Re: [PATCH v4 2/2] cpufreq: Pass the policy to cpufreq_driver->adjust_perf()

[Gary Guo]

On Mon Mar 16, 2026 at 8:18 AM GMT, K Prateek Nayak wrote:
> cpufreq_cpu_get() can sleep on PREEMPT_RT in presence of concurrent
> writer(s), however amd-pstate depends on fetching the cpudata via the
> policy's driver data which necessitates grabbing the reference.
>
> Since schedutil governor can call "cpufreq_driver->update_perf()"
> during sched_tick/enqueue/dequeue with rq_lock held and IRQs disabled,
> fetching the policy object using the cpufreq_cpu_get() helper in the
> scheduler fast-path leads to "BUG: scheduling while atomic" on
> PREEMPT_RT [1].
>
> Pass the cached cpufreq policy object in sg_policy to the update_perf()
> instead of just the CPU. The CPU can be inferred using "policy->cpu".
>
> The lifetime of cpufreq_policy object outlasts that of the governor and
> the cpufreq driver (allocated when the CPU is onlined and only reclaimed
> when the CPU is offlined / the CPU device is removed) which makes it
> safe to be referenced throughout the governor's lifetime.
>
> Fixes: 1d215f0319c2 ("cpufreq: amd-pstate: Add fast switch function for AMD P-State")
> Reported-by: Bert Karwatzki <spasswolf@xxxxxx>
> Closes:https://lore.kernel.org/all/20250731092316.3191-1-spasswolf@xxxxxx/ [1]
> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

Acked-by: Gary Guo <gary@xxxxxxxxxxx> # Rust

Looks like a sensible thing to do, interestingly I noted that this (as an
effect) aligns the C API to what the Rust-side cpufreq API already has.

