sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

[PATCH] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by Yury Norov 3 weeks ago

From: Yury Norov (NVIDIA) <yury.norov@gmail.com>

cpumask_empty() call is O(N) and useless, because the previous
cpumask_and() returns false for empty 'cpus'.

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
 kernel/sched/fair.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b173a059315c..7229339cbb1b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8320,9 +8320,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 		int max_spare_cap_cpu = -1;
 		int fits, max_fits = -1;
 
-		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
-
-		if (cpumask_empty(cpus))
+		if (!cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask))
 			continue;
 
 		/* Account external pressure for the energy estimation */
-- 
2.43.0

Re: [PATCH] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by Markus Elfring 3 weeks ago

> cpumask_empty() call is O(N) and useless, because the previous
> cpumask_and() returns false for empty 'cpus'.

It seems to be more helpful to reuse a return value from a previous function call.
How do you think about to add any tags (like “Fixes” and “Cc”) accordingly?

See also:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.17-rc5#n94

Regards,
Markus

Re: [PATCH] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by Vincent Guittot 3 weeks ago

On Thu, 11 Sept 2025 at 04:34, Yury Norov <yury.norov@gmail.com> wrote:
>
> From: Yury Norov (NVIDIA) <yury.norov@gmail.com>
>
> cpumask_empty() call is O(N) and useless, because the previous
> cpumask_and() returns false for empty 'cpus'.
>
> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/fair.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..7229339cbb1b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8320,9 +8320,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                 int max_spare_cap_cpu = -1;
>                 int fits, max_fits = -1;
>
> -               cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
> -
> -               if (cpumask_empty(cpus))
> +               if (!cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask))
>                         continue;
>
>                 /* Account external pressure for the energy estimation */
> --
> 2.43.0
>

Re: [PATCH] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by Madadi Vineeth Reddy 3 weeks ago

On 11/09/25 08:04, Yury Norov wrote:
> From: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> 
> cpumask_empty() call is O(N) and useless, because the previous
> cpumask_and() returns false for empty 'cpus'.
> 

LGTM

Reviewed-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>

Thanks,
Madadi Vineeth Reddy

> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> ---
>  kernel/sched/fair.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..7229339cbb1b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8320,9 +8320,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  		int max_spare_cap_cpu = -1;
>  		int fits, max_fits = -1;
>  
> -		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
> -
> -		if (cpumask_empty(cpus))
> +		if (!cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask))
>  			continue;
>  
>  		/* Account external pressure for the energy estimation */

Re: [PATCH] sched: fair: drop useless cpumask_empty() in find_energy_efficient_cpu()

Posted by K Prateek Nayak 3 weeks ago

Hello Yury,

On 9/11/2025 8:04 AM, Yury Norov wrote:
> From: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> 
> cpumask_empty() call is O(N) and useless, because the previous
> cpumask_and() returns false for empty 'cpus'.

I agree. Please feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>

> 
> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> ---
>  kernel/sched/fair.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..7229339cbb1b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8320,9 +8320,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  		int max_spare_cap_cpu = -1;
>  		int fits, max_fits = -1;
>  
> -		cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
> -
> -		if (cpumask_empty(cpus))
> +		if (!cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask))
>  			continue;
>  
>  		/* Account external pressure for the energy estimation */

-- 
Thanks and Regards,
Prateek