[lttng-dev] Wrong timestamps in trace

[Thomas McGuire]

Hi Mathieu,

On 12.06.2017 18:08, Mathieu Desnoyers wrote:
>> I'm struggling to get lttng to run on an i.MX 25 board. I got everything
>> working, however the timestamps in the resulting trace file are all the
>> same. Because of the timestamps, TraceCompass fails at visualizing the
>> schedule.
>> I record on the device and then copy the trace over to my host to
>> examine it with babeltrace. See attached text files for the start and
>> the end of the trace, showing the timestamps being the same, despite the
>> trace running for a few seconds.
>>
>> The output of lttng-sessiond -v is also attached.
>>
>> I started everything the following way:
>>  cd /lib/modules/4.9.28-20170428-1/kernel/extra
>>  find . -type f -exec insmod {} \;
>>  find . -type f -exec insmod {} \;
>>  find . -type f -exec insmod {} \;
>>  find . -type f -exec insmod {} \;
>> lttng-sessiond -v --consumerd32-path=/usr/bin/lttng-consumerd &>
>> sessiond-log.txt
>>  lttng create my-session --output=/home/lttng/my-trace
>>  lttng enable-event --kernel --syscall --all --channel channel0
>>  lttng enable-event --kernel --channel channel0 sched*
>>  lttng start
>>  sleep 5 # do something
>>  lttng stop
>>
>> The Linux distribution is Pengutronix's ptxdist, which doesn't have
>> lttng rules, so I manually cross-compiled lttng-tools 2.9.4 and
>> lttng-modules 2.9.2. Kernel is 4.9.28-20170428-1, babeltrace on the host
>> is 1.2.4.
>>
>> What do I need to do to get correct timestamps?
>
> Hi Thomas,
>
> It appears like you may have the lttng-modules tests/clock-plugin
> kernel module loaded, which overrides the clock source with a
> "dummy" clock for testing purposes.
>
> Can you provide the output of lsmod ? Or did you build lttng
> within your kernel image ? If so, what is your kernel config ?
>
> Can you also provide the "metadata" file generated as attachment ?
> It will allow us to confirm whether it's using the kernel clock
> ("monotonic") or the dummy test clock ("lttng_test_clock_override").

Thanks for the quick response, that was exactly my problem! I simply 
issued an "insmod" on every lttng-related .ko file that lttng built, and 
that included the test clock plugin.
The reason why I used "insmod" is that for me, lttng-sessiond didn't 
load the lttng modules itself. I looked at this further now, and the 
reason was that I didn't issue a "depmod -a". With that, everything now 
works perfectly, the timestamps are correct and TraceCompass shows me a 
nice schedule :)

Thanks again for your help and every developer here for providing lttng, 
it's a great tool.

Regards,
Thomas
-- 
Thomas McGuire | thomas.mcguire at kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH & Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts

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