Re: [PATCH v3 17/20] sched/core: Introduce default arch handling code for inc/dec preferred CPUs

[Shrikanth Hegde]

On 6/9/26 12:10 AM, Ilya Leoshkevich wrote:
> On 5/14/26 17:22, Shrikanth Hegde wrote:
> > Define default handlers for high/low steal time. If arch has better
> > decision logic, may override the default implementation.
> >
> > - If the steal time higher than threshold, reduce the number of preferred
> >    CPUs by 1 core. The last core in the intersection of online and
> >    preferred CPUs will be marked as non-preferred.
> >    Ensure at least one core is left as preferred always.
> >
> > - If the steal time lower than threshold, increase the number of 
> > preferred
> >    CPUs by 1 core. First online core which is not in 
> > cpu_preferred_mask will
> >    be marked as preferred.
> >    If all cores are already set to preferred, bail out.
> >
> > Increase/Decrease may need to modify the splicing across NUMA nodes. 
> > It is
> > being kept simple for now.
> >
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >   include/linux/sched.h |  2 ++
> >   kernel/sched/core.c   | 58 +++++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 60 insertions(+)
>
> [...]
>
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index a3f65e9c7d30..195e3648b1b5 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -11368,6 +11368,64 @@ void sched_init_steal_monitor(void)
> >       steal_mon.sampling_period_ms  = 1000;        /* once per second */
> >   }
> > +/*
> > + * Default implementation of decrementing the preferred CPUs based on 
> > steal
> > + * time. This is simple logic and decrease the preferred CPUs by 1 core.
> > + * It takes out the last core in the online & preferred.
> > + *
> > + * Ensure at least one housekeeping core is always kept as preferred
> > + *
> > + * Could be overwritten by arch specific handling.
> > + */
> > +#ifndef arch_dec_preferred_cpus
> > +void arch_dec_preferred_cpus(struct steal_monitor_t *sm, u64 
> > steal_ratio)
> > +{
> > +    int last_cpu, tmp_cpu;
> > +    int this_cpu = raw_smp_processor_id();
> > +
> > +    cpumask_and(sm->tmp_mask, cpu_online_mask, cpu_preferred_mask);
>
> Since preferred is always a subset of online, do we even need this?
> Dropping tmp_mask will also make the error handling concern I posted
> earlier go away.

You are right. Decrement doesn't need this. Thanks for catching this.

I ran through the cases mentioned here and some more. I don't see a good reason
to have the tmp_mask.
https://lore.kernel.org/all/0d8412de-e18a-476f-9eb6-9a977f4474a3@xxxxxxxxxxxxx/