linux-kernel - Re: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace

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Date:	Fri, 20 Mar 2009 20:54:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Fri, Mar 20, 2009 at 08:27:21PM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > On Fri, Mar 20, 2009 at 07:38:49PM +0100, Ingo Molnar wrote:
> > > > 
> > > > * Ingo Molnar <mingo@...e.hu> wrote:
> > > > 
> > > > > > > This looks like it is RCU/stop_machine related. The CPU is 
> > > > > > > stuck in in stop_machine? I see that rcu_torture is running. 
> > > > > > > Does this go away if you turn off rcu_torture?
> > > > > > 
> > > > > > Grasping at straws...  Does Lai's rcu_barrier() fix help?
> > > > > 
> > > > > which one is that?
> > > > 
> > > > ok, found it. Will know in about ~24 hours whether it helps.
> > > 
> > > http://lkml.org/lkml/2009/3/20/71 was the one I was thinking of, 
> > > just to double-check.
> > 
> > Yeah - i just picked it up into tip:core/rcu.
> > 
> > I didnt immediately connect the two things, as 'tracer self-test' 
> > does not lend itself to 'CPU hotplug and RCU race fix' - but indeed 
> > in the case of the function tracer there's a dependency due to 
> > stop_machine_run(). Thanks for point it out!
> > 
> > 	Ingo
> 
> 
> I've successfully triggered a crash, the problem is that I can't be sure this is
> the same because I don't have a serial line on my x86-64, and the trace goes too far.
> Moreover boot_delay=N make it disappear.
> 
> I can trigger it each time I boot with ftrace=function_graph and
> the following patch applied:
> 
> diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
> index 7c4142a..e9914a8 100644
> --- a/kernel/rcutorture.c
> +++ b/kernel/rcutorture.c
> @@ -616,7 +616,7 @@ rcu_torture_writer(void *arg)
>  	static DEFINE_RCU_RANDOM(rand);
>  
>  	VERBOSE_PRINTK_STRING("rcu_torture_writer task started");
> -	set_user_nice(current, 19);
> +	set_user_nice(current, -1);
>  
>  	do {
>  		schedule_timeout_uninterruptible(1);
> @@ -736,7 +736,7 @@ rcu_torture_reader(void *arg)
>  	struct timer_list t;
>  
>  	VERBOSE_PRINTK_STRING("rcu_torture_reader task started");
> -	set_user_nice(current, 19);
> +	set_user_nice(current, -1);
>  	if (irqreader && cur_ops->irqcapable)
>  		setup_timer_on_stack(&t, rcu_torture_timer, 0);

i dont have a reproducer right now. Can you trigger it with latest 
-tip, which has this commit included:

04cb9ac: rcu: rcu_barrier VS cpu_hotplug: Ensure callbacks in dead cpu are migrated to o

?

	Ingo
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