Re: [RFC PATCH V2 2/9] perf: Extend ABI to support post-processing monotonic raw conversion

[John Stultz]

On Tue, Feb 14, 2023 at 12:38 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> On 2023-02-14 3:11 p.m., John Stultz wrote:
> > On Tue, Feb 14, 2023 at 9:00 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> >> On 2023-02-14 9:51 a.m., Liang, Kan wrote:
> >>> If I understand correctly, the idea is to let the user space tool run
> >>> the above interpoloation algorithm several times to 'guess' the atomic
> >>> mapping. Using the mapping information to covert the TSC from the PEBS
> >>> record. Is my understanding correct?
> >>>
> >>> If so, to be honest, I doubt we can get the accuracy we want.
> >>>
> >>
> >> I implemented a simple test to evaluate the error.
> >
> > Very cool!
> >
> >> I collected TSC -> CLOCK_MONOTONIC_RAW mapping using the above algorithm
> >> at the start and end of perf cmd.
> >>         MONO_RAW        TSC
> >> start   89553516545645  223619715214239
> >> end     89562251233830  223641517000376
> >>
> >> Here is what I get via mult/shift conversion from this patch.
> >>         MONO_RAW        TSC
> >> PEBS    89555942691466  223625770878571
> >>
> >> Then I use the time information from start and end to create a linear
> >> function and 'guess' the MONO_RAW of PEBS from the TSC. I get
> >> 89555942692721.
> >> There is a 1255 ns difference.
> >> I tried several different PEBS records. The error is ~1000ns.
> >> I think it should be an observable error.
> >
> > Interesting. That's a good bit higher than I'd expect as I'd expect a
> > clock_gettime() call to take ~ double digit nanoseconds range on
> > average, so the error should be within that.
> >
> > Can you share your logic?
> >
>
> I run the algorithm right before and after the perf command as below.
> (The source code of time is attached.)
>
> $./time
> $perf record -e cycles:upp --clockid monotonic_raw $some_workaround
> $./time
>
> The time will dump both MONO_RAW and TSC. That's where "start" and "end"
> from.
> The perf command print out both TSC and converted MONO_RAW (using the
> mul/shift from this patch series). That's where "PEBS" value from.
>
> Than I use the below formula to calculate the guessed MONO_RAW of PEBS TSC.
> Guessed_MONO_RAW = (PEBS_TSC - start_TSC) / (end_TSC - start_TSC) *
> (end_MONO_RAW - start_MONO_RAW) + start_MONO_RAW.
>
> The guessed_MONO_RAW is 89555942692721.
> The PEBS_MONO_RAW is 89555942691466.
> The difference is 1255.
>
> Is the calculation correct?

Thanks for sharing it. The equation you have there looks ok at a high
level for the values you captured (there's small tweaks like doing the
mult before the div to make sure you don't hit integer precision
issues, but I didn't see that with your results).

I've got a todo to try to see how the calculation changes if we do
provide atomic TSC/RAW stamps, here but I got a little busy with other
work and haven't gotten to it.
So my apologies, but I'll try to get back to this soon.

thanks
-john