[lttng-dev] Wrong procname for userspace trace of app with different thread names

[David.OShea@quantum.com (David OShea)]

Hi all,

I performed a userspace trace of a multi-threaded application that uses prctl(PR_SET_NAME, ...) to give its threads names.  From the output of 'lttng view' below (I removed timestamps to make it shorter), it appears that UST is populating the "procname" by getting the process/thread name from the first thread it sees for a given PID, then assuming that name applies to all threads in the process:

mydaemon:22593 mydaemon:start: { <cpu_id> }, { pthread_id = 1496398144, procname = "mythread31", vtid = 22761, vpid = 22593 }, { <payload> }
mydaemon:22593 mydaemon:end: { <cpu_id> }, { pthread_id = 1496398144, procname = "mythread31", vtid = 22761, vpid = 22593 }, { <payload> }
myclient:28277 myclient:end: { <cpu_id> }, { pthread_id = 47216641758592, vtid = 28277, vpid = 28277, procname = "myclient" }, { <payload> }
myclient:28277 myclient:start: { <cpu_id> }, { pthread_id = 47216641758592, vtid = 28277, vpid = 28277, procname = "myclient" }, { <payload> }
mydaemon:22593 mydaemon:start: { <cpu_id> }, { pthread_id = 1485908288, procname = "mythread31", vtid = 22760, vpid = 22593 }, { <payload> }

On the first line, the procname contains the thread name for vtid = 22761.  On the last line, the procname is still shown as "mythread31", but the vtid is different, and I can see from /proc/22593/task/22760/stat that the thread's name is "mythread30".  However, the <process name>:<PID> at the start of each line - the pair before the <trace provider>:<tracepoint name> - does show the actual process name rather than any thread name.

I'm using LTTng 2.0.4 and babeltrace 1.0.0.rc5.  I set up my trace using these steps:

lttng create
lttng enable-channel channel0 --userspace
lttng add-context --userspace -t vpid -t vtid -t procname -t pthread_id
lttng enable-event --userspace --all

It would be great if "procname" was replaced by "threadname" and showed each thread's name correctly, since the process's name is already shown, but the thread names would be very useful.  Otherwise, at the very least, "procname" should probably show the same thing that is shown at the start of the line rather than incorrectly using the name of one of the threads.

Incidentally, why do I have to do "enable-channel" to get the context to actually appear?  If I don't do this, the "add-context" still says that each of the four contexts was "added to all channels", but they don't appear in 'lttng view'.

Thanks in advance,
David

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