Messages in this thread | | | Date | Fri, 01 May 2020 16:29:27 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer |
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On 2020-05-01 16:14, Leo Yan wrote: > 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).
I think Peter's point was *not* to use __sched_clock_offset which appears to be only there for the purpose of an x86 workaround (and not available to other architecture), but to instead expose the relevant field (epoch_cyc) to the perf subsystem.
I feel it makes sense to make this a generic API, and see whether x86 can move over to it rather than the other way around.
Thanks,
M. -- Jazz is not dead. It just smells funny...
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