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

[Sumit Gupta]

On 15/06/23 00:29, Sumit Gupta wrote:
> 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@xxxxxxxxxx/
> [2] 
> https://lore.kernel.org/lkml/cde1d8a9-3a21-e82b-7895-40603a14d898@xxxxxxxxxx/T/#mb898a75fd0c72d166b26b04da3ad162afe068a82

Observed another issue where CPUFREQ is coming too high when the
performance governor is set as default.

Below change solves that by using the new API arch_freq_get_on_cpu()
if present over the existing one, while verifying the currently set 
frequency.

 diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
 index 62face349fd2..2c74e70f701e 100644
 --- a/drivers/cpufreq/cpufreq.c
 +++ b/drivers/cpufreq/cpufreq.c
 @@ -1761,9 +1761,12 @@ static unsigned int 
cpufreq_verify_current_freq(struct cpufreq_policy *policy, b
  {
 		unsigned int new_freq;

 -       new_freq = cpufreq_driver->get(policy->cpu);
 -       if (!new_freq)
 -               return 0;
 +       new_freq = arch_freq_get_on_cpu(policy->cpu);
 +       if (!new_freq) {
 +               new_freq = cpufreq_driver->get(policy->cpu);
 +               if (!new_freq)
 +                       return 0;
 +       }


Best Regards,
Sumit Gupta