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

[Liang, Kan]

Hi John,

On 2023-02-17 6:11 p.m., John Stultz wrote:
> On Tue, Feb 14, 2023 at 12:38 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>> On 2023-02-14 3:11 p.m., John Stultz wrote:
>>> On Tue, Feb 14, 2023 at 9:00 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>>> On 2023-02-14 9:51 a.m., Liang, Kan wrote:
>>>>> If I understand correctly, the idea is to let the user space tool run
>>>>> the above interpoloation algorithm several times to 'guess' the atomic
>>>>> mapping. Using the mapping information to covert the TSC from the PEBS
>>>>> record. Is my understanding correct?
>>>>>
>>>>> If so, to be honest, I doubt we can get the accuracy we want.
>>>>>
>>>>
>>>> I implemented a simple test to evaluate the error.
>>>
>>> Very cool!
>>>
>>>> I collected TSC -> CLOCK_MONOTONIC_RAW mapping using the above algorithm
>>>> at the start and end of perf cmd.
>>>>         MONO_RAW        TSC
>>>> start   89553516545645  223619715214239
>>>> end     89562251233830  223641517000376
>>>>
>>>> Here is what I get via mult/shift conversion from this patch.
>>>>         MONO_RAW        TSC
>>>> PEBS    89555942691466  223625770878571
>>>>
>>>> Then I use the time information from start and end to create a linear
>>>> function and 'guess' the MONO_RAW of PEBS from the TSC. I get
>>>> 89555942692721.
>>>> There is a 1255 ns difference.
>>>> I tried several different PEBS records. The error is ~1000ns.
>>>> I think it should be an observable error.
>>>
>>> Interesting. That's a good bit higher than I'd expect as I'd expect a
>>> clock_gettime() call to take ~ double digit nanoseconds range on
>>> average, so the error should be within that.
>>>
>>> Can you share your logic?
>>>
>>
>> I run the algorithm right before and after the perf command as below.
>> (The source code of time is attached.)
>>
>> $./time
>> $perf record -e cycles:upp --clockid monotonic_raw $some_workaround
>> $./time
>>
>> The time will dump both MONO_RAW and TSC. That's where "start" and "end"
>> from.
>> The perf command print out both TSC and converted MONO_RAW (using the
>> mul/shift from this patch series). That's where "PEBS" value from.
>>
>> Than I use the below formula to calculate the guessed MONO_RAW of PEBS TSC.
>> Guessed_MONO_RAW = (PEBS_TSC - start_TSC) / (end_TSC - start_TSC) *
>> (end_MONO_RAW - start_MONO_RAW) + start_MONO_RAW.
>>
>> The guessed_MONO_RAW is 89555942692721.
>> The PEBS_MONO_RAW is 89555942691466.
>> The difference is 1255.
>>
>> Is the calculation correct?
> 
> Thanks for sharing it. The equation you have there looks ok at a high
> level for the values you captured (there's small tweaks like doing the
> mult before the div to make sure you don't hit integer precision
> issues, but I didn't see that with your results).
> 
> I've got a todo to try to see how the calculation changes if we do
> provide atomic TSC/RAW stamps, here but I got a little busy with other
> work and haven't gotten to it.
> So my apologies, but I'll try to get back to this soon.
> 

Have you got a chance to try the idea?

I just want to check whether the userspace interpolation approach works.
Should I prepare V3 and go back to the kernel solution?


Thanks,
Kan