Re: [PATCH v5] arm64: topology: fix arch_freq_get_on_cpu() overflow above 4.19 GHz

[Dietmar Eggemann]

On 17.09.26 20:20, Oleg Keri wrote:
> arch_freq_get_on_cpu() computes the product of the frequency scale and
> the reference frequency as a u64, but assigns it to an unsigned int
> before shifting it back down:
> 
> 	freq = scale * arch_scale_freq_ref(cpu);
> 	freq >>= SCHED_CAPACITY_SHIFT;
> 
> The product is truncated to 32 bits before the shift, so the result
> wraps once arch_scale_freq_ref() exceeds 2^32 / SCHED_CAPACITY_SCALE,
> i.e. 4194304 kHz.
> 
> On a Snapdragon X2 Elite (Glymur) laptop, whose boost OPP is 4723200
> kHz, cpuinfo_avg_freq reports 524283 kHz instead of ~4723200 kHz while
> the CPU demonstrably runs at the boost frequency: a fixed workload
> completes in 1.72 s at the 4723200 kHz OPP versus 2.01 s at 4032000
> kHz, matching the 1.171 frequency ratio.
> 
> Compute it with cap_scale(), turned into a static inline taking u64 and
> moved to <linux/sched/topology.h> so it is usable outside kernel/sched.
> 
> Fixes: 16d1e27475f6 ("arm64: Provide an AMU-based version of arch_freq_get_on_cpu")
> Signed-off-by: Oleg Keri <okerixx@xxxxxxxxx>

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>

> ---
> Changes in v5:
> - cap_scale() becomes a static inline taking u64 arguments instead of
>   a macro, as Dietmar proposed and Peter agreed, after Peter pointed out
>   that the macro relies on one operand being u64.  It now lives in
>   <linux/sched/topology.h>, where SCHED_CAPACITY_SHIFT is visible,
>   rather than <linux/topology.h>.
> - Patch 2/2 of v4 is dropped: it duplicated Ananthu C V's series [1],
>   which I tested instead.  This fix is needed with that series, since
>   it puts the reference above 4194304 kHz from boot.
> - v4: https://lore.kernel.org/all/20260917125112.2283-1-okerixx@xxxxxxxxx/
> 
> [1] https://lore.kernel.org/all/20260908-schedutil-boost-frequency-handling-v2-0-25312a713699@xxxxxxxxxxxxxxxx/
> 
>  arch/arm64/kernel/topology.c   | 6 ++----
>  include/linux/sched/topology.h | 5 +++++
>  kernel/sched/sched.h           | 2 --
>  3 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index d28438f8b83f..39dd7f8575cd 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -19,6 +19,7 @@
>  #include <linux/init.h>
>  #include <linux/percpu.h>
>  #include <linux/sched/isolation.h>
> +#include <linux/sched/topology.h>
>  #include <linux/xarray.h>
>  
>  #include <asm/cpu.h>
> @@ -186,7 +187,6 @@ int arch_freq_get_on_cpu(int cpu)
>  	struct amu_cntr_sample *amu_sample;
>  	unsigned int start_cpu = cpu;
>  	unsigned long last_update;
> -	unsigned int freq = 0;
>  	u64 scale;
>  
>  	if (!amu_fie_cpu_supported(cpu) || !arch_scale_freq_ref(cpu))
> @@ -245,9 +245,7 @@ int arch_freq_get_on_cpu(int cpu)
>  	 * (see amu_scale_freq_tick for details)
>  	 */
>  	scale = arch_scale_freq_capacity(cpu);
> -	freq = scale * arch_scale_freq_ref(cpu);
> -	freq >>= SCHED_CAPACITY_SHIFT;
> -	return freq;
> +	return cap_scale(arch_scale_freq_ref(cpu), scale);
>  }
>  
>  static void amu_fie_setup(const struct cpumask *cpus)
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index b5d9d7c2b8ad..922b2f015e89 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -234,6 +234,11 @@ static inline void rebuild_sched_domains_energy(void)
>  }
>  #endif
>  
> +static inline u64 cap_scale(u64 value, u64 scale)
> +{
> +	return value * scale >> SCHED_CAPACITY_SHIFT;
> +}
> +
>  #ifndef arch_scale_cpu_capacity
>  /**
>   * arch_scale_cpu_capacity - get the capacity scale factor of a given CPU.
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 6c3ad70e58b8..45796fccdc84 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -236,8 +236,6 @@ static inline int task_has_dl_policy(struct task_struct *p)
>  	return dl_policy(p->policy);
>  }
>  
> -#define cap_scale(v, s)		((v)*(s) >> SCHED_CAPACITY_SHIFT)
> -
>  static inline void update_avg(u64 *avg, u64 sample)
>  {
>  	s64 diff = sample - *avg;
> 
> base-commit: 0d9d0dbf2fddcff5859d623e90ca73c4054276e1