linux-kernel - Re: [PATCH 2/2] nohz: Add basic tracing

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Message-ID: <20130423210156.GD3427@linux.vnet.ibm.com>
Date:	Tue, 23 Apr 2013 14:01:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] nohz: Add basic tracing

On Tue, Apr 23, 2013 at 04:00:23PM -0400, Steven Rostedt wrote:
> On Tue, 2013-04-23 at 18:12 +0000, Christoph Lameter wrote:
> > On Mon, 22 Apr 2013, Frederic Weisbecker wrote:
> > 
> > > It's not obvious to find out why the full dynticks subsystem
> > > doesn't always stop the tick: whether this is due to kthreads,
> > > posix timers, perf events, etc...
> > >
> > > These new tracepoints are here to help the user diagnose
> > > the failures and test this feature.
> > 
> > Very good. This will help a lot.
> 
> You can also do:
> 
> cd /sys/kernel/debug/tracing
> echo 1 > max_graph_depth
> echo function_graph > current_tracer
> 
> And then run your code, and look to see what happens on the cpu in
> question:
> 
> cat per_cpu/cpuX/trace
> 
> The "max_graph_depth" of one will make the function graph tracer just
> trace the first function that enters the kernel. You'll be able to see
> if the kernel did anything to your userspace application that wasn't
> planned.
> 
> "max_graph_depth" was added in 3.9-rc1

Very cool!  I have added this to the documentation.

							Thanx, Paul

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