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Subject: Re: Ftrace: can trace kthread?
From: Steven Rostedt <srostedt@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>, rostedt@goodmis.org
Cc: "Figo.zhang" <zhangtianfei@leadcoretech.com>,
        Yong Zhang <yong.zhang0@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>,
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Date: Thu, 21 Oct 2010 09:31:41 -0400
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On Thu, 2010-10-21 at 12:12 +0200, Peter Zijlstra wrote:
> On Thu, 2010-10-21 at 16:59 +0800, Figo.zhang wrote:
> > > I just
> > > echo  0 > tracing_enabled

I need to remove that file. "tracing_enabled" is pretty much deprecated,
just use tracing_on to enable and disable the trace recording.

> > > echo function > current_tracer
> > > echo 374(pid of flush) > set_ftrace_pid
> > > echo 1 > tracing_enabled
> > > wait some time
> > > echo 0 > tracing_enabled
> > > cat trace
> > > 
> > > So I think this answer your question "could kthread be traced?"
> > 
> > i do it the same of you.
> 
> > but i canot trace nothing. 
> 
> Your initial attempt had:
> 
>         echo kswapd > /sys/kernel/debug/tracing/set_graph_function

Yeah, that wont work. That's because kswapd() is called at system
startup and goes into a loop. The function graph tracer will never see
that function again (it's not called again). It can't know what function
it is in, only what functions it enters.

-- Steve

> 
> have you reset that state?