Re: [PATCH 3/3] ftrace: add ability to only trace swapper tasks

[ebiederm@xmission.com (Eric W. Biederman)]

Steven Rostedt <rostedt@goodmis.org> writes:

> On Thu, 4 Dec 2008, Eric W. Biederman wrote:
>> >> >
>> >> >> > +static struct pid * const ftrace_swapper_pid = (struct pid *)1;
>> >> >> 
>> >> >> eh?
>> >> >
>> >> > all side-effects of getting rid of the integer based PID namespace and 
>> >> > replacing them with struct pid pointers.
>> >> 
>> >> Thanks for asking Andrew it looks like an unnecessary side effect.
>> >
>> > Well, it was necessary without hacking fork.c ;-)
>> 
>> The (struct pid *)1 has always been unnecessary.
>
> Well, I could set it to the &init_struct_pid as you said, but it will not 
> change any of the code below it. So it does not matter what 
> ftrace_swapper_pid is set to, as long as it is not set to something that 
> can be a legitimate pid struct for something not the swapper task.
>
> It will only matter when we fix the fork code.

Well that and if someone dereferences.  

>> As for fork.  It would be nice to remove most of the special cases
>> for the idle thread.  At least the counts are significant.  The rest
>> is pretty much a don't care at this point.
>
> Well, the swapper task should still have a pid of zero. That is probably 
> important.

Right.  I simply meant most of the
if (likely(p->pid)) conditional except for the counts is pretty much a don't
care.  Keeping the idle tasks off of the process list and out of the counts
is useful.

For this particular case what problem did you see with calling attach_pid
with PIDTYPE_PID on init_struct_pid?

Eric