Re: [PATCH v5 7/7] arm64/amu: Use capacity_ref_freq to set AMU ratio

[Will Deacon]

On Sat, Nov 04, 2023 at 11:59:07AM +0100, Vincent Guittot wrote:
> Use the new capacity_ref_freq to set the ratio that is used by AMU for
> computing the arch_scale_freq_capacity().
> This helps to keep everything aligned using the same reference for
> computing CPUs capacity.
> 
> The default value of the ratio (stored in per_cpu(arch_max_freq_scale))
> ensures that arch_scale_freq_capacity() returns max capacity until it is
> set to its correct value with the cpu capacity and capacity_ref_freq.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/topology.c  | 26 ++++++++++++++------------
>  drivers/base/arch_topology.c  | 12 +++++++++++-
>  include/linux/arch_topology.h |  1 +
>  3 files changed, 26 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 817d788cd866..615c1a20129f 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -82,7 +82,12 @@ int __init parse_acpi_topology(void)
>  #undef pr_fmt
>  #define pr_fmt(fmt) "AMU: " fmt
>  
> -static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale);
> +/*
> + * Ensure that amu_scale_freq_tick() will return SCHED_CAPACITY_SCALE until
> + * the CPU capacity and its associated frequency have been correctly
> + * initialized.
> + */
> +static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) =  1UL << (2 * SCHED_CAPACITY_SHIFT);

This doesn't seem to match the comment? SCHED_CAPACITY_SCALE doesn't have
the '2 *' multiplier.

>  static DEFINE_PER_CPU(u64, arch_const_cycles_prev);
>  static DEFINE_PER_CPU(u64, arch_core_cycles_prev);
>  static cpumask_var_t amu_fie_cpus;
> @@ -112,14 +117,14 @@ static inline bool freq_counters_valid(int cpu)
>  	return true;
>  }
>  
> -static int freq_inv_set_max_ratio(int cpu, u64 max_rate, u64 ref_rate)
> +void freq_inv_set_max_ratio(int cpu, u64 max_rate)
>  {
> -	u64 ratio;
> +	u64 ratio, ref_rate = arch_timer_get_rate();
>  
>  	if (unlikely(!max_rate || !ref_rate)) {
> -		pr_debug("CPU%d: invalid maximum or reference frequency.\n",
> +		WARN_ONCE(1, "CPU%d: invalid maximum or reference frequency.\n",
>  			 cpu);
> -		return -EINVAL;
> +		return;
>  	}
>  
>  	/*
> @@ -139,12 +144,12 @@ static int freq_inv_set_max_ratio(int cpu, u64 max_rate, u64 ref_rate)
>  	ratio = div64_u64(ratio, max_rate);
>  	if (!ratio) {
>  		WARN_ONCE(1, "Reference frequency too low.\n");
> -		return -EINVAL;
> +		return;
>  	}
>  
> -	per_cpu(arch_max_freq_scale, cpu) = (unsigned long)ratio;
> +	WRITE_ONCE(per_cpu(arch_max_freq_scale, cpu), (unsigned long)ratio);

Why is WRITE_ONCE() now needed?

> -	return 0;
> +	return;

Pointless 'return;' statement.

Will