netdev - Re: [RFC v5 3/6] Add history to cross timestamp interface supporting slower devices

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Date:	Fri, 8 Jan 2016 15:04:28 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Christopher Hall <christopher.s.hall@...el.com>
cc:	John Stultz <john.stultz@...aro.org>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	"Stanton, Kevin B" <kevin.b.stanton@...el.com>
Subject: Re: [RFC v5 3/6] Add history to cross timestamp interface supporting
 slower devices

On Thu, 7 Jan 2016, Christopher Hall wrote:
> On Wed, 06 Jan 2016 11:37:23 -0800, John Stultz <john.stultz@...aro.org>
> wrote:
> > I've not done a super close reading here. But is it very likely the
> > the history_ref->cs_seq is the same as the captured seq? I thought
> > this history_ref was to allow old cross stamps to be used to improve
> > the back-calculation of the time at the given cycle value. So throwing
> > them out if they are older then the last tick seems strange.
> 
> Maybe this needs more explanation. The clocksource sequence (cs_seq) is
> incremented for each change in clocksource. I use this to detect a rare corner
> case where the clocksource is changed from (on x86 anyway) TSC and then back.
> If the history crosses one of these changes then interpolation shouldn't be
> attempted (return error). It's not really enough when using the history to
> just check that the current clocksource is equal to the one used at the start
> of the history. The clocksource must not have changed at all. To answer your
> question, it's not at all likely that this would occur.

You can flush the history when a clocksource change happens. No need to add
extra data to the core structures.

Thanks,

	tglx