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

[John Stultz]

On Tue, Feb 14, 2023 at 7:56 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.

Well, CLOCK_MONOTONIC_RAW is at least a fixed function, we don't
change its frequency. Whereas other clocks will likely be adjusted
over their lifetime, so deriving the frequency has to be continually
re-calculated, so they aren't ideal for this sort of interpolation.

thanks
-john