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

[John Stultz]

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?

thanks
-john