Re: cpufreq on demand governor sampling rate restricted to HZ even on NO_HZ kernels

[Thomas Renninger]

On Friday 30 January 2009 18:28:16 Pallipadi, Venkatesh wrote:
> On Fri, 2009-01-30 at 06:59 -0800, Thomas Renninger wrote:
> > Hi,
> >
> > depending on HZ set to:
> >
> > 100
> > 250
> > 1000
> >
> > the ondemand governor is currently limited to poll the CPU load
> > and adjust the frequency (sampling rate sysfs variable) every:
> >
> > 200ms
> > 80ms
> > 20ms
> >
> > This limitation does not consider NO_HZ which looks wrong?
> > If this is correct, can someone give me a pointer, I'd like
> > to understand why.
>
> That is wrong.
I think I got it now. I first thought my above assumptions are wrong.
Double checking tells me that above assumptions are right, but you
agree that the ondemand minimum sampling is wrong, is that correct?

Can a system fall back to periodic timers, once NO_HZ is active?
Or is NO_HZ always active, once no_hz=off boot param and timer
requirements are analyzed?
Then a rather low value could just be added to ondemand if no_hz is
active, checking what is allowed to be written to ondemand/sampling_rate
and that's it.
What could be sane minimum sampling rate value, the ondemand governor
would set the deferrable timer to?

> ondemand sampling_rate should not limit the sampling rate
> based on HZ when NO_HZ is configured. The idle statistics is not limited
> by HZ rate with NO_HZ, as we will have idle microaccounting.
>
> > If NO_HZ can/should go down to 20ms polling and more (current
> > CPUs are able to switch fast enough, so that the ondemand governor
> > would calculate the default polling interval below 80ms for them),
> > this would hurt in respect of C-states at some point.
> >
> > For performance reasons, one wants to poll as much as possible, for
> > powersaving reasons (C-states), one wants to poll as seldom as
> > possible.
> >
> > I wonder whether it makes sense to dynamically adjust the polling
> > interval (e.g. by a hint (and initial wakeup) from the scheduler or
> > taking C-states into account) to:
> >   - increase the sampling rate, e.g. based on context switching
> >     activity
> >   - lower sampling rate when the system is idle (to gain
> >     full C-state efficiency)
> > Or in what other way deep C-states could be taken into account
> > in respect of ondemand polling?
>
> ondemand polling uses deferrable timer and hence will not be called
> frequently on a totally idle CPU. The main reason we did not do the
> dynamic sampling_rate is because it increases the ondemand response time
> with a sudden increase of load, which is not liked by most workloads.

Neat. I didn't know about the deferrable timer, thanks.

    Thomas
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