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

[Yang Shi]

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