linux-kernel - Re: [QUESTION] The delay between sampling for CPPC

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Date:   Fri, 24 Mar 2023 08:55:44 -0700
From:   Yang Shi <yang@...amperecomputing.com>
To:     rafael@...nel.org, viresh.kumar@...aro.org,
        george.cherian@...ium.com
Cc:     scott@...amperecomputing.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [QUESTION] The delay between sampling for CPPC

Gently ping...


On 3/20/23 10:21 AM, Yang Shi wrote:
> Hi folks,
>
> When testing CPPC cpufreq on our platform, we noticed the error may be 
> quite high and it (high error) may happen quite often. For example, 
> our platform max freq is 2.8GHz, when the CPUs were fully loaded (100% 
> load), we saw cpuinfo_cur_freq may show 4GHz, it means > 40% error. 
> And the high error (> 1%) happened 256 times out of 2127 samples 
> (sampled every 3 seconds) in 2hrs test.
>
> We tried to change the delay to 100us, things got improved although we 
> still saw high error, but it happened much less often (22 times out of 
> 3623 samples).
>
> So why was 2us used as the delay? Does it make sense to use a larger 
> delay, for example, 100us used by our test, or even larger, for 
> example, ms granularity?
>
> Thanks,
> Yang