Best,
Gary

> ---
> changelog v3..v4:
>
> o Added the Fixes tag. (Gautham, Chris Mason's review-prompts)
> ---
>  drivers/cpufreq/amd-pstate.c     |  3 +--
>  drivers/cpufreq/cpufreq.c        |  6 +++---
>  drivers/cpufreq/intel_pstate.c   |  4 ++--
>  include/linux/cpufreq.h          |  4 ++--
>  kernel/sched/cpufreq_schedutil.c |  5 +++--
>  rust/kernel/cpufreq.rs           | 13 ++++++-------
>  6 files changed, 17 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index 5faccb3d6b14..ad4b5f84773a 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -710,13 +710,12 @@ static unsigned int amd_pstate_fast_switch(struct cpufreq_policy *policy,
>  	return policy->cur;
>  }
>  
> -static void amd_pstate_adjust_perf(unsigned int cpu,
> +static void amd_pstate_adjust_perf(struct cpufreq_policy *policy,
>  				   unsigned long _min_perf,
>  				   unsigned long target_perf,
>  				   unsigned long capacity)
>  {
>  	u8 max_perf, min_perf, des_perf, cap_perf;
> -	struct cpufreq_policy *policy __free(put_cpufreq_policy) = cpufreq_cpu_get(cpu);
>  	struct amd_cpudata *cpudata;
>  	union perf_cached perf;
>  
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 2082a9e4384f..17a5b8e0ea1e 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2231,7 +2231,7 @@ EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
>  
>  /**
>   * cpufreq_driver_adjust_perf - Adjust CPU performance level in one go.
> - * @cpu: Target CPU.
> + * @policy: cpufreq policy object of the target CPU.
>   * @min_perf: Minimum (required) performance level (units of @capacity).
>   * @target_perf: Target (desired) performance level (units of @capacity).
>   * @capacity: Capacity of the target CPU.
> @@ -2250,12 +2250,12 @@ EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
>   * parallel with either ->target() or ->target_index() or ->fast_switch() for
>   * the same CPU.
>   */
> -void cpufreq_driver_adjust_perf(unsigned int cpu,
> +void cpufreq_driver_adjust_perf(struct cpufreq_policy *policy,
>  				 unsigned long min_perf,
>  				 unsigned long target_perf,
>  				 unsigned long capacity)
>  {
> -	cpufreq_driver->adjust_perf(cpu, min_perf, target_perf, capacity);
> +	cpufreq_driver->adjust_perf(policy, min_perf, target_perf, capacity);
>  }
>  
>  /**
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 51938c5a47ca..1552b2d32a34 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -3239,12 +3239,12 @@ static unsigned int intel_cpufreq_fast_switch(struct cpufreq_policy *policy,
>  	return target_pstate * cpu->pstate.scaling;
>  }
>  
> -static void intel_cpufreq_adjust_perf(unsigned int cpunum,
> +static void intel_cpufreq_adjust_perf(struct cpufreq_policy *policy,
>  				      unsigned long min_perf,
>  				      unsigned long target_perf,
>  				      unsigned long capacity)
>  {
> -	struct cpudata *cpu = all_cpu_data[cpunum];
> +	struct cpudata *cpu = all_cpu_data[policy->cpu];
>  	u64 hwp_cap = READ_ONCE(cpu->hwp_cap_cached);
>  	int old_pstate = cpu->pstate.current_pstate;
>  	int cap_pstate, min_pstate, max_pstate, target_pstate;
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index cc894fc38971..4317c5a312bd 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -372,7 +372,7 @@ struct cpufreq_driver {
>  	 * conditions) scale invariance can be disabled, which causes the
>  	 * schedutil governor to fall back to the latter.
>  	 */
> -	void		(*adjust_perf)(unsigned int cpu,
> +	void		(*adjust_perf)(struct cpufreq_policy *policy,
>  				       unsigned long min_perf,
>  				       unsigned long target_perf,
>  				       unsigned long capacity);
> @@ -617,7 +617,7 @@ struct cpufreq_governor {
>  /* Pass a target to the cpufreq driver */
>  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>  					unsigned int target_freq);
> -void cpufreq_driver_adjust_perf(unsigned int cpu,
> +void cpufreq_driver_adjust_perf(struct cpufreq_policy *policy,
>  				unsigned long min_perf,
>  				unsigned long target_perf,
>  				unsigned long capacity);
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 153232dd8276..ae9fd211cec1 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -461,6 +461,7 @@ static void sugov_update_single_perf(struct update_util_data *hook, u64 time,
>  				     unsigned int flags)
>  {
>  	struct sugov_cpu *sg_cpu = container_of(hook, struct sugov_cpu, update_util);
> +	struct sugov_policy *sg_policy = sg_cpu->sg_policy;
>  	unsigned long prev_util = sg_cpu->util;
>  	unsigned long max_cap;
>  
> @@ -482,10 +483,10 @@ static void sugov_update_single_perf(struct update_util_data *hook, u64 time,
>  	if (sugov_hold_freq(sg_cpu) && sg_cpu->util < prev_util)
>  		sg_cpu->util = prev_util;
>  
> -	cpufreq_driver_adjust_perf(sg_cpu->cpu, sg_cpu->bw_min,
> +	cpufreq_driver_adjust_perf(sg_policy->policy, sg_cpu->bw_min,
>  				   sg_cpu->util, max_cap);
>  
> -	sg_cpu->sg_policy->last_freq_update_time = time;
> +	sg_policy->last_freq_update_time = time;
>  }
>  
>  static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
> diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
> index 76faa1ac8501..a83aec198336 100644
> --- a/rust/kernel/cpufreq.rs
> +++ b/rust/kernel/cpufreq.rs
> @@ -1256,18 +1256,17 @@ impl<T: Driver> Registration<T> {
>      /// # Safety
>      ///
>      /// - This function may only be called from the cpufreq C infrastructure.
> +    /// - The pointer arguments must be valid pointers.
>      unsafe extern "C" fn adjust_perf_callback(
> -        cpu: c_uint,
> +        ptr: *mut bindings::cpufreq_policy,
>          min_perf: c_ulong,
>          target_perf: c_ulong,
>          capacity: c_ulong,
>      ) {
> -        // SAFETY: The C API guarantees that `cpu` refers to a valid CPU number.
> -        let cpu_id = unsafe { CpuId::from_u32_unchecked(cpu) };
> -
> -        if let Ok(mut policy) = PolicyCpu::from_cpu(cpu_id) {
> -            T::adjust_perf(&mut policy, min_perf, target_perf, capacity);
> -        }
> +        // SAFETY: The `ptr` is guaranteed to be valid by the contract with the C code for the
> +        // lifetime of `policy`.
> +        let policy = unsafe { Policy::from_raw_mut(ptr) };
> +        T::adjust_perf(policy, min_perf, target_perf, capacity);
>      }
>  
>      /// Driver's `get_intermediate` callback.