Re: [PATCH 0/3] Convert TSC to monotonic clock for PEBS

[John Stultz]

On Mon, Jan 23, 2023 at 10:27 AM <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>
> A Processor Event Based Sampling (PEBS) record includes a field that
> provide the time stamp counter value when the counter was overflowed
> and the PEBS record was generated. The accurate time stamp can be used
> to reconcile user samples. However, the current PEBS codes only can
> convert the time stamp to sched_clock, which is not available from user
> space. A solution to convert a given TSC to user visible monotonic
> clock is required.
>
> The perf_event subsystem only converts the TSC in a NMI handler. The
> converter function must be fast and NMI safe.
>
> Considered the below two existing functions, but none of them fulfill
> the above requirements.
> - The ktime_get_mono_fast_ns() is NMI safe, but it can only return the
>   current clock monotonic rather than a given time's monotonic.
> - The get_device_system_crosststamp() can calculate the system time from
>   a given device time. But it's not fast and NMI safe.

So, apologies if this is a silly question (my brain quickly evicts the
details on get_device_system_crosststamp every time I look at it), but
rather then introducing a new interface, what would it take to rework
the existing get_device_system_crosststamp() logic to be usable for
both use cases?

thanks
-john