Date:   Tue, 14 Feb 2023 11:43:33 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     John Stultz <jstultz@google.com>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
        linux-kernel@vger.kernel.org, sboyd@kernel.org, eranian@google.com,
        namhyung@kernel.org, ak@linux.intel.com, adrian.hunter@intel.com
Subject: Re: [RFC PATCH V2 2/9] perf: Extend ABI to support post-processing
 monotonic raw conversion
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References: <20230213190754.1836051-1-kan.liang@linux.intel.com>
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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.