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

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Message-ID: <CALAqxLXphoiigM2Bk9GTJaOYM_F3BGW==cmNiuHSP1dBR56AbA@mail.gmail.com>
Date:	Thu, 7 Jan 2016 17:12:16 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Christopher Hall <christopher.s.hall@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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, Jan 7, 2016 at 5:07 PM, Christopher Hall
<christopher.s.hall@...el.com> wrote:
> On Wed, 06 Jan 2016 11:37:23 -0800, John Stultz <john.stultz@...aro.org>
> wrote:
>> On Mon, Jan 4, 2016 at 4:45 AM, Christopher S. Hall
>>>
>>> +               if (!history_ref || history_ref->cs_seq != cs_seq ||
>>
>>
>> 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.

Yes. Apologies. I had mis-skimmed the cs_seq increment as happening in
the update_wall_time not setup_internals.

Instead of cs_seq, which is easily confused as being related to the
seqcount lock, could you instead call it something more explicit like
clocksource_changed_count?

And yea, having it as part of the timekeeper structure would be the
better place for it.

thanks
-john