Re: [PATCH V3 3/9] cpufreq: Cap the default transition delay value to 10 ms

[Leonard Crestez]

On Fri, 2017-07-28 at 10:58 +0530, Viresh Kumar wrote:
> On 27-07-17, 19:54, Leonard Crestez wrote:
> > On Wed, 2017-07-26 at 11:36 +0530, Viresh Kumar wrote:
> > > Without this patch the sampling rate of ondemand governor will be 109
> > > ms. And after this patch it would be capped at 10 ms. Why would that
> > > screw up anyone's setup ? I don't have an answer to that right now.
> > On a closer look it seems that most of the time is actually spent at
> > low cpufreq though (90%+).
> > 
> > Your change makes it so that even something like "sleep 1; cat
> > scaling_cur_freq" raises the frequency to the maximum.
> Why?
> 
> > 
> > This happens
> > enough that even if you do it in a loop you will never seeÂthe minimum
> > frequency. It seems there is enough internal bookkeeping on such a
> > wakeup that it takes more than 10ms and enough for a reevaluation of
> > cpufreq until cat returns the value?!

> At this point I really feel that this is a hardware specific problem
> and it was working by chance until now. And I am not sure if we
> shouldn't be stopping this patch from getting merged just because of
> that.

Yes, I agree. Something is fishy here but most likely your patch just
expose the problem.

> At least you can teach your distribution to go increase the sampling
> rate from userspace to make it all work.
> 
> Can you try one more thing? Try using schedutil governor and see how
> it behaves ?

I don't have the time to investigate this properly right now.

> > I found this by enabling the power:cpu_frequency tracepoint event and
> > checking for deltas with a script. Enabling CPU_FREQ_STATÂshow this:
> > 
> > time_in_state:
> > 
> > 396000 1609
> So we still stay at the lowest frequency most of the time.

Yes

> Maybe can you compare these values with and without this patch to let
> us know?

Without the patch it is always at low freq. Sampling at a lower
frequency means spikes get ignored.