Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Wed, 29 Mar 2023 20:43:34 +0200 | Subject | Re: [PATCH] cpufreq: CPPC: use 10ms delay instead of 2us to avoid high error |
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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.
> 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.
> ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1); > if (ret) > --
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