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

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Message-ID: <CALCETrUOpU8QT6zG4QUtD8sfBZMDEMbNstz0f0EiW99=vrFOYQ@mail.gmail.com>
Date:	Tue, 2 Jun 2015 17:58:25 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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 5:39 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> On Tue, Jun 02, 2015 at 05:21:27PM -0700, Andy Lutomirski wrote:
>> On Tue, Jun 2, 2015 at 4:38 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> > On Tue, Jun 02, 2015 at 11:03:26PM +0200, Thomas Gleixner wrote:
>> >>
>> >>
>> >> On Tue, 2 Jun 2015, Andi Kleen wrote:
>> >>
>> >> > > There's the code in tsc_msr.c.  It should be relatively
>> >> > > straightforward to extend it to cover everything that intel_pstate
>> >> > > supports.
>> >> >
>> >> > That's a good idea, but we still need an always working fallback when the
>> >> > model number is not available. So Adrian's patch is needed in any
>> >> > case.
>> >>
>> >> Nonsense. The slow calibration is already a working fallback.
>> >
>> > Please read Adrian's description again. It's not working when the PIT read is
>> > too slow. That is when the new algorithm is needed.
>> >
>>
>> tglx's suggestion was to use slow calibration as a fallback.
>
> You mean the last fallback we have today?
>
> That one doesn't work if the PIT read is too slow.
>
> And Adrian's patch is fixing that.

Then the changelog should say that I think.  The current text says
"Fast TSC calibration will always fail", which, to me, suggests that
either the slow calibration will work or that the changelog message
should be changed.

--Andy
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