Re: [PATCH v2 6/10] cpufreq: Support for fast frequency switching

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Mar 06, 2016 at 03:17:09AM +0100, Rafael J. Wysocki wrote:
> > Agreed, fail the stuff hard.
> >
> > Simply make cpufreq_register_notifier a __must_check function and add
> > error handling to all call sites.
> 
> Quite frankly, I don't see a compelling reason to do anything about
> the notifications at this point.
> 
> The ACPI driver is the only one that will support fast switching for
> the time being and on practically all platforms that can use the ACPI
> driver the transition notifications cannot be relied on anyway for a
> few reasons.  First, if intel_pstate or HWP is in use, they won't be
> coming at all.  Second, anything turbo will just change frequency at
> will without notifying (like HWP).  Finally, if they are coming,
> whoever receives them is notified about the frequency that is
> requested and not the real one, which is misleading, because (a) the
> request may just make the CPU go into the turbo range and then see
> above or (b) if the CPU is in a platform-coordinated package, its
> request will only be granted if it's the winning one.

Sure I know all that. But that, to me, seems like an argument for why
you should have done this a long time ago.

Someone registering a notifier you _know_ won't be called reliably is a
sure sign of borkage. And you want to be notified (pun intended) of
borkage.

So the alternative option to making the registration fail, is making the
registration WARN (and possibly disable fast support in the driver).

But I do think something wants to be done here.

> > No no no, that's just horrible. Why would you want to keep this
> > notification stuff alive? If your platform can change frequency 'fast'
> > you don't want notifiers.
> 
> I'm not totally sure about that.

I am, per definition, if you need to call notifiers, you're not fast.

I would really suggest making that a hard rule and enforcing it.

> > What's the point of a notification that says: "At some point in the
> > random past my frequency has changed, and it likely has changed again
> > since then, do 'something'."
> >
> > That's pointless. If you have dependent clock domains or whatever, you
> > simply _cannot_ be fast.
> >
> 
> What about thermal?  They don't need to get very accurate information,
> but they need to be updated on a regular basis.  It would do if they
> get averages instead of momentary values (and may be better even).

Thermal, should be an integral part of cpufreq, but if they need a
callback from the switching hook (and here I would like to remind
everyone that this is inside scheduler hot paths and the more code you
stuff in the harder the performance regressions will hit you in the
face) it can get a direct function call. No need for no stinking
notifiers.