Re: [PATCH 01/16 v3] function_graph: Convert ret_stack to a series of longs

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, May 28, 2019 at 08:58:26AM -0400, Steven Rostedt wrote:
> On Tue, 28 May 2019 05:50:43 -0400
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> > On Fri, May 24, 2019 at 11:16:34PM -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > > 
> > > In order to make it possible to have multiple callbacks registered with the
> > > function_graph tracer, the retstack needs to be converted from an array of
> > > ftrace_ret_stack structures to an array of longs. This will allow to store
> > > the list of callbacks on the stack for the return side of the functions.
> > > 
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > > ---
> > >  include/linux/sched.h |   2 +-
> > >  kernel/trace/fgraph.c | 124 ++++++++++++++++++++++++------------------
> > >  2 files changed, 71 insertions(+), 55 deletions(-)
> > > 
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index 11837410690f..1850d8a3c3f0 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -1113,7 +1113,7 @@ struct task_struct {
> > >  	int				curr_ret_depth;
> > >  
> > >  	/* Stack of return addresses for return function tracing: */
> > > -	struct ftrace_ret_stack		*ret_stack;
> > > +	unsigned long			*ret_stack;  
> > 
> > Can it be converted to an array of unsigned int so the shadown stack space
> > can be better used? This way stack overflow chance is lesser if there are too
> > many registered fgraph users and the function call depth is too deep.
> > AFAICS from patch 5/13, you need only 32-bits for the ftrace_ret_stack
> > offset, type and array index, so the upper 32-bit would not be used.
> > 
> 
> We can look to improve that later on. This is complex enough and I kept
> some features (like 4 byte minimum) out of this series to keep the
> complexity down. I believe there are some archs that require 64bit
> aligned access for 64 bit words and pointers. And the retstack
> structure still has longs on it. If we need to adapt to making sure we
> are aligned, I rather keep that complexity out for now.
> 
> That said, I just did a git grep HAVE_64BIT_ALIGNED_ACCESS and only
> found the kconfig where it is defined and the ring buffer code that
> deals with it. We recently removed a bunch of archs, and it could very
> well be that this requirement no longer exists.
> 
> Regardless, I've been testing this code quite heavily, and changing the
> stack from moving up to moving down already put me behind. I'd like to
> pull this code into linux-next soon. Converting to ints can be done for
> the release after we get this in.

Ok sure, I agree the conversion to ints can be done at a later time. thanks!

 - Joel