linux-kernel - Re: [PATCH] rcu: convert rcupreempt trace to seq file

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Date:	Sun, 11 Jan 2009 06:01:31 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] rcu: convert rcupreempt trace to seq file

On Sun, Jan 11, 2009 at 02:39:40AM +0100, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Sat, Jan 10, 2009 at 03:48:46PM +0800, Li Zefan wrote:
> > > Impact: also fix a bug in show_rcustats()
> > > 
> > > Use seq file for simplification, and the global buffer and mutex can be
> > > removed.
> > 
> > Looks very good, and passes my tests.
> > 
> > > While doing this, I found rcustats will never show 'ggp' and 'rcc' fields
> > > due to a bug in show_rcustats().
> > 
> > Good eyes!
> > 
> > The only change I ask is that you pull the contents of
> > rcupreempt_debugfs_init() into the now-one-line rcupreempt_trace_init().
> > 
> > With that change:
> > 
> > Tested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >
> > > ---
> > >  kernel/rcupreempt_trace.c |  155 ++++++++++++++++++++-------------------------
> 
> i think the whole kernel/rcupreempt_trace.c file might be a good candidate 
> for the 'statistical tracing' extensions to ftrace that Frederice is 
> working on.
> 
> That way there would just be a few tracepoints, and the rest would be done 
> in kernel/tracing/trace_rcupreempt.c, or so.
> 	Ingo

Indeed!

The debugfs and seqfile handling and handled in the statistical
tracing engine.

You just have to provide an iterator for your stat entries through two callbacks:

_ stat_start() -> gives the first entry
_ stat_next() -> iterates over the next entry

And an output callback

_ stat_show() -> print one entry from your stat list

And two optional things:

_ stat_cmp() -> compare two entries, useful if you want your stats to be sorted
_ stat_headers() -> provide the first line in your stat file, typically to describe your columns

The last thing you need is to give a name to your trace file.
You will retrieve it into /debugfs/tracing/trace_stat/your_file_name as a current snapshot
of your stats.

It is currently used by the branch tracer, and by a pending patch for a new workqueue
tracer which will provide you a simple example.

If you have any question about how to use it, don't hesitate to ask.

Frederic.

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