Re: [PATCH] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD

From	Thomas Graziadei <thomas.graziadei@omicronenergy.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD
Date	2016-06-01 14:10 +0200
Message-ID	<rFd8e-8fg-45@gated-at.bofh.it> (permalink)
References	<rESwO-3JM-13@gated-at.bofh.it> <rF173-wS-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 06/01/2016 01:11 AM, John Stultz wrote:
> On Tue, May 31, 2016 at 6:06 AM, Thomas Graziadei
> <thomas.graziadei@omicronenergy.com> wrote:
>> From: Thomas Graziadei <thomas.graziadei@omicronenergy.com>
>>
>> The user notices the problem in a raw and real time drift, calling
>> clock_gettime with CLOCK_REALTIME / CLOCK_MONOTONIC_RAW on a system
>> with no ntp correction taking place (no ntpd or ptp stuff running).
>
> Hmm.. Curious. Was it actually drifting, or was it just
> oscillating/ringing near the RAW clock's value?

It is actually drifting.

This is the output from a little test program:

realtime  : 1464775074:846282133
raw time  : 1054:851963700
drift_real: 999402ns

total duration: 1000s 158517540ns

>
>> The problem is, that old_vsyscall_fixup adds an extra 1ns even though
>> xtime_nsec is already held in full nsecs and the remainder in this
>> case is 0. Do the rounding up buisness only if needed.
>
> The patch looks ok. But I'm curious what architecture you were seeing
> this on (ia64, powerpc?), as it would be much nicer to have those
> architectures migrate off of the old low-res vsyscall calculation and
> use the newer method with sub-ns precision, instead of trying to
> further fix up the deprecated method.
>
> I had submitted a patch to convert ia64 awhile back, but I don't
> recall getting much feedback.
>

We are using a powerpc architecture.

I guess you are right, it would be nicer to use the new method but then 
on the other hand, this timing topic is rather new to me.

> thanks
> -john
>

thanks,
Thomas

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[PATCH] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD Thomas Graziadei <thomas.graziadei@omicronenergy.com> - 2016-05-31 16:10 +0200
  Re: [PATCH] timekeeping: Fix 1ns/tick drift with GENERIC_TIME_VSYSCALL_OLD John Stultz <john.stultz@linaro.org> - 2016-06-01 01:20 +0200
    Re: [PATCH] timekeeping: Fix 1ns/tick drift with  GENERIC_TIME_VSYSCALL_OLD Thomas Graziadei <thomas.graziadei@omicronenergy.com> - 2016-06-01 14:10 +0200

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