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

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Date:	Fri, 4 Mar 2016 23:32:22 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2 6/10] cpufreq: Support for fast frequency switching

On Fri, Mar 4, 2016 at 11:18 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
> On 03/03/2016 07:07 PM, Rafael J. Wysocki wrote:
>> +void cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>> +                             unsigned int target_freq, unsigned int relation)
>> +{
>> +     unsigned int freq;
>> +
>> +     freq = cpufreq_driver->fast_switch(policy, target_freq, relation);
>> +     if (freq != CPUFREQ_ENTRY_INVALID) {
>> +             policy->cur = freq;
>> +             trace_cpu_frequency(freq, smp_processor_id());
>> +     }
>> +}
>
> Even if there are platforms which may change the CPU frequency behind
> cpufreq's back, breaking the transition notifiers, I'm worried about the
> addition of an interface which itself breaks them. The platforms which
> do change CPU frequency on their own have probably evolved to live with
> or work around this behavior. As other platforms migrate to fast
> frequency switching they might be surprised when things don't work as
> advertised.

Well, intel_pstate doesn't do notifies at all, so anything depending
on them is already broken when it is used.  Let alone the hardware
P-states coordination mechanism (HWP) where the frequency is
controlled by the processor itself entirely.

That said I see your point.

> I'm not sure what the easiest way to deal with this is. I see the
> transition notifiers are the srcu type, which I understand to be
> blocking. Going through the tree and reworking everyone's callbacks and
> changing the type to atomic is obviously not realistic.

Right.

> How about modifying cpufreq_register_notifier to return an error if the
> driver has a fast_switch callback installed and an attempt to register a
> transition notifier is made?

That sounds like a good idea.

There also is the CPUFREQ_ASYNC_NOTIFICATION driver flag that in
principle might be used as a workaround, but I'm not sure how much
work that would require ATM.

> In the future, perhaps an additional atomic transition callback type can
> be added, which platform/driver owners can switch to if they wish to use
> fast transitions with their platform.

I guess you mean an atomic notification mechanism based on registering
callbacks?  While technically viable that's somewhat risky, because we
are in a fast path and allowing anyone to add stuff to it would be
asking for trouble IMO.

Thanks,
Rafael