linux-kernel - Re: [PATCH] tracing, sched: mark preempt

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Date:	Tue, 18 Aug 2009 12:28:47 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing, sched: mark preempt_schedule() notrace



On Tue, 18 Aug 2009, Frederic Weisbecker wrote:

> On Tue, Aug 18, 2009 at 04:01:57PM +0800, Lai Jiangshan wrote:
> > 
> > Current preempt_schedule() is not marked notrace. It may be
> > infinite recursion in __trace_graph_return().
> > 
> > preempt_schedule()
> >   __trace_graph_return()
> >     ftrace_preempt_disable() (!!return false!!)
> >     ftrace_preempt_enable()
> >       preempt_enable_notrace()
> >          preempt_schedule() (need_resched() may be true again)
> 
> 
> 
> It would happen in __trace_graph_return() , when preempt_schedule()
> has finished its job. It's very unlikely the TIF_NEED_RESCHED is
> set just after (because it has just been cleared).
> But why not. In that case, preempt_schedule() is called again but it's
> not a real tracing recursion.
> 
> That seems like a normal behaviour actually.

In fact, this is actually something to be traced. I would not add a 
notrace to it. This is not saving anything, the trace_graph_return does 
not need to worry about stack overflow, since the return code already 
freed the stack.

-- Steve
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