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

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Date:	Thu, 11 Aug 2011 09:21:49 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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

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.

> 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!

Thank you!

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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