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

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2023-02-13 2:37 p.m., John Stultz wrote:
> On Mon, Feb 13, 2023 at 11:08 AM <kan.liang@linux.intel.com> wrote:
>>
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> The monotonic raw clock is not affected by NTP/PTP correction. The
>> calculation of the monotonic raw clock can be done in the
>> post-processing, which can reduce the kernel overhead.
>>
>> Add hw_time in the struct perf_event_attr to tell the kernel dump the
>> raw HW time to user space. The perf tool will calculate the HW time
>> in post-processing.
>> Currently, only supports the monotonic raw conversion.
>> Only dump the raw HW time with PERF_RECORD_SAMPLE, because the accurate
>> HW time can only be provided in a sample by HW. For other type of
>> records, the user requested clock should be returned as usual. Nothing
>> is changed.
>>
>> Add perf_event_mmap_page::cap_user_time_mono_raw ABI to dump the
>> conversion information. The cap_user_time_mono_raw also indicates
>> whether the monotonic raw conversion information is available.
>> If yes, the clock monotonic raw can be calculated as
>> mono_raw = base + ((cyc - last) * mult + nsec) >> shift
> 
> Again, I appreciate you reworking and resending this series out, I
> know it took some effort.
> 
> But oof, I'd really like to make sure we're not exporting timekeeping
> internals to userland.
> 
> I think Thomas' suggestion of doing the timestamp conversion in
> post-processing was more about interpolating collected system times
> with the counter (tsc) values captured.
>

Thomas, could you please clarify your suggestion regarding "the relevant
conversion information" provided by the kernel?
https://lore.kernel.org/lkml/87ilgsgl5f.ffs@tglx/

Is it only the interpolation information or the entire conversion
information (Mult, shift etc.)?

If it's only the interpolation information, the user space will be lack
of information to handle all the cases. If I understand John's comments
correctly, it could also bring some interpolation error which can only
be addressed by the mult/shift conversion.

If the suggestion is to dump the entire conversion information into the
user space, we have to expose the timekeeping internals.

Considering the above difficulties, could we use the kernel conversion?
(The current perf already uses the kernel conversion for monotonic raw.
It should not bring extra overhead.)

Thanks,
Kan

> I get the interpolation can be difficult as the counter value and
> system time can't currently atomically collected, so potentially there
> may be a need for a way to tie two together (see my previous email's
> thought of ktime_get_raw_monotonic_from_timestamp()), but we'd
> probably want a clear understanding of the benefit (quantitative
> reduction in interpolation error, and what real benefit that brings),
> and would also want the driver to generate and share those pairs
> rather than having userland have access.
> 
> thanks
> -john