Re: Tracing thread name (was: ltt comm tracking)

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2009-08-03 at 09:48 -0400, Mathieu Desnoyers wrote:
> * Matthieu CASTET (matthieu.castet@parrot.com) wrote:
> > Hi,
> > 
> > I use ltt on a 2.6.27 on an arm architecture.
> > It works quite well, but I have a minor problem :
> > 
> > my application sets thread name with prctl PR_SET_NAME. But ltt viewer
> > doesn't seem to saw it.
> >
> 
> Hrm, I guess we might want to instrument set_task_comm as you propose to
> get the correct process name, but if this gets us the thread name, I
> think it's only an implementation side-effect:
> 
> If I look at the man page:
> 
> PRCTL(2)
> 
>        PR_SET_NAME (since Linux 2.6.9)
>               Set the process name for the calling process, using the value in
>               the location pointed to by (char *) arg2.  The name can be up to
>               16  bytes  long,  and  should  be null terminated if it contains
>               fewer bytes.
> 
> It seems to officially set the process name, not thread name.

Then the man page is wrong, it really only sets the task (thread) name.

> The way LTTng handles thread names is by adding a userspace "thread
> branding" event. It should be executed at thread startup. The downside
> of the current LTTng approach is that we cannot know the name of threads
> already executing before we started tracing.
> 
> Ideally, having something like prctl PR_SET/GET_THREAD_NAME would
> probably make sense.

Going by the current behaviour, you'd need to add process name.

fs/exec.c:

        void set_task_comm(struct task_struct *tsk, char *buf)
        {
                task_lock(tsk);
                strlcpy(tsk->comm, buf, sizeof(tsk->comm));
                task_unlock(tsk);
                perf_counter_comm(tsk);
        }