linux-kernel - Re: timer: lockup in run_timer

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Date:	Thu, 11 Jul 2013 19:11:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Dave Jones <davej@...hat.com>, Tejun Heo <tj@...nel.org>,
	tglx@...utronix.de, LKML <linux-kernel@...r.kernel.org>,
	trinity@...r.kernel.org
Subject: Re: timer: lockup in run_timer_softirq()

On Thu, Jul 11, 2013 at 12:59:32PM -0400, Steven Rostedt wrote:
> On Thu, 2013-07-11 at 12:55 -0400, Steven Rostedt wrote:
> 
> > > 
> > > Other than that, a function tracer environment that is safer to use might be
> > > useful for other people as well.
> > 
> > Not sure how to make the environment safe, as the main purpose of the
> > function trace is to debug those hard to debug locations, like NMIs,
> > RCU, dynamic ticks, etc. To ensure a "safe" environment, it would
> > cripple the tracer.
> > 
> > Hmm, what would you state as a safe environment? How can we detect if
> > the environment is safe to trace or not?
> 
> Maybe I misunderstood you. You mean to have this environment be
> something for not just perf, and have the macro be:
> 
> NONSAFE_TRACE(__local_bh_enable);
> 
> ?
> 
> Then, any ftrace user could set a flag in the registering of its ops to
> 'safe_only_functions'. And it will ignore all of these locations.
> There's really not many of them, so it may not be too hard to weed out.

Yah, like that. But that doesn't invalidate your question as to what 'safe'
would encompass. I think RCU/lockdep would be the big thing for perf, not
sure it should be wider than that.
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