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

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Message-ID: <1298041659.23343.956.camel@gandalf.stny.rr.com>
Date:	Fri, 18 Feb 2011 10:07:39 -0500
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>,
	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)

On Fri, 2011-02-18 at 20:45 +0900, Masami Hiramatsu wrote:
> (2011/02/18 5:11), Steven Rostedt wrote:
> > On Fri, 2011-02-18 at 01:07 +0900, Masami Hiramatsu wrote:
> > 
> >> I just thought that frequent stop-machine is not so good from the user's
> >> POV. I agree that disabled probe ignoring the call is enough.
> >> Maybe, it could be done with the similar mechanism of jump optimization.
> > 
> > I thought jump optimization still calls stop_machine too?
> 
> Yes, but now it does batch optimization.
> Even if hundreds kprobes are registered separately, jump optimization
> has been done in background with a stop_machine per every 256 probes.
> (Until optimizing, kprobes can use breakpoints instead)

But a single optimized kprobe still must use stopmachine.

But it is true that the function tracer does it as one big shot. That
is, it will do all functions in a single stop machine that needs to be
changed. It too is batched, but there is not a limit to that batch.

I would be interested in hearing from users and real use cases that
someone would like to trace functions but stopmachine is too big of a
hammer.

-- Steve


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