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

[Liang, Kan]

On 2023-02-14 2:37 p.m., John Stultz wrote:
> 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.

The updating of the mul/shift for sched_clock should be done in the
set_cyc2ns_scale() in tsc.c

The perf user space tool mmap a page to retrieve the enabling
time/running time from the kernel. On X86 and Arm, the conversion
information from HW time (TSC) to sched_clock/perf_time is also stored
in the page. Please see the arch_perf_update_userpage(). In the perf
mmap, it only retrieve the current mul/shift information and write them
into the page for the user space tool.

This V2 patch series try to do the same thing for the monotonic raw
conversion. So the kernel internal mul/shift information has to be exposed.


Thanks,
Kan