[ltt-dev] trace-clock-userspace.patch for ARM/PowerPC ?

[Abbas Raza]

On 07/26/2011 10:17 PM, Mathieu Desnoyers wrote:
> * Abbas Raza (abbas_raza at mentor.com) wrote:
> [...]
>> Hi Mathieu,
>>
>> Some confusion here :(
>>
>> In case of LTTng 0.x, when we take kernel traces then timestamps for
>> these traces is based on TSC/TB . So after that we view these traces by
>> lttv and it shows timestamps with all events which occurred.
>> I print the timestamp in kernel ltt function 'ltt_trace_alloc()'  and it
>> comes out to be 22944596907 (by calling trace_clock_read64() ). While
>> lttv show Birth sec of first event to 367 second. How this conversion is
>> made from TSC to the Birth sec/nsec value displayed in lttv? just want
>> to confirm that whether timestamps shown in lttv (Birth sec/nsec) are
>> based on TSC values... if yes then how TSC values are converted to Birth
>> sec/nsec and i no then from where lttv gets timestamps to be displayed?
>> Or in simple words, do kernel traces which we get contain timestamps
>> based simply on TSC values or it is something else?
> Yes, we write, in the lttng 0.x headers, the following information:
>
> struct ltt_subbuffer_header {
>   [...]
>          uint64_t start_freq;            /*
>                                           * Frequency at trace start,
>                                           * used all along the trace.
>                                           */
>          uint32_t freq_scale;            /* Frequency scaling (divisor) */
> [...]
> }
>
> so by using the start frequency and the scale, we can convert from TSC
> values to nanoseconds. Please note that LTTng 0.x requires the clock
> source to appear as if it has a constant rate.
>
> These values map to trace_clock_frequency() and trace_clock_freq_scale()
> trace clock functions.
>
> Best regards,
>
> Mathieu
>
>> Thanks a lot again for help :)
>>
>>
>> Abbas Raza
>>

Just to confirm that TSC's just have raw values of clock cycles and lttv 
converts these values into sec/nsec by reading ltt_subbuffer_header ?


Thanks!

Abbas Raza