linux-kernel - Re: [PATCH 0/5][RFC] kprobes/ftrace: Have kprobes use ftrace on ftrace nops

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Date:	Wed, 10 Aug 2011 20:34:02 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jason Baron <jbaron@...hat.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH 0/5][RFC] kprobes/ftrace: Have kprobes use ftrace on
 ftrace nops

On Thu, 2011-08-11 at 09:21 +0900, Masami Hiramatsu wrote:
> Hi Steven,
> 
> Thanks for this nice feature!
> 
> (2011/08/11 1:22), Steven Rostedt wrote:
> > Hi All,
> >
> > I started working on adding the -mfentry switch to ftrace, which
> > allows us to remove the frame pointers requirement from function tracing
> > as well as makes mcount (fentry) work just better.
> >
> > But when I did this in another branch, I noticed that I broke kprobes
> > in its most common usage. The attaching a probe at the beginning of
> > a function to use get to its parameters.
> >
> > So I started this branch. This branch is to have kprobes use ftrace
> > directly when a probe is attached to a ftrace nop. Currently, kprobes
> > will just error when that happens. With this patch set, it will hook
> > into the ftrace infrastructure and use ftrace instead. This is more
> > like an optimized probe as no breakpoints need to be set. A call to
> > the function is done directly via the mcount trampoline. If ftrace
> > pt_regs is implemented for an arch, kprobes gets this feature for free.
> 
> I agreed this idea, this looks good to me too :)
> With -fentry, this can improve dynamic trace events very much.
> 
> BTW (OT), it seems that current kprobe data structure becomes a bit
> fat. Maybe what we need is just a "holder of hooking handler" as
> what ftrace provides, not a full storage data structure of copied
> instrucutions. Perhaps, we'd better diet the kprobe structure for
> transparency of hooking infrastructure.

Sure, I can make the ftrace_ops field in kprobes dynamically allocated
instead. That shouldn't be an issue.

> 
> > The first patch is just a clean up that I need to push out to get rid
> > of the annoying compile warning about initialized variables that
> > gcc can't tell have been initialized.
> >
> > The next two patches have ftrace pass both the ftrace_ops structure
> > and the pt_regs to the callback function that is registered with ftrace.
> >
> > The last two patches have kprobes interact with ftrace and use the
> > ftrace infrastructure instead.
> >
> > I only did this for x86_64, and will do it for x86_32 and PPC64 if everyone
> > agrees with this approach. Then I could find people to do it for other
> > archs :)
> 
> OK, I'll review this soon!

Thanks!

-- Steve


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