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SubjectRe: [PATCH] arm64: perf_event: Fix time_offset for arch timer
Hi all,

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!

Thanks,
Leo

[1] https://lkml.org/lkml/2020/1/23/571

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