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

[John Stultz]

On Tue, Feb 14, 2023 at 12:09 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> 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:
> >> 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

Thanks for the pointer!

> 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.

Ugh. Well, I think perf may have made a bad API choice here, so I'm
still going to push back on exposting timekeeping internals to
userland.

But I do suspect that with ways to provide paired TSC/CLOCK_MONOTONIC
values, you should be able to get the same functionality in userland
as if the underlying data was shared.

thanks
-john