Re: [PATCH 2/2] sched/fair: Honor asymmetric SMT priority in idle selection

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

On 9/7/26 9:27 AM, K Prateek Nayak wrote:
> Hello Andrea,
> 
> On 9/4/2026 2:48 PM, Andrea Righi wrote:
>> +/*
>> + * Return true when @cpu has a higher asymmetric-packing priority than
>> + * @other in their shared SMT scheduling domain.
>> + */
>> +static bool sched_smt_asym_prefer(int cpu, int other)
>> +{
>> +	struct sched_domain *sd = rcu_dereference_all(cpu_rq(cpu)->sd);
>> +
>> +	if (!sd)
>> +		return false;
>> +
>> +	if (!(sd->flags & SD_SHARE_CPUCAPACITY) ||
>> +	    !(sd->flags & SD_ASYM_PACKING))
>> +		return false;
>> +
>> +	if (!cpumask_test_cpu(other, sched_domain_span(sd)))
>> +		return false;
>> +
>> +	return sched_asym_prefer(cpu, other);
>> +}
>> +
>> +/*
>> + * Return the highest-priority available CPU in @cpu's SMT core that is also in @cpus.
>> + */
>> +static int __select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
>> +{
>> +	int best = cpu;
>> +	int sibling;
>> +
>> +	for_each_cpu_and(sibling, cpu_smt_mask(cpu), cpus) {
>> +		if (sibling == best || !choose_idle_cpu(sibling, p))
>> +			continue;
>> +
>> +		if (sched_smt_asym_prefer(sibling, best))
>> +			best = sibling;
> 
> nit. Since sched_smt_asym_prefer() is only used here, and we know rq->sd
> is the one that can have SD_SHARE_CPUCAPACITY | SD_ASYM_PACKING, perhaps
> you can inline the check here do a:
> 
>      sd = rcu_dereference_all(cpu_rq(cpu)->sd);
> 
>      if (!sd)
>           return cpu;
> 
>     if (!(sd->flags & SD_SHARE_CPUCAPACITY) || !(sd->flags & SD_ASYM_PACKING))
>         return cpu;
> 
>     for_each_cpu_and (sibling, sched_domain_span(sd), cpus) {
>         ...
>     }
> 
>    ...
> 
> 
> That way, you don't need to dereference cpu_rq(cpu)->sd every time in
> sched_smt_asym_prefer() and check cpumask_test_cpu(). Both, domain
> span and task affinity will be covered at once.
> 
> Thoughts?
> 
>> +	}
>> +
>> +	return best;
>> +}
>> +
>> +static inline int
>> +select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
>> +{
>> +	if (!sched_smt_asym_active())
>> +		return cpu;
>> +
>> +	return __select_idle_smt_cpu(p, cpu, cpus);
>> +}
>> +
>> +/*
>> + * Redirect an available SMT CPU to a higher-priority available sibling allowed by task affinity.
>> + */
>> +static inline int select_idle_smt_priority(struct task_struct *p, int cpu)
>> +{
>> +	return select_idle_smt_cpu(p, cpu, p->cpus_ptr);
>> +}
>> +
>>   /*
>>    * Scans the local SMT mask to see if the entire core is idle, and records this
>>    * information in sd_balance_shared->has_idle_cores.
>> @@ -8645,7 +8702,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
>>   	}
>>   
>>   	if (idle)
>> -		return core;
>> +		return select_idle_smt_cpu(p, core, cpus);
>>   
>>   	cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
>>   	return -1;
>> @@ -8668,7 +8725,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>>   		if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
>>   			continue;
>>   		if (choose_idle_cpu(cpu, p))
>> -			return cpu;
>> +			return select_idle_smt_priority(p, cpu);
>>   	}
>>   
>>   	return -1;
>> @@ -8720,7 +8777,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>   						return -1;
>>   					idle_cpu = __select_idle_cpu(cpu, p);
>>   					if ((unsigned int)idle_cpu < nr_cpumask_bits)
>> -						return idle_cpu;
>> +						return select_idle_smt_priority(p, idle_cpu);
> 
> Question for Shrikanth: On larger SMT (SMT-4, SMT-8), does the ranking
> make that big of a difference if the core is already busy?
> 

Only on Power7 we had AYSM PACKING.
There IPC of CPU0 > CPU1 > CPU2 > CPU4 for the four siblings IIRC irrespective of busy
or idle.

