[lttng-dev] TMF: inconsistencies in the context vtid reporting

[Sébastien Barthélémy]

On Fri, Mar 22, 2013 at 10:24 PM, Alexandre Montplaisir
<alexmonthy at voxpopuli.im> wrote:
> Hi Sébastien,
>
> On 13-03-22 06:02 AM, Sébastien Barthélémy wrote:
>> Hello,
>>
>> I'm using simultaneous kernel and userspace tracing in order to
>> understand how a multi threaded application (mis)behaves at shutdown.
>>
>> The idea is to use userspace tracepoints in the application, augmented
>> with the vtid context, in order to know which thread is calling what
>> and to relate that with the threads lifespan as reconstructed from the
>> kernel scheduler traces.
>>
>> First, is the approach ok? Am I right to expect the vtid attached to
>> ust event to match the tid shown in the reconstructed kernel scheduler
>> state?
>
> It should work, yes. However note that the concept of "vtid" (virtual
> thread ID) is used to differentiate the tid assigned to a process when
> used within a container (LXC) vs. its tid on the host. You could use
> context.tid too, but if you don't use containers those two should be the
> same.

Thank you for the clarification. I didn't knew tip could be attached
to a UST event.

>> I believe my kernel trace is complete, since the following command
>> shows no output.
>>
>>     $ babeltrace lttng-traces/auto-20130321-172308/kernel/ > /dev/null
>>
>> My userspace trace is not complete. But I believe it does not matter,
>> because the missing events are seconds before the time window I'm
>> interested in, and AFAIK, there is no state reconstruction involved
>> (but may be there is?).
>
> It could happen ; maybe the tracer missed an important sched_switch
> event that set a process in a particular state. I wouldn't recommend
> running analyses on traces with missing events, some information could
> be all wrong.

My kernel trace is *not* reported as incomplete, so I do not think I'm
missing sched_switch event. Should I?

> Unless you are on an embedded platform with very limited memory, it's
> usually safe to increase the buffer size to a couple MBs. See
> --enable-channel and --subbuf-size in the LTTng man page. I have to warn
> you about https://bugs.lttng.org/issues/228 in advance though...

ok, thank you for the tip.

> I still think the default value should be higher (and have the embedded
> guys use --subbuf-size to reduce it), but I don't call those shots ;)
>
>> I eventually came to a more blatant inconsistency: the displayed
>> context vtid of an event is not always the same.
>>
>> [...]
>>
>> I'm using TMF 2.0.0.201303211612 (nightly build from yesterday).
>
> Are your traces available somewhere (if they can be made public)? I
> would like to take a look at it, it seems the CTF parser gets confused
> between vtid and vpid for some weird reason...

No, sorry, I cannot easily share the trace. And I have not found a simple way
to trig the bug.