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

[Thomas Gleixner]

On Thu, 7 Jan 2016, Christopher Hall wrote:
> On Wed, 06 Jan 2016 11:37:23 -0800, John Stultz <john.stultz@xxxxxxxxxx>
> 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