Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86_64

[Ingo Molnar <mingo@kernel.org>]

* H. Peter Anvin <hpa@zytor.com> wrote:

> On 08/07/2012 12:38 PM, Steven Rostedt wrote:
> >
> >This is an RFC patch set that makes gcc use the -mfentry option with
> >-pg. This will set the ftrace 'hook' to the beginning of the function
> >and also remove the requirement that -pg enables frame pointers.
> >
> >This has a couple of benefits (and probably more).
> >
> >1) removal of the frame pointer requirement makes for smaller and faster code.
> >
> >2) Having the function trace hook at the beginning of the function instead
> >  of after the frame is set up, gives the function tracing callbacks access
> >  to the parameters. This means that kprobes can take advantage of this.
> >  When a kprobe is set on top of a ftrace hook (nop), it will automatically
> >  use the function tracing callback. This makes it into an 'optimized' probe
> >  as there's no need to hit a breakpoint and trigger the probe that way.
> >  The function tracing code can do the work for it. Note, optimized probes
> >  are only allowed with !PREEMPT, but a ftrace optimize probe is allowed
> >  in any context (another benefit).
> >
> >This only implements fentry for x86_64.
> >
> 
> Acked-by: H. Peter Anvin <hpa@linux.intel.com>

Acked-by: Ingo Molnar <mingo@kernel.org>

Nice feature!

Thanks,

	Ingo