Re: [PATCH 3/3] ftrace: trace single pid for function graph tracer

From: Steven Rostedt
Date: Wed Dec 03 2008 - 23:32:36 EST

On Wed, 3 Dec 2008, Eric W. Biederman wrote:

> Steven Rostedt <rostedt@xxxxxxxxxxx> writes:
> > On Wed, 3 Dec 2008, Eric W. Biederman wrote:
> >
> >> Steven Rostedt <rostedt@xxxxxxxxxxx> writes:
> >>
> >> The way patch 2 uses pids is just stupid. It has nothing to do with
> >> pids aren't unique. You do a full walk of the process list instead
> >> of using the hash table.
> >
> > The way patch 2 uses the pids is stupid, it was just the easiest way to
> > implement it correctly ;-) I work with, do it stupid but correct first,
> > then optimize.
> Which is a reasonable and very unix approach. As long as I don't need to
> do more than advise about what to do.

No prob. I already wrote the patch and I'm just about ready to post.

> >> It makes me think that task->pid really should go away because with it
> >> there people don't bother to look and see how things normally work.
> >
> > This is far from a fast path, and I can easily fix it. The hard work was
> > the rest of the patch not this part. I even did it stupid knowing that I
> > would be rewriting it to handle namespaces. I stated that this needed to
> > be fixed in the patch itself.
> >
> > One thing I never got an answer for, using the namespace pid path, can I
> > still select the idle task to trace, i.e. pid == 0.
> Sorry I didn't see this question. As I recall we have a special pid that
> is not hashed that is used by the idle task.
> So it should be possible but it would need to be a special case. Although
> please note we have one idle task per cpu.

Yep, I'm almost finish with this patch. It is a special case.

> Is the idle task doing anything interesting enough to warrant tracing?
> We don't report the idle tasks in proc or in any of our other user space
> interfaces.

Yes, because we can see what wakes things up from idle. That is, if we
trace the idle task, we can trace the interrupts that happen while the CPU
is idle.


-- Steve

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