       2        0        0
>> >   5:       2        0        0
>> >   6:       8        0        0
>> >   7:       8        0        0
>> >   8:       9        0        0
>> >   9:       9        0        0
>> >  10:      10        0        0
>> > 
>> > which looks fine to me, apart from the missing info on CPU 11.
>> 
>> I can change the patch to print the information for the offline cpus
>> (including the now missing ones), too.
>> 
> 
> That is fine too. I just don't like inconsistent output. :p
> 
> P.S. you probably want to add a new field to the existing interface to
> indicate if a cpu is online.

And if we extend the interface anyway, also the thread ID (as iirc
pointed out as missing recently by George).

Jan



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