Re: [PATCH v2 3/3] sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n

[K Prateek Nayak]

Hello Shrikanth,

Thank you for cleaning these bits up! Couple comments below:

On 5/12/2026 8:51 PM, Shrikanth Hegde wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 353e31ecaadc..b6f9592b31fd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1586,6 +1586,9 @@ static inline bool is_core_idle(int cpu)
>  {
>  	int sibling;
>  
> +	if (!IS_ENABLED(CONFIG_SCHED_SMT))
> +		return true;

I think !sched_smt_active() can also return early here. That said ...

> +
>  	for_each_cpu(sibling, cpu_smt_mask(cpu)) {
>  		if (cpu == sibling)
>  			continue;
> @@ -12003,7 +12006,8 @@ static int should_we_balance(struct lb_env *env)

... I think the if() above can simply check for sched_smt_active()
before calling is_core_idle() like:

    if (sched_smt_active() &&
        !(env->sd->flags & SD_SHARE_CPUCAPACITY) &&
        !is_core_idle(cpu)) {
        ...
    }

That way, we ensure we don't call is_core_idle() and the cpumask_and()
unnecessarily on topologies that don't have SMT domain similar to how
sched_use_asym_prio() guards the call to is_core_idle().

Thoughts?

>  			 * idle has been found, then its not needed to check other
>  			 * SMT siblings for idleness:
>  			 */
> -			cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
> +			if (IS_ENABLED(CONFIG_SCHED_SMT))
> +				cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
>  			continue;
>  		}
>  

-- 
Thanks and Regards,
Prateek