[PATCH] cpufreq: optimize policy_is_shared()

[PATCH] cpufreq: optimize policy_is_shared()

[Yury Norov]

The switch to cpumask_nth() over cpumask_weight(), as it may return
earlier - as soon as the function counts the required number of CPUs.

Signed-off-by: Yury Norov <ynorov@nvidia.com>
---
 include/linux/cpufreq.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index cc894fc38971..8ca2bcb3d7ae 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -232,7 +232,7 @@ static inline bool policy_is_inactive(struct cpufreq_policy *policy)
 
 static inline bool policy_is_shared(struct cpufreq_policy *policy)
 {
-	return cpumask_weight(policy->cpus) > 1;
+	return cpumask_nth(1, policy->cpus) < nr_cpumask_bits;
 }
 
 #ifdef CONFIG_CPU_FREQ
-- 
2.43.0

Re: [PATCH] cpufreq: optimize policy_is_shared()

[Zhongqiu Han]

On 3/15/2026 3:25 AM, Yury Norov wrote:
> The switch to cpumask_nth() over cpumask_weight(), as it may return
> earlier - as soon as the function counts the required number of CPUs.
> 
> Signed-off-by: Yury Norov <ynorov@nvidia.com>
> ---
>   include/linux/cpufreq.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index cc894fc38971..8ca2bcb3d7ae 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -232,7 +232,7 @@ static inline bool policy_is_inactive(struct cpufreq_policy *policy)
>   
>   static inline bool policy_is_shared(struct cpufreq_policy *policy)
>   {
> -	return cpumask_weight(policy->cpus) > 1;
> +	return cpumask_nth(1, policy->cpus) < nr_cpumask_bits;
>   }
>   
>   #ifdef CONFIG_CPU_FREQ


Hi Yury,

Thanks for the patch.

I see the intent of switching from cpumask_weight() to cpumask_nth() to
allow an early exit once we know there are at least two CPUs in the
mask. Functionally this looks equivalent.

That said, policy_is_shared() appears to be used mostly during governor
/driver bring-up paths (e.g. sugov_start(),cpufreq_dbs_governor_start(),
acpi_cpufreq_cpu_init(), etc.), rather than a hot runtime path. Given
that, I'm not sure the micro-optimization is worth the readability cost.

Also, cpumask_nth()'s documented return value is ">= nr_cpu_ids if such
cpu doesn't exist", so comparing against nr_cpu_ids would match the API
contract more directly than nr_cpumask_bits (even if they are currently
equivalent). [1][2]


[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/cpumask.h#n540
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/cpumask.h#n78




-- 
Thx and BRs,
Zhongqiu Han

Re: [PATCH] cpufreq: optimize policy_is_shared()

[Viresh Kumar]

On 14-03-26, 15:25, Yury Norov wrote:
> The switch to cpumask_nth() over cpumask_weight(), as it may return
> earlier - as soon as the function counts the required number of CPUs.
> 
> Signed-off-by: Yury Norov <ynorov@nvidia.com>
> ---
>  include/linux/cpufreq.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index cc894fc38971..8ca2bcb3d7ae 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -232,7 +232,7 @@ static inline bool policy_is_inactive(struct cpufreq_policy *policy)
>  
>  static inline bool policy_is_shared(struct cpufreq_policy *policy)
>  {
> -	return cpumask_weight(policy->cpus) > 1;
> +	return cpumask_nth(1, policy->cpus) < nr_cpumask_bits;
>  }
>  
>  #ifdef CONFIG_CPU_FREQ

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

-- 
viresh

Re: [PATCH] cpufreq: optimize policy_is_shared()

[Zhongqiu Han]

On 3/15/2026 3:25 AM, Yury Norov wrote:
> The switch to cpumask_nth() over cpumask_weight(), as it may return
> earlier - as soon as the function counts the required number of CPUs.
> 
> Signed-off-by: Yury Norov <ynorov@nvidia.com>
> ---
>   include/linux/cpufreq.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index cc894fc38971..8ca2bcb3d7ae 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -232,7 +232,7 @@ static inline bool policy_is_inactive(struct cpufreq_policy *policy)
>   
>   static inline bool policy_is_shared(struct cpufreq_policy *policy)
>   {
> -	return cpumask_weight(policy->cpus) > 1;
> +	return cpumask_nth(1, policy->cpus) < nr_cpumask_bits;
>   }
>   
>   #ifdef CONFIG_CPU_FREQ

Thanks. While I had a minor readability/API-nit earlier, I'm fine with
the patch as-is.

Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>

-- 
Thx and BRs,
Zhongqiu Han

Re: [PATCH] cpufreq: optimize policy_is_shared()

[Rafael J. Wysocki]

On Tue, Mar 17, 2026 at 7:47 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>
> On 14-03-26, 15:25, Yury Norov wrote:
> > The switch to cpumask_nth() over cpumask_weight(), as it may return
> > earlier - as soon as the function counts the required number of CPUs.
> >
> > Signed-off-by: Yury Norov <ynorov@nvidia.com>
> > ---
> >  include/linux/cpufreq.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> > index cc894fc38971..8ca2bcb3d7ae 100644
> > --- a/include/linux/cpufreq.h
> > +++ b/include/linux/cpufreq.h
> > @@ -232,7 +232,7 @@ static inline bool policy_is_inactive(struct cpufreq_policy *policy)
> >
> >  static inline bool policy_is_shared(struct cpufreq_policy *policy)
> >  {
> > -     return cpumask_weight(policy->cpus) > 1;
> > +     return cpumask_nth(1, policy->cpus) < nr_cpumask_bits;
> >  }
> >
> >  #ifdef CONFIG_CPU_FREQ
>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

Applied as 7.1 material, thanks!