Re: [PATCH v5 07/24] sched/fair: Select preferred CPU at wakeup when possible

[Shrikanth Hegde]

Hi Peter, Thank you very much for going through the patches.

On 6/26/26 3:29 PM, Peter Zijlstra wrote:
> On Thu, Jun 25, 2026 at 06:16:31PM +0530, Shrikanth Hegde wrote:
> > Update available_idle_cpu to consider preferred CPUs. This takes care of
> > lot of decisions at wakeup to use only preferred CPUs. There is no need to
> > put those explicit checks everywhere.
> >
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/sched.h | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 5d009c2529b2..148fe6145f1a 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1434,6 +1434,9 @@ static inline bool available_idle_cpu(int cpu)
> >   	if (!idle_rq(cpu_rq(cpu)))
> >   		return 0;
> >   
> > +	if (!cpu_preferred(cpu))
> > +		return 0;
> > +
> >   	if (vcpu_is_preempted(cpu))
> >   		return 0;
> >   
>
> This one might hurt, it is a whole extra cacheline in otherwise already
> sensitive (wakeup) paths.

Yes, this could be costly. If wakeup returns a non-preferred CPU,
is_cpu_allowed would catch it.
So, i think we can avoid repeated computation of it in available_idle_cpu.

Let me see if removing it still achieves the functionality of moving out
fast enough and numbers are close enough to with it.