Subject: Re: [PATCH] arm64: Utilize for_each_cpu_wrap for reference lookup

From: Yury Norov <[email-masked]>
Date: Tue, 18 Feb 2025 15:14:23 -0500

On Tue, Feb 18, 2025 at 07:24:12PM +0000, Beata Michalska wrote:
> While searching for a reference CPU within a given policy,
> arch_freq_get_on_cpu relies on cpumask_next_wrap to iterate over
> all available CPUs and to ensure each is verified only once.
> Recent changes to cpumask_next_wrap will handle the latter no more,
> so switching to for_each_cpu_wrap, which  preserves expected behavior
> while ensuring compatibility with the updates.

This is technically correct, but I would rather point that for
iterating over each CPU, it's better to use a dedicated iterator
instead of opencoded loop.

> Fixes: 16d1e27475f6 ("arm64: Provide an AMU-based version of arch_freq_get_on_cpu")
> Signed-off-by: Beata Michalska <[email-masked]>
> ---
>  based on arm64 for-next/amuv1-avg-freq
> 
>  arch/arm64/kernel/topology.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index a09b0551ec59..9e3583720668 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -254,7 +254,7 @@ int arch_freq_get_on_cpu(int cpu)
>  		if (!housekeeping_cpu(cpu, HK_TYPE_TICK) ||
>  		    time_is_before_jiffies(last_update + msecs_to_jiffies(AMU_SAMPLE_EXP_MS))) {
>  			struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> -			int ref_cpu = cpu;
> +			int ref_cpu;
>  
>  			if (!policy)
>  				return -EINVAL;
> @@ -265,11 +265,15 @@ int arch_freq_get_on_cpu(int cpu)
>  				return -EOPNOTSUPP;
>  			}
>  
> -			do {
> -				ref_cpu = cpumask_next_wrap(ref_cpu, policy->cpus,
> -							    start_cpu, true);
> -
> -			} while (ref_cpu < nr_cpu_ids && idle_cpu(ref_cpu));
> +			for_each_cpu_wrap(ref_cpu, policy->cpus, cpu + 1) {
> +				if (ref_cpu == start_cpu) {
> +					/* Prevent verifying same CPU twice */
> +					ref_cpu = nr_cpu_ids;
> +					break;

If start_cpu == cpu, and you begin with 'cpu + 1', you will never
check the 'cpu' for idle, right? Maybe like this?

        unsigned int start_cpu = cpu + 1;

> +				}
> +				if (!idle_cpu(ref_cpu))
> +					break;
> +			}
>  
>  			cpufreq_cpu_put(policy);
>  
> -- 
> 2.25.1