Re: [PATCH 1/2] sched/fair: consider hk_mask early in triggering ilb

[K Prateek Nayak]

Hello Shrikanth,

On 3/19/2026 12:23 PM, Shrikanth Hegde wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b19aeaa51ebc..02cca2c7a98d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7392,6 +7392,7 @@ static inline unsigned int cfs_h_nr_delayed(struct rq *rq)
>  static DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
>  static DEFINE_PER_CPU(cpumask_var_t, select_rq_mask);
>  static DEFINE_PER_CPU(cpumask_var_t, should_we_balance_tmpmask);
> +static DEFINE_PER_CPU(cpumask_var_t, kick_ilb_tmpmask);

nit. We can rename and reuse select_rq_mask. Wakeups happen with IRQs
disabled and kick happens from the hrtimer handler so it should be safe
to reuse that and save some space.

Thoughts?

[..snip..]

> @@ -12715,27 +12716,41 @@ static void nohz_balancer_kick(struct rq *rq)
>  	 */
>  	nohz_balance_exit_idle(rq);
>  
> +	/* ILB considers only HK_TYPE_KERNEL_NOISE housekeeping CPUs */
> +
>  	if (READ_ONCE(nohz.has_blocked_load) &&
> -	    time_after(now, READ_ONCE(nohz.next_blocked)))
> +	    time_after(now, READ_ONCE(nohz.next_blocked))) {
>  		flags = NOHZ_STATS_KICK;
> +		cpumask_and(ilb_cpus, nohz.idle_cpus_mask,
> +			    housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
> +	}
>  
>  	/*
> -	 * Most of the time system is not 100% busy. i.e nohz.nr_cpus > 0
> -	 * Skip the read if time is not due.
> +	 * Most of the time system is not 100% busy. i.e there are idle
> +	 * housekeeping CPUs.
> +	 *
> +	 * So, Skip the reading idle_cpus_mask if time is not due.
>  	 *
>  	 * If none are in tickless mode, there maybe a narrow window
>  	 * (28 jiffies, HZ=1000) where flags maybe set and kick_ilb called.
>  	 * But idle load balancing is not done as find_new_ilb fails.
> -	 * That's very rare. So read nohz.nr_cpus only if time is due.
> +	 * That's very rare. So check (idle_cpus_mask & HK_TYPE_KERNEL_NOISE)
> +	 * only if time is due.
> +	 *
>  	 */
>  	if (time_before(now, nohz.next_balance))
>  		goto out;
>  
> +	/* Avoid the double computation */
> +	if (flags != NOHZ_STATS_KICK)
> +		cpumask_and(ilb_cpus, nohz.idle_cpus_mask,
> +			    housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
> +
>  	/*
>  	 * None are in tickless mode and hence no need for NOHZ idle load
>  	 * balancing
>  	 */
> -	if (unlikely(cpumask_empty(nohz.idle_cpus_mask)))
> +	if (unlikely(cpumask_empty(ilb_cpus)))
>  		return;

We can just use the return value from the previous cpumask_and() for
this and save on another cpumask iteration.

>  
>  	if (rq->nr_running >= 2) {
-- 
Thanks and Regards,
Prateek