Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer

[Leo Yan]

On Fri, May 01, 2020 at 04:26:09PM +0100, Will Deacon wrote:

[...]

> > > > Let me try and understand your particular problem better.
> > > 
> > > I think the long and short of it is that userspace needs a way to convert
> > > the raw counter cycles into a ns value that can be compared against values
> > > coming out of sched_clock. To do this accurately, I think it needs the
> > > cycles value at the point when sched_clock was initialised.
> > 
> > Will's understanding is exactly what I want to resolve in this patch.
> > 
> > The background info is for the ARM SPE [1] decoding with perf tool, if
> > the timestamp is enabled, it uses the generic timer's counter as
> > timestamp source.  SPE trace data only contains the raw counter cycles,
> > as Will mentioned, the perf tool needs to convert it to a coordinate
> > value with sched_clock.  This is why this patch tries to calculate the
> > offset between the raw counter's ns value and sched_clock, eventually
> > this offset value will be used by SPE's decoding code in Perf tool to
> > calibrate a 'correct' timestamp.
> > 
> > Based on your suggestions, I will use __sched_clock_offset to resolve
> > the accuracy issue in patch v2.  (I noticed Peter suggested to use a
> > new API for wrapping clock_data structure, IIUC, __sched_clock_offset
> > is more straightforward for this case).
> > 
> > Please correct if I miss anything.  Thank you for reviewing and
> > suggestions!
> 
> I don't think you can use __sched_clock_offset without selecting
> HAVE_UNSTABLE_SCHED_CLOCK, and we really don't want to do that just
> for this. So Peter's idea about exposing what we need is better, although
> you'll probably need to take care with the switch-over from jiffies.
> 
> It needs some thought, but one possibility would be to introduce a new
> variant of sthe ched_clock_register() function that returns the cycle
> offset, and then we could fish that out of the timer driver.

Thanks a lot for you and Marc for correction.

> If we're
> crossing all the 'i's and dotting all the 't's then we'd want to disable the
> perf userpage if sched_clock changes clocksource too (a bit like we do for
> the vDSO).

To be honest, one thing is not clear for me is how the perf tool to
update the arch timer's parameters in the middle of tracing after
disable and re-enable per userpage.  I will note for this and look
into detailed implementation for this part.

Thanks for sharing comprehensive thoughts!

Leo