linux-kernel - Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is for x86

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Message-ID: <4D5D4013.4070602@hitachi.com>
Date:	Fri, 18 Feb 2011 00:34:43 +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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andi Kleen <andi@...stfloor.org>,
	2nddept-manager@....hitachi.co.jp
Subject: Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is
 for x86_64 right now)

(2011/02/17 22:18), Steven Rostedt wrote:
> On Thu, 2011-02-17 at 21:37 +0900, Masami Hiramatsu wrote:
> 
>> Oops! with this change, current kprobes might not be able to probe
>> the entry of functions, because that is always reserved by ftrace!
>> I think we need to have some new interface for replacing each other
>> safely...
> 
> Good point. I suspect that this wont be ready till .40 anyway. When I
> get a chance to work more an this, I'll also include patches where if
> -mfentry is activated kprobes will just hook to the mcount caller
> instead. Or ftrace itself :)

Ah, that's a good idea! :) it could be done without -mfentry too.
But is that possible to modify just one mcount entry? I also worry
about the latency of enabling/disabling one entry.

BTW, without dynamic ftrace (no code modifying), I think we don't
need to reserve mcount code, because no one modifies it.

> 
> I'm also working on making the function tracer a bit more flexible. That
> is, to let multiple clients control the dynamic trace instead of just
> one big global one.

Nice!


-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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