PS: I haven't seen the patches in detail yet.

> Does the overehead of additional search get offset by the benefit of
> being placed on a better ranked thread? If not, maybe the paths for
> !has_idle_core can stay as is?
> 
>>   				}
>>   			}
>>   			cpumask_andnot(cpus, cpus, sched_group_span(sg));
>> @@ -8745,7 +8802,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>   	if (has_idle_core)
>>   		set_idle_cores(target, false);
>>   
>> -	return idle_cpu;
>> +	return (unsigned int)idle_cpu < nr_cpumask_bits ?
>> +		select_idle_smt_priority(p, idle_cpu) : idle_cpu;
> 
> Since every path does a select_idle_smt_priority() - be it coming from
> select_idle_core(), the early-return from the cluster scan, or just an
> idle CPU from the LLc scan, can't we simply just do it once in
> select_idle_sibling()?
> 
> Something like:
> 
>    (Only build tested)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f79fcba4afec..7c97585141dd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8964,7 +8964,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   
>   	if (choose_idle_cpu(target, p) &&
>   	    asym_fits_cpu(task_util, util_min, util_max, target))
> -		return target;
> +		goto out;
>   
>   	/*
>   	 * If the previous CPU is cache affine and idle, don't be stupid:
> @@ -8974,8 +8974,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   	    asym_fits_cpu(task_util, util_min, util_max, prev)) {
>   
>   		if (!static_branch_unlikely(&sched_cluster_active) ||
> -		    cpus_share_resources(prev, target))
> -			return prev;
> +		    cpus_share_resources(prev, target)) {
> +			target = prev;
> +			goto out;
> +		}
>   
>   		prev_aff = prev;
>   	}
> @@ -8993,7 +8995,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   	    prev == smp_processor_id() &&
>   	    this_rq()->nr_running <= 1 &&
>   	    asym_fits_cpu(task_util, util_min, util_max, prev)) {
> -		return prev;
> +		target = prev;
> +		goto out;
>   	}
>   
>   	/* Check a recently used CPU as a potential idle candidate: */
> @@ -9007,8 +9010,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   	    asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
>   
>   		if (!static_branch_unlikely(&sched_cluster_active) ||
> -		    cpus_share_resources(recent_used_cpu, target))
> -			return recent_used_cpu;
> +		    cpus_share_resources(recent_used_cpu, target)) {
> +			target = recent_used_cpu;
> +			goto out;
> +		}
>   
>   	} else {
>   		recent_used_cpu = -1;
> @@ -9030,7 +9035,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   		 */
>   		if (sd) {
>   			i = select_idle_capacity(p, sd, target);
> -			return ((unsigned)i < nr_cpumask_bits) ? i : target;
> +			target = ((unsigned)i < nr_cpumask_bits) ? i : target;
> +			goto out;
>   		}
>   	}
>   
> @@ -9043,27 +9049,31 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   
>   		if (!has_idle_core && cpus_share_cache(prev, target)) {
>   			i = select_idle_smt(p, sd, prev);
> -			if ((unsigned int)i < nr_cpumask_bits)
> -				return i;
> +			if ((unsigned int)i < nr_cpumask_bits) {
> +				target = i;
> +				goto out;
> +			}
>   		}
>   	}
>   
>   	i = select_idle_cpu(p, sd, has_idle_core, target);
>   	if ((unsigned)i < nr_cpumask_bits)
> -		return i;
> -
> +		target = i;
>   	/*
>   	 * For cluster machines which have lower sharing cache like L2 or
>   	 * LLC Tag, we tend to find an idle CPU in the target's cluster
>   	 * first. But prev_cpu or recent_used_cpu may also be a good candidate,
>   	 * use them if possible when no idle CPU found in select_idle_cpu().
>   	 */
> -	if ((unsigned int)prev_aff < nr_cpumask_bits)
> -		return prev_aff;
> -	if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
> -		return recent_used_cpu;
> +	else if ((unsigned int)prev_aff < nr_cpumask_bits)
> +		target = prev_aff;
> +	else if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
> +		target = recent_used_cpu;
> +out:
> +	if (!sched_smt_asym_active())
> +		return target;
>   
> -	return target;
> +	return select_idle_smt_priority(p, target);
>   }
>   
>   /**
> ---
> 
> That way, it lives in a single place, and we don't have to pepper
> select_idle_smt_priority() everywhere. Thoughts?
>