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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 3/20/26 9:07 AM, K Prateek Nayak wrote:
> 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?

May be. but it could be a confusing name. sched_tmpmask?

We could similar stuff already to load_balance_mask, select_rq_mask.
So, i would prefer to keep it separate.

> 
> [..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.
> 

Makes sense. Will do.

>>   
>>   	if (rq->nr_running >= 2) {