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

Peter Zijlstra
2023-02-14T15:57:04Z

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.