linux-kernel - Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit

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Message-ID: <CALCETrVQX8Z-oEhNZvgkGOSZskyxoPmG3Mdz+xcZTeVRHhZtyA@mail.gmail.com>
Date:	Tue, 2 Jun 2015 13:03:25 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit_calibrate()

On Tue, Jun 2, 2015 at 12:58 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 2 Jun 2015, Andi Kleen wrote:
>> On Tue, Jun 02, 2015 at 12:41:27PM -0700, Andy Lutomirski wrote:
>> > On Tue, Jun 2, 2015 at 12:33 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > > On Thu, 21 May 2015, Adrian Hunter wrote:
>> > >
>> > >> If it takes longer than 12us to read the PIT counter lsb/msb,
>> > >> then the error margin will never fall below 500ppm within 50ms,
>> > >> and Fast TSC calibration will always fail.
>> > >
>> > > So finally the legacy PIT emulation takes longer than the 30 years old
>> > > hardware implementation. Progress!
>> > >
>> > >> This patch detects when that will happen and switches to using
>> > >> a slightly different algorithm that takes advantage of the PIT's
>> > >> latch comand.
>> > >
>> > > Is there really no smarter way to figure out the TSC frequency on
>> > > modern systems?
>> >
>> > I just asked Len this question yesterday.  intel_pstate can do it,
>> > although the algorithm is a bit gross.
>
> Well, the algorithm for that slow PIT emulation thing is definitely
> not from the straight forward kind either.
>
>> intel_pstate needs to know the model number. If you know the
>> model number sure you can do a lot better (e.g. using the
>> ref-clock fixed counter or some other methods)
>>
>> But if you don't you need something else. And at some point
>> the only thing left over is the PIT.
>
> Right, and if we dont have a way to readout the stuff because the
> kernel does not yet have support for that particular model it falls
> back to PIT slow path in the worst case.
>
> That's better than having another weird calibration routine which
> might be outdated with the next generation of PIT emulation. We've
> been there with the HPET deferred writes already. No need to have more
> of this nonsense.
>
> It's about time that Intel gets its act together and provides a proper
> architected way to figure that out. Though I fear that will require
> another 15 years of yelling (IIRC that was the time it took to get a
> constant frequency TSC).

There's the code in tsc_msr.c.  It should be relatively
straightforward to extend it to cover everything that intel_pstate
supports.

But yes, Intel should add an MSR that gives the frequency in Hz.

--Andy
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