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

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Message-ID: <1377981737.26624.6.camel@pippen.local.home>
Date:	Sat, 31 Aug 2013 16:42:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
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, 2013-08-31 at 12:55 -0700, Paul E. McKenney wrote:
> 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?

Ideally, (for 3.13) I plan on having this be a sysctl switch. That will
be 1 or 0, depending on if it is enabled or not. But yeah, I can add a
printk to show that the kernel changed it. It doesn't happen often, and
mainly by a user (or selftest).

-- Steve

> 
> 							Thanx, Paul
> 
> > ---


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