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

Yury Norov
2025-02-18T20:14:26Z

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 <beata.michalska@arm.com>
> ---
> 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