Re: [PATCH 08/13 v2] ftrace: do not trace init sections

From: Steven Rostedt
Date: Thu Oct 23 2008 - 07:36:51 EST

On Thu, 23 Oct 2008, Jan Kiszka wrote:

> Steven Rostedt wrote:
> > The recordmcount script is now robust enough not to process any sections
> > but the .text section. But the gcc compiler still adds a call to mcount.
> >
> > Note: The function mcount looks like:
> >
> > ENTRY(mcount)
> > ret
> > END(mcount)
> >
> > Which means the overhead is just a return.
> >
> > This patch adds notrace to the init sections to not even bother calling
> > mcount (which simply returns).
> Sorry for a potentially dumb question (didn't follow all recent ftrace
> developments), but doesn't this mean that code in such sections is now
> invisible for all tracers, even with dynamic tracing disabled (in which
> case they should cause no problem)? What if you *do* want to have such
> functions in your trace as they may contribute to problem or give other
> useful hints?

Not a dumb question, I've thought about this too.

Well, you can still add tracing into those functions, just the mcount call
will not be there. I've thought about other ways to handle this. Perhaps
add it only when DYNAMIC_FTRACE is configured. But that to me is a new
feature. The patches I'm submitting is to help with performance, bug
fixes, and sanity checks. So I left out any optional notraces.

e.g. in init.h I could do something like.

# define init_notrace notrace
# define init_notrace

And then use the init_notrace throughout the file. If this is considered
something that is acceptible for adding into 28, I would be happy to
supply a patch.

-- Steve

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