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

[John Stultz]

On Tue, Feb 14, 2023 at 9:46 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> On 2023-02-14 5:43 a.m., Peter Zijlstra wrote:
> > On Mon, Feb 13, 2023 at 02:22:39PM -0800, John Stultz wrote:
> >> The interpoloation is pretty easy to do:
> >>
> >> do {
> >>     start= readtsc();
> >>     clock_gett(CLOCK_MONOTONIC_RAW, &ts);
> >>     end = readtsc();
> >>     delta = end-start;
> >> } while (delta  > THRESHOLD)   // make sure the reads were not preempted
> >> mid = start + (delta +(delta/2))/2; //round-closest
> >>
> >> and be able to get you a fairly close matching of TSC to
> >> CLOCK_MONOTONIC_RAW value.
> >>
> >> Once you have that mapping you can take a few samples and establish
> >> the linear function.
> >
> > Right, this is how we do the TSC calibration in the first place, and if
> > NTP can achieve high correctness over a network, then surely we can do
> > better locally.
> >
> > That is, this scheme should work for all CLOCKs, not only MONOTONIC_RAW.
>
> If I understand correctly, the TSC calibration is done in the kernel.
> The kernel keeps updating the mul/shift. We dump the mul/shift into the
> perf mmap page for the user tools.

Where is that done in the perf mmap? I wasn't aware.

thanks
-john