Re: [PATCH v2 RSEND] sched/fair: Optimize EAS energy calculation complexity from O(N) to O(1) inside inner loop

[Christian Loehle <christian.loehle@arm.com>]

On 2/2/26 03:05, Qiliang Yuan wrote:
> Pre-calculate the base maximum utilization of each performance domain during the
> main loop of find_energy_efficient_cpu() and cache it in the local
> 'energy_env' structure.
> 
> By caching this base value, the maximum utilization for candidate CPU
> placements (such as prev_cpu and max_spare_cap_cpu) can be determined in
> O(1) time, eliminating redundant scans of the performance domain. This
> optimizes the energy estimation path by reducing the number of scans per
> performance domain from three to one.

Which is still O(n), I think the title is misleading.

> 
> This change significantly reduces wake-up latency on systems with high core
> counts or complex performance domain topologies by minimizing the overall
> complexity of the Energy-Aware Scheduling (EAS) calculation.

I don't think this is actually true. EAS doesn't really work with a large
number of PDs because of the expensive wakeup path.
I don't think there's an EAS system out there where this would actually make a
measurable impact. Most have 2 or 3, the highest number of PDs I'm aware of
is 5, but FWIW the actual change looks correct to me.

> 
> Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
> Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
> ---
> v2:
>  - Ensure RCU safety by using local 'energy_env' for caching instead of
>    modifying the shared 'perf_domain' structure.
>  - Consolidate pre-calculation into the main loop to avoid an extra pass
>    over the performance domains.
> v1:
>  - Optimize energy calculation by pre-calculating performance domain max utilization.
>  - Add max_util and max_spare_cap_cpu to struct perf_domain.
>  - Reduce inner loop complexity from O(N) to O(1) for energy estimation.
> 
>  kernel/sched/fair.c | 36 ++++++++++++++++++------------------
>  1 file changed, 18 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e71302282671..5c114c49c202 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8148,6 +8148,7 @@ struct energy_env {
>  	unsigned long pd_busy_time;
>  	unsigned long cpu_cap;
>  	unsigned long pd_cap;
> +	unsigned long pd_max_util;
>  };
>  
>  /*
> @@ -8215,41 +8216,32 @@ static inline void eenv_pd_busy_time(struct energy_env *eenv,
>   * exceed @eenv->cpu_cap.
>   */
>  static inline unsigned long
> -eenv_pd_max_util(struct energy_env *eenv, struct cpumask *pd_cpus,
> +eenv_pd_max_util(struct energy_env *eenv, struct perf_domain *pd,
>  		 struct task_struct *p, int dst_cpu)
>  {
> -	unsigned long max_util = 0;
> -	int cpu;
> +	unsigned long max_util = eenv->pd_max_util;
>  
> -	for_each_cpu(cpu, pd_cpus) {
> -		struct task_struct *tsk = (cpu == dst_cpu) ? p : NULL;
> -		unsigned long util = cpu_util(cpu, p, dst_cpu, 1);
> +	if (dst_cpu >= 0 && cpumask_test_cpu(dst_cpu, perf_domain_span(pd))) {
> +		unsigned long util = cpu_util(dst_cpu, p, dst_cpu, 1);
>  		unsigned long eff_util, min, max;
>  
> -		/*
> -		 * Performance domain frequency: utilization clamping
> -		 * must be considered since it affects the selection
> -		 * of the performance domain frequency.
> -		 * NOTE: in case RT tasks are running, by default the min
> -		 * utilization can be max OPP.
> -		 */
> -		eff_util = effective_cpu_util(cpu, util, &min, &max);
> +		eff_util = effective_cpu_util(dst_cpu, util, &min, &max);
>  
>  		/* Task's uclamp can modify min and max value */
> -		if (tsk && uclamp_is_used()) {
> +		if (uclamp_is_used()) {
>  			min = max(min, uclamp_eff_value(p, UCLAMP_MIN));
>  
>  			/*
>  			 * If there is no active max uclamp constraint,
>  			 * directly use task's one, otherwise keep max.
>  			 */
> -			if (uclamp_rq_is_idle(cpu_rq(cpu)))
> +			if (uclamp_rq_is_idle(cpu_rq(dst_cpu)))
>  				max = uclamp_eff_value(p, UCLAMP_MAX);
>  			else
>  				max = max(max, uclamp_eff_value(p, UCLAMP_MAX));
>  		}
>  
> -		eff_util = sugov_effective_cpu_perf(cpu, eff_util, min, max);
> +		eff_util = sugov_effective_cpu_perf(dst_cpu, eff_util, min, max);
>  		max_util = max(max_util, eff_util);
>  	}
>  
> @@ -8265,7 +8257,7 @@ static inline unsigned long
>  compute_energy(struct energy_env *eenv, struct perf_domain *pd,
>  	       struct cpumask *pd_cpus, struct task_struct *p, int dst_cpu)
>  {
> -	unsigned long max_util = eenv_pd_max_util(eenv, pd_cpus, p, dst_cpu);
> +	unsigned long max_util = eenv_pd_max_util(eenv, pd, p, dst_cpu);
>  	unsigned long busy_time = eenv->pd_busy_time;
>  	unsigned long energy;
>  
> @@ -8376,12 +8368,20 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  
>  		eenv.cpu_cap = cpu_actual_cap;
>  		eenv.pd_cap = 0;
> +		eenv.pd_max_util = 0;
>  
>  		for_each_cpu(cpu, cpus) {
>  			struct rq *rq = cpu_rq(cpu);
> +			unsigned long util_b, eff_util_b, min_b, max_b;
>  
>  			eenv.pd_cap += cpu_actual_cap;
>  
> +			/* Pre-calculate base max utilization for the performance domain */
> +			util_b = cpu_util(cpu, p, -1, 1);
> +			eff_util_b = effective_cpu_util(cpu, util_b, &min_b, &max_b);
> +			eff_util_b = sugov_effective_cpu_perf(cpu, eff_util_b, min_b, max_b);
> +			eenv.pd_max_util = max(eenv.pd_max_util, eff_util_b);
> +
>  			if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
>  				continue;
>