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

From: Steven Rostedt
Date: Fri Dec 05 2008 - 07:23:20 EST

On Thu, 4 Dec 2008, Eric W. Biederman wrote:

> Steven Rostedt <rostedt@xxxxxxxxxxx> writes:
> > On Thu, 4 Dec 2008, Eric W. Biederman wrote:
> >>
> >> 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?
> >
> > On boot up, the CPU 0 idle task is attached to init_struct_pid, and not
> > the others. If you do a "attach_pid" on the next idle task that is created,
> > it will become the attched process, bumping off CPU 0's idle task from the
> > init_struct_pid.
> It should form a linked list. For other pid types we don't have a problem.

Other pids get allocated per task. In the beginning of copy_process we

if (pid != &init_struct_pid) {
retval = -ENOMEM;
pid = alloc_pid(task_active_pid_ns(p));

Where alloc_pid allocates a pid structure. But this is only done if it is
not a swapper task.

> > When doing the code you suggested, I end up with only marking the last
> > idle task to be created.
> Odd. It is all a linked list through the task structures.
> I'm guessing the initialization isn't quite right.
> Weird.

Do I need to change the loop to do_each_pid_thread?

I'll try that later today.

-- Steve

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