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

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

On 5/13/26 11:37 AM, K Prateek Nayak wrote:
> 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 ...
> 

If we do below, the rest users already have the checks.
So we should be good.
I will add a comment saying this expects to be called under sched_smt_active.

>> +
>>   	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?

Ack. Let me edit the v3. I was about to send.

> 
>>   			 * 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;
>>   		}
>>   
>