linux-kernel - Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86

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Date:	Mon, 13 Aug 2012 10:42:45 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC PATCH 0/4] ftrace: Add use of -mfentry for x86_64


* H. Peter Anvin <hpa@...or.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@...ux.intel.com>

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

Nice feature!

Thanks,

	Ingo
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