Re: [PATCH] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Sumit Gupta <sumitg@nvidia.com>]

On 06/06/23 21:27, Beata Michalska wrote:
> External email: Use caution opening links or attachments
> 
> 
> With the Frequency Invariance Engine (FIE) being already wired up with
> sched tick and making use of relevant (core counter and constant
> counter) AMU counters, getting the current frequency for a given CPU
> on supported platforms, can be achieved by utilizing the frequency scale
> factor which reflects an average CPU frequency for the last tick period
> length.
> 
> With that at hand, arch_freq_get_on_cpu dedicated implementation
> gets enrolled into cpuinfo_cur_freq policy sysfs attribute handler,
> which is expected to represent the current frequency of a given CPU,
> as obtained by the hardware. This is exactly the type of feedback that
> cycle counters provide.
> 
> In order to avoid calling arch_freq_get_on_cpu from the scaling_cur_freq
> attribute handler for platforms that do provide cpuinfo_cur_freq, and
> yet keeping things intact for those platform that do not, its use gets
> conditioned on the presence of cpufreq_driver (*get) callback (which also
> seems to be the case for creating cpuinfo_cur_freq attribute).
> 

Tested the change with frequency switch stress test but was getting big 
delta between set and get freq.
After passing "nohz=off" and commenting "wfi" in "cpu_do_idle()", the
delta is less. This confirms that more delta is due to AMU counters
stopping at "WFI".

   +++ b/arch/arm64/kernel/idle.c
   @@ -27,7 +27,7 @@ void noinstr cpu_do_idle(void)
           arm_cpuidle_save_irq_context(&context);

           dsb(sy);
   -       wfi();
   +//     wfi();

I am not sure if the expected behavior here is right.
In our tests, we compare the last set frequency against the re-generated
value from counters to confirm that the CPU is actually running at the
requested frequency and the counters are working correct. But that won't
happen with this change.

In [1] and later in the updated patch within [2], we are busy looping
on the target CPU and avoid WFI to get the actual frequency.

Please share what you think is the right expected behavior.

[1] https://lore.kernel.org/lkml/20230418113459.12860-7-sumitg@nvidia.com/
[2] 
https://lore.kernel.org/lkml/cde1d8a9-3a21-e82b-7895-40603a14d898@nvidia.com/T/#mb898a75fd0c72d166b26b04da3ad162afe068a82