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

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Date:   Tue, 4 Apr 2023 12:07:17 -0700
From:   Yang Shi <yang@...amperecomputing.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Pierre Gondois <pierre.gondois@....com>
Cc:     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 3/29/23 11:43 AM, Rafael J. Wysocki wrote:
> 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.

Per Pierre's comment, the delay may still be ms. Is it still too long? A 
quick look at the code shows cpufreq_quick_get() is the only caller with 
irq off IIRC. So can we have another callback for it, for example, 
get_quick() which does spin for shorter time (for example, keep 2us 
delay). Then have ->get() callback use longer delay?


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