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

[Will Deacon <will@kernel.org>]

On Fri, May 01, 2020 at 11:14:48PM +0800, Leo Yan wrote:
> On Thu, Apr 30, 2020 at 05:18:15PM +0100, Will Deacon wrote:
> > On Thu, Apr 30, 2020 at 06:04:36PM +0200, Peter Zijlstra wrote:
> > > On Thu, Apr 30, 2020 at 04:29:23PM +0100, Marc Zyngier wrote:
> > > 
> > > > I wonder if we could/should make __sched_clock_offset available even when
> > > > CONFIG_HAVE_UNSTABLE_SCHED_CLOCK isn't defined. It feels like it would
> > > > help with this particular can or worm...
> > > 
> > > Errrgh. __sched_clock_offset is only needed on x86 because we transition
> > > from one clock device to another on boot. It really shouldn't exist on
> > > anything sane.
> > 
> > I think we still transition from jiffies on arm64, because we don't register
> > with sched_clock until the timer driver probes. Marc, is that right?
> > 
> > > 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. 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).

Will