Re: [PATCH 1/5] x86/smpboot: Add the missing description of possible_cpus

[Dou Liyang]

At 03/21/2018 05:34 PM, Dou Liyang wrote:
> Hi Peter,
>
> At 03/21/2018 05:08 PM, Peter Zijlstra wrote:
> > On Wed, Mar 21, 2018 at 01:33:24PM +0800, Dou Liyang wrote:
> > > How about:
> > >
> > > possible_cpus=ÂÂÂ [s390,x86_64] Set the number of possible CPUs which
> > > ÂÂÂÂÂÂÂ are determined by the ACPI tables MADT or mptables by
> > > ÂÂÂÂÂÂÂ default. possible_cpus=n : n >= 1 enforces the possible
> > > ÂÂÂÂÂÂÂ number to be 'n'.
> > > ÂÂÂÂÂÂÂ While nr_cpus is also be set: nr_cpus=m, choice the
> > > ÂÂÂÂÂÂÂ minimum one for the number of possible CPUs.
> >
> > So what is the exact difference between possible_cpus and nr_cpus ? I
>
> the possible_cpus= can reset the number of possible CPUs, even bigger
> than 'num_processors + disabled_cpus', But nr_cpus= can't.
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      the maximum number kernel gets from ACPI/mptables, no matter what
number nr_cpus= is, the number of possible CPUs will not bigger than it.

> > konw maxcpus= limits the number of CPUs we bring up, and possible_cpus
> > limits the possible_map, but I'm not entirely sure what nr_cpus does
> > here.
>
> nr_cpus can limited the maximum CPUs that the kernel could support.
>
> Here is a double check in case of using them at the same time, even if I
> think just using possible_cpus= is enough. :-)
>
> Thanks,
>  ÂÂÂÂdou.
> >