Re: [RFC PATCH 18/19] sched/fair: Optimize global "nohz.nr_cpus" tracking

[Shrikanth Hegde]

On 9/4/25 9:45 AM, K Prateek Nayak wrote:
> Optimize "nohz.nr_cpus" by tracking number of "sd_nohz->shared" with
> non-zero "nr_idle_cpus" count via "nohz.nr_doms" and only updating at
> the boundary of "sd_nohz->shared->nr_idle_cpus" going from 0 -> 1 and
> back from 1 -> 0.
>
> This also introduces a chance of double accounting when a nohz idle
> entry or the tick races with hotplug or cpuset as described in
> __nohz_exit_idle_tracking().
>
> __nohz_exit_idle_tracking() called when the sched_domain_shared nodes
> tracking idle CPUs are freed is used to correct any potential double
> accounting which can unnecessarily trigger nohz idle balances even when
> all the CPUs have tick enabled.
Is it possible to get rid of this nr_cpus or nr_doms altogether?

The reason being, with current code, one updates the nohz.idle_cpus_mask and
then inc/dec nr_cpus.

The only use it to decide to do periodic idle balancing or not.
If instead, could use cpumask_empty(nohz.idle_cpus_mask) check no?
It may not be every tick accurate, but that may be ok.

I haven't gone through your series in detail yet, but similar thing is doable,
check if the list is empty or not.