[ltt-dev] Problem on combine kernel trace and userspace trace.

[phyomh@gmail.com (Sam Liao)]

Hi Julien,

Thanks for your answer.

On Wed, Nov 3, 2010 at 10:04 PM, Julien Desfossez
<julien.desfossez at polymtl.ca> wrote:
> Hi Sam,
>
> Thanks for posting this, regarding the issue with time synchronization
> of the traces, we are actually working exactly on it right now.
> The main problem for now is that the events are recorded in cycles in
> the trace which means we need to know the TSC frequency to convert back
> in sec.nsec in LTTV. From UST we don't have a clear way to access this
> information, so we are now working on a new option in clock_gettime to
> use the LTTng trace clock infrastructure to make it easy to synchronise
> kernel and ust traces. I will post more details soon.
>

So using different clock sources brings this problem.

> Regarding the possibility of combining the userspace and kernelspace
> events for the same process I'm not sure exactly how it works, from what
> I can see the PID is 0 in UST traces so the traces cannot really be
> merged but I will let the people more familiar with UST answer this one.
>

I think that even there is no PID related information in the UST trace, if
the kernel trace's timestamp is consistent with the ust's, then we can get the
ust event's pid context from context switch("sched_change") related kernel
events after combining the kernel and ust trace.

Thanks,
-Sam

> Thanks,
>
> Julien
>
>
> On 10-11-03 05:24 AM, Sam Liao wrote:
>> Hi,
>>
>> I'm trying to combine the kernel trace and userspace trace, and it
>> seems that the
>> the userspace events can not be recognized to the right process with the kernel
>> events. I'm not sure if this is a problem of timestamp between kernel
>> and userspace.
>>
>> Here is the environment I'm using:
>> ----
>> Linux Trace Toolkit Visualizer 0.12.35-20082010
>> linux kernel 2.6.34.6
>> ust-0.4. ( I update the clock to use clock_gettime).
>> ----
>>
>> Is there anything I missed? Also I'm thinking about one problem, if
>> ust using clock_gettime to
>> get the nonseconds timestamp which is actually the timestamp that the user
>> application is doing the clock_gettime system call instead the
>> userspace event, how ust
>> deal with such difference and get this fixed when trace combination?
>>
>> Thanks,
>> -Sam
>>
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>