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

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Date:	Mon, 7 Mar 2016 14:42:10 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	"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>,
	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 Mon, Mar 7, 2016 at 2:32 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Mar 07, 2016 at 02:15:47PM +0100, Rafael J. Wysocki wrote:
>> On Mon, Mar 7, 2016 at 9:00 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
>> > Sure I know all that. But that, to me, seems like an argument for why
>> > you should have done this a long time ago.
>>
>> While I generally agree with this, I don't quite see why cleaning that
>> up necessarily has to be connected to the current patch series which
>> is my point.
>
> Ah OK, fair enough I suppose. But someone should stick this on their
> TODO list, we should not 'forget' about this (again).

Sure.

>> > But I do think something wants to be done here.
>>
>> So here's what I can do for the "fast switch" thing.
>>
>> There is the fast_switch_possible policy flag that's necessary anyway.
>> I can make notifier registration fail when that is set for at least
>> one policy and I can make the setting of it fail if at least one
>> notifier has already been registered.
>>
>> However, without spending too much time on chasing code dependencies i
>> sort of suspect that it will uncover things that register cpufreq
>> notifiers early and it won't be possible to use fast switch without
>> sorting that out.
>
> The two x86 users don't register notifiers when CONSTANT_TSC, which
> seems to be the right thing.
>
> Much of the other users seem unlikely to be used on x86, so I suspect
> the initial fallout will be very limited.

OK, let me try this then.

> *groan* modules, cpufreq allows drivers to be modules, so init sequences
> are poorly defined at best :/ Yes that blows.

Yup.

>> And that won't even change anything apart from
>> removing some code that has not worked for quite a while already and
>> nobody noticed.
>
> Which is always a good thing, but yes, we can do this later.
>
>> It is doable for the "fast switch" thing, but it won't help in all of
>> the other cases when notifications are not reliable.
>
> Right, you can maybe add a 'NOTIFIERS_BROKEN' flag to the intel_p_state
> and HWP drivers or so, and trigger off of that.

Something like that, yes.