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

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Date:	Fri, 18 Feb 2011 10:19:54 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Dominique Toupin <dominique.toupin@...csson.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	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>, 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)

[Adding Dominique Toupin, from Ericsson, to CC list]

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 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.

Hi Steven,

Telecom end users are one of such cases where the latency induced by stop
machine while the system is running is a problem. Dominique Toupin could
certainly tell us more about Ericsson's use-cases.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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