Re: [PATCH 08/23] arm64: topology: Use RCU to protect access to HK_TYPE_TICK cpumask

Chen Ridong
2026-04-22T09:35:02Z

On 2026/4/21 11:03, Waiman Long wrote:
> As the HK_TYPE_TICK cpumask is going to be changeable at run time, we
> need to use RCU to protect access to the cpumask to prevent it from
> going away in the middle of the operation.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> arch/arm64/kernel/topology.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index b32f13358fbb..48f150801689 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -173,6 +173,7 @@ void arch_cpu_idle_enter(void)
> if (!amu_fie_cpu_supported(cpu))
> return;
>
> + guard(rcu)();
> /* Kick in AMU update but only if one has not happened already */
> if (housekeeping_cpu(cpu, HK_TYPE_TICK) &&
> time_is_before_jiffies(per_cpu(cpu_amu_samples.last_scale_update, cpu)))
> @@ -187,11 +188,16 @@ int arch_freq_get_on_cpu(int cpu)
> unsigned int start_cpu = cpu;
> unsigned long last_update;
> unsigned int freq = 0;
> + bool hk_cpu;
> u64 scale;
>
> if (!amu_fie_cpu_supported(cpu) || !arch_scale_freq_ref(cpu))
> return -EOPNOTSUPP;
>
> + scoped_guard(rcu) {
> + hk_cpu = housekeeping_cpu(cpu, HK_TYPE_TICK);
> + }
> +

Should we put this into a while loop, since cpu might be changed to ref_cpu?

> while (1) {
>
> amu_sample = per_cpu_ptr(&cpu_amu_samples, cpu);
> @@ -204,16 +210,21 @@ int arch_freq_get_on_cpu(int cpu)
> * (and thus freq scale), if available, for given policy: this boils
> * down to identifying an active cpu within the same freq domain, if any.
> */
> - if (!housekeeping_cpu(cpu, HK_TYPE_TICK) ||
> + if (!hk_cpu ||
> time_is_before_jiffies(last_update + msecs_to_jiffies(AMU_SAMPLE_EXP_MS))) {
> struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> + bool hk_intersects;
> int ref_cpu;
>
> if (!policy)
> return -EINVAL;
>
> - if (!cpumask_intersects(policy->related_cpus,
> - housekeeping_cpumask(HK_TYPE_TICK))) {
> + scoped_guard(rcu) {
> + hk_intersects = cpumask_intersects(policy->related_cpus,
> + housekeeping_cpumask(HK_TYPE_TICK));
> + }
> +
> + if (!hk_intersects) {
> cpufreq_cpu_put(policy);
> return -EOPNOTSUPP;
> }

--
Best regards,
Ridong