Re: [PATCH] Perf: bug fix: distinguish between rename and exec

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2012-02-13 at 20:56 -0800, Luigi Semenzato wrote:
> This makes it possible for "perf report" to distinguish between an exec and
> a rename (for instance from prctl(PR_SET_NAME)).  Currently a similar COMM
> record is produced for the two events.  Perf report assumes all COMM records
> are execs and discards the old mappings.  Without mappings, perf report
> can no longer correlate sampled IPs to the functions containing them,
> and collapses all samples into a single bucket.
> 
> This change maintains backward compatibility in both directions, i.e. old
> version of perf will continue to work on new perf.data files in the same
> way, and new versions of perf on old data files.
> 
> Another solution would be to not send COMM records for renames.  Although
> it seems reasonable, it might break existing setups.

Uhm, didn't you argue its already broken?

> +++ b/fs/exec.c
> @@ -1052,7 +1052,7 @@ char *get_task_comm(char *buf, struct task_struct *tsk)
>  }
>  EXPORT_SYMBOL_GPL(get_task_comm);
>  
> -void set_task_comm(struct task_struct *tsk, char *buf)
> +void set_task_comm(struct task_struct *tsk, char *buf, bool is_rename)
>  {
>  	task_lock(tsk);
>  
> @@ -1068,7 +1068,7 @@ void set_task_comm(struct task_struct *tsk, char *buf)
>  	wmb();
>  	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
>  	task_unlock(tsk);
> -	perf_event_comm(tsk);
> +	perf_event_comm(tsk, is_rename);
>  }

I really dislike changing generic code purely for the purpose of
instrumentation like this. Better to pull perf_event_comm() out of here
if you want to change semantics.

Personally I couldn't care less about renames, I think they're a waste
of time, so I'm ok with the simple patch moving the perf_event_comm()
into setup_new_exec() and possibly renaming it to perf_event_exec().

Acme, do you care about renames?