linux-kernel - Re: [RFC][PATCH 12/18 v2] ftrace: Disable RCU unsafe checker when function graph is enabled

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Date:	Sat, 31 Aug 2013 12:55:50 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC][PATCH 12/18 v2] ftrace: Disable RCU unsafe checker when
 function graph is enabled

On Sat, Aug 31, 2013 at 01:11:29AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> 
> Having the RCU unsafe checker running when function graph is enabled
> can cause a live lock. That's because the RCU unsafe checker enables
> full lockdep debugging on RCU which does a lot of interal calls that
> may be traced by the function graph tacer. This adds quite a bit of

s/tacer/tracer/  (Yeah, yeah, picky, picky!)

> overhead and can possibly live lock the system.
> 
> Just do not do the RCU unsafe checks when function graph tracer is
> enabled.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

One question: How does the user/tester/developer know that RCU-unsafe
checks have been disabled by function-graph tracing?  Would it make
sense to print something to dmesg calling this out?  Or do the transitions
happen too often?

							Thanx, Paul

> ---
>  kernel/trace/ftrace.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 69b7f62..310b727 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5118,6 +5118,12 @@ int register_ftrace_graph(trace_func_graph_ret_t retfunc,
>  		goto out;
>  	}
> 
> +	/*
> +	 * The Unsafe RCU checker can live lock function graph tracing.
> +	 * It's best to just disable it while doing the fgraph tracing.
> +	 */
> +	ftrace_unsafe_rcu_checker_disable();
> +
>  	ftrace_graph_return = retfunc;
>  	ftrace_graph_entry = entryfunc;
> 
> @@ -5141,6 +5147,7 @@ void unregister_ftrace_graph(void)
>  	ftrace_shutdown(&global_ops, FTRACE_STOP_FUNC_RET);
>  	unregister_pm_notifier(&ftrace_suspend_notifier);
>  	unregister_trace_sched_switch(ftrace_graph_probe_sched_switch, NULL);
> +	ftrace_unsafe_rcu_checker_enable();
> 
>   out:
>  	mutex_unlock(&ftrace_lock);
> -- 
> 1.7.10.4
> 
> 

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