linux-kernel - Re: [PATCH] cpufreq: CPPC: use 10ms delay instead of 2us to avoid high error

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Message-ID: <CAJZ5v0gQ7vak9DaEmLKe6un60Gcpj7VtmxdjPwuXTi=P=KJjbA@mail.gmail.com>
Date:   Wed, 29 Mar 2023 20:43:34 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Yang Shi <yang@...amperecomputing.com>
Cc:     rafael@...nel.org, viresh.kumar@...aro.org,
        scott@...amperecomputing.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: CPPC: use 10ms delay instead of 2us to avoid
 high error

On Tue, Mar 28, 2023 at 9:39 PM Yang Shi <yang@...amperecomputing.com> wrote:
>
> When testing CPPC cpufreq on our platform, we noticed the error may be quite
> high and the high error may happen quite often.  For example, on a platform
> with a maximum frequency of 2.8GHz when the CPUs were fully loaded (100% load),
> we saw cpuinfo_cur_freq may show 4GHz, it means the error is > 40%.  And the
> high error (> 1%) happened 256 times out of 2127 samples (sampled every 3
> seconds) in an approximate 2hrs test.

The description above is a bit cryptic IMV.  For example, it is not
particularly clear what "high error" means.

> We tried to enlarge the delay, and tested with 100us, 1ms and 10ms.  The
> below is the results.
>
> 100us:
> The highest error is 4GHz, 22 times out of 3623 samples
>
> 1ms:
> The highest error is 3.3GHz, 3 times out of 2814 samples
>
> 10ms:
> No high error anymore
>
> Increase the measurement delay in cppc_cpufreq_get_rate to 10ms to avoid
> high measurement errors.
>
> Signed-off-by: Yang Shi <yang@...amperecomputing.com>
> ---
>  drivers/cpufreq/cppc_cpufreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 022e3555407c..c2bf65448d3d 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -851,7 +851,7 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>         if (ret)
>                 return ret;
>
> -       udelay(2); /* 2usec delay between sampling */
> +       mdelay(10); /* 10msec delay between sampling */

This function can be called with interrupts off, so it cannot spin for 10 ms.

>         ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
>         if (ret)
> --