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

[David Ahern <dsahern@gmail.com>]

On 2/15/12 5:48 AM, Peter Zijlstra wrote:
> 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?
>

I'm not Acme, but I do care. We use a lot of processes with named 
threads that give users an idea about the function of a particular thread.

I do not understand the use case targeted by the patch -- what kind of 
processes run for some amount of time and then decide to change task names?

David