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

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Message-ID: <1313290684.18583.362.camel@gandalf.stny.rr.com>
Date:	Sat, 13 Aug 2011 22:58:04 -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,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH 0/5][RFC] kprobes/ftrace: Have kprobes use ftrace on
 ftrace nops

On Sat, 2011-08-13 at 19:09 +0900, Masami Hiramatsu wrote:
> (2011/08/12 22:08), Steven Rostedt wrote:
> > On Fri, 2011-08-12 at 11:57 +0900, Masami Hiramatsu wrote:
> >
> >> I don't think it won't work. It can work but on a long way.
> >> Could you tell me your "bigger ideas"? Perhaps, we are on the different
> >> way but aim to same goal.
> >
> > Part of the bigger ideas is to have things like function graph tracing
> > use this, as it will simplify the entry.S code. There's other things
> > that may come out of this too.
> 
> Hmm, I think that the current function graph tracing implementation
> is more scalable than kretprobes, because kretprobe requires
> spinlock on every hit. Moreover, you can't probe NMI handler with
> kprobe, and kprobes on irq-handler are also possible to fail
> because of recursive-call.
> So I don't recommend using kretprobe for function-graph tracer :-(

Sorry for the confusion. My idea is not to use kretprobe with function
graph tracer, but to use the ftrace hooks with the pt_regs and friends
for function graph tracer instead of what it does today, which is to add
function graph code directly into entry.S.

The point I was making is, if I need to get ftrace function tracing
being good enough for function graph tracer, then it should work with
kprobes without any issues. If I need to do the work anyway (for
function graph tracing) then why not use it directly with kprobes
instead of doing more hooks just in the kprobe_trace?

-- Steve


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