linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Thomas Gleixner <tglx@linutronix.de>
To: Christopher Hall <christopher.s.hall@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	jesus.sanchez-palencia@intel.com, gavin.hindman@intel.com,
	liam.r.girdwood@intel.com, Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	John Stultz <john.stultz@linaro.org>
Subject: Re: TSC to Mono-raw Drift
Date: Fri, 19 Oct 2018 17:25:02 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1810191720590.6075@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20181015160945.5993-1-christopher.s.hall@intel.com>

Christopher,

Please Cc LKML on such issues in the future.

On Mon, 15 Oct 2018, Christopher Hall wrote:

Leaving context around for new readers:

> Problem Statement:
> 
> The TSC clocksource mult/shift values are derived from CPUID[15H], but the
> monotonic raw clock value is not equal to TSC in nominal nanoseconds, i.e.
> the timekeeping code is not accurately transforming TSC ticks to nominal
> nanoseconds based on CPUID[15H}.
> 
> The included code calculates the drift between nominal TSC nanoseconds and
> the monotonic raw clock.
> 
> Background:
> 
> Starting with 6th generation Intel CPUs, the TSC is "phase locked" to the
> Always Running Timer (ART). The relation between TSC and ART is read from
> CPUID[15H]. Details of the TSC-ART relation are in the "Invariant
> Timekeeping" section of the SDM.
> 
> CPUID[15H].ECX returns the nominal frequency of ART (or crystal frequency).
> CPU feature TSC_KNOWN_FREQ indicates that tsc_khz (tsc.c) is derived from
> CPUID[15H]. The calculation is in tsc.c:native_calibrate_tsc().
> 
> When the TSC clocksource is selected, the timekeeping code uses mult/shift
> values to transform TSC into nanoseconds. The mult/shift value is determined
> using tsc_khz.
> 
> Example Output:
> 
> Running for 3 seconds trial 1
> Scaled TSC delta: 3000328845
> Monotonic raw delta: 3000329117
> Ran for 3 seconds with 272 ns skew
> 
> Running for 3 seconds trial 2
> Scaled TSC delta: 3000295209
> Monotonic raw delta: 3000295482
> Ran for 3 seconds with 273 ns skew
> 
> Running for 3 seconds trial 3
> Scaled TSC delta: 3000262870
> Monotonic raw delta: 3000263142
> Ran for 3 seconds with 272 ns skew
> 
> Running for 300 seconds trial 4
> Scaled TSC delta: 300000281725
> Monotonic raw delta: 300000308905
> Ran for 300 seconds with 27180 ns skew
> 
> The skew between tsc and monotonic raw is about 91 PPB. 
> 
> System Information:
> 
> CPU model string: Intel(R) Core(TM) i5-6600 CPU @ 3.30GHz
> Kernel version tested: 4.14.71-rt44
> 	NOTE: The skew seems to be insensitive to kernel version after
> 		introduction of TSC_KNOWN_FREQ capability
> 
> >From CPUID[15H]:
> 	Time Stamp Counter/Core Crystal Clock Information (0x15):
> 		TSC/clock ratio = 276/2
> 		nominal core crystal clock = 24000000 Hz (table lookup)
> 
> TSC kHz used to calculate mult/shift value: 3312000

Now the most interesting information here would be the resulting mult/shift
value which the kernel uses. Can you please provide that?

Thanks,

	Thomas

       reply	other threads:[~2018-10-19 15:25 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20181015160945.5993-1-christopher.s.hall@intel.com>
2018-10-19 15:25 ` Thomas Gleixner [this message]
2018-10-19 18:34   ` John Stultz
2018-10-19 18:39     ` John Stultz
2018-10-19 18:37   ` Thomas Gleixner
2018-10-19 18:48     ` John Stultz
2018-10-19 18:57       ` Thomas Gleixner
2018-10-19 19:21         ` John Stultz
2018-10-19 20:50           ` Thomas Gleixner
2018-10-19 22:36             ` John Stultz
2018-10-23 18:31               ` John Stultz
2018-10-24 14:51                 ` Miroslav Lichvar
2018-10-24 17:32                   ` Christopher Hall
2018-10-25 11:49                     ` Miroslav Lichvar
2018-11-01 17:41                   ` Thomas Gleixner
2018-11-02 10:26                     ` Miroslav Lichvar
2018-11-02 11:27                       ` Thomas Gleixner
2018-11-01 17:44                 ` Thomas Gleixner
2018-11-01 17:56                   ` John Stultz
2018-11-01 18:03                     ` Thomas Gleixner
2018-11-02 11:20                       ` Miroslav Lichvar
2018-11-02 11:25                         ` Thomas Gleixner
2018-11-02 12:31                           ` Miroslav Lichvar

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=alpine.DEB.2.21.1810191720590.6075@nanos.tec.linutronix.de \
    --to=tglx@linutronix.de \
    --cc=christopher.s.hall@intel.com \
    --cc=gavin.hindman@intel.com \
    --cc=hpa@zytor.com \
    --cc=jesus.sanchez-palencia@intel.com \
    --cc=john.stultz@linaro.org \
    --cc=liam.r.girdwood@intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-rt-users@vger.kernel.org \
    --cc=peterz@infradead.org \
    --subject='Re: TSC to Mono-raw Drift' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).