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

[Yang Shi <yang@os.amperecomputing.com>]

On 3/29/23 11:43 AM, Rafael J. Wysocki wrote:
> On Tue, Mar 28, 2023 at 9:39 PM Yang Shi <yang@os.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.

The max freq is approximately 2.8GHz, but reading cpuinfo_cur_freq 
showed 3.x GHz ~ 4GHz sometimes. The error seems high.

>
>> 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@os.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.

I didn't notice it may be called with irq off, if so 10ms spin sounds 
too long. Is 100us fine? It can't eliminate the high error completely, 
but alleviate it a lot. Or any other suggestion?

Thanks,
Yang

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