Re: [PATCH v14 04/13] cpumask: Introduce cpu_preferred_mask

[Yury Norov <ynorov@nvidia.com>]

On Thu, Oct 29, 2026 at 05:36:28PM +0100, Mete Durlu wrote:
> On 29/10/2026 17:28, Yury Norov wrote:
> > On Thu, Oct 29, 2026 at 02:32:08PM +0100, Mete Durlu wrote:
> > > On 28/09/2026 07:37, Shrikanth Hegde wrote:
> > > > Provide the preferred CPU infrastructure. Define get/set macros
> > > > which could be used to get/set CPU state as preferred.
> > > > 
> > > > CONFIG_PREFERRED_CPU will be selected by the driver which handles
> > > > steal time values. It is going to set/clear preferred CPU state.
> > > > This driver will be called steal_governor and it is introduced in
> > > > subsequent patches. It periodically computes the steal ratio and
> > > > decides on preferred CPU state.
> > > > 
> > > > A CPU is set to preferred when it becomes active. Later it may be
> > > > marked as non-preferred depending on steal ratio by the steal_governor.
> > > > 
> > > > Always maintain design construct of preferred is subset of active.
> > > > i.e. preferred ⊆ active ⊆ online ⊆ present ⊆ possible
> > > > 
> > > > With CONFIG_PREFERRED_CPU=n, ensure set_cpu_preferred is a nop and get
> > > > method returns the active state in that case.
> > > > 
> > > > Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> > > > ---
> > > >    include/linux/cpumask.h | 24 ++++++++++++++++++++++++
> > > >    kernel/Kconfig.preempt  |  4 ++++
> > > >    kernel/cpu.c            |  6 ++++++
> > > >    kernel/sched/core.c     |  5 +++++
> > > >    4 files changed, 39 insertions(+)
> > > 
> > > [..snip..]
> > > 
> > > >    #define cpu_is_offline(cpu)	unlikely(!cpu_online(cpu))
> > > > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > > > index 985aea617cfe..edc067a0c422 100644
> > > > --- a/kernel/Kconfig.preempt
> > > > +++ b/kernel/Kconfig.preempt
> > > > @@ -194,3 +194,7 @@ config SCHED_CLASS_EXT
> > > >    	  For more information:
> > > >    	    Documentation/scheduler/sched-ext.rst
> > > >    	    https://github.com/sched-ext/scx
> > > > +
> > > > +config PREFERRED_CPU
> > > > +	bool
> > > > +	depends on SMP && PARAVIRT
> > > FWICT, PREFERRED_CPU mechanism can work perfectly fine without PARAVIRT
> > > selected. It would be annoying to enable it and implement a dummy
> > > paravirt_steal_clock() just for that.
> > > 
> > > Shrikanth, would it be possible to drop that dependency?
> > 
> > Do you have a non-paravirt case in mind?
> 
> s390 LPARs are exactly that. s390 does not select PARAVIRT by default
> but enabling PREFERRED_CPU is strongly considered.
> 
> I removed the PARAVIRT dependency and gave the whole thing a quick
> spin. Builds and functions w/o any issues on s390!
> > I'm actually about to submit an RFC that drops the PARAVIRT dependency
> > and provides write access to the preferred mask. But I want to make
> > sure that we need it, and we've got no reasonable mechanism to put
> > unneeded CPUs on idle already.
> 
> You can mention s390 LPARs as valid case on your RFC if need be.

Sure, will do. I'm about submitting this before the end of week; will
add you in CC.

Thanks,
Yury