Messages in this thread | | | Date | Fri, 1 May 2020 18:13:53 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer |
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On Sat, May 02, 2020 at 12:10:50AM +0800, Leo Yan wrote: > 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.
I don't fully understand the concern but, generally, the seqlock should take care of any inconsistencies in the data page.
Will
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