linux-kernel - Re: [PATCHv2 03/10] ftrace: Add enable/disable ftrace

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Date:	Tue, 20 Dec 2011 15:57:27 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	fweisbec@...il.com, mingo@...hat.com, paulus@...ba.org,
	acme@...stprotocols.net, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, aarapov@...hat.com,
	Christoph Lameter <christoph@...eter.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv2 03/10] ftrace: Add enable/disable ftrace_ops control
 interface

On Mon, Dec 19, 2011 at 02:35:35PM -0500, Steven Rostedt wrote:
> On Mon, 2011-12-05 at 18:22 +0100, Jiri Olsa wrote:
> >  
> > +static inline void enable_ftrace_function(struct ftrace_ops *ops)
> > +{
> > +	atomic_t *disabled = this_cpu_ptr(ops->disabled);
> > +	atomic_dec(disabled);
> > +}
> > +
> > +static inline void disable_ftrace_function(struct ftrace_ops *ops)
> > +{
> > +	atomic_t *disabled = this_cpu_ptr(ops->disabled);
> > +	atomic_inc(disabled);
> > +}
> > +
> 
> The above should be renamed to ftrace_function_enable/disable(), and
> they should pass in the cpu. There may be a case we want to disable
> ftrace functions on another CPU.
> 
> Not to mention, this is the perfect example of "this_cpu_ptr" being used
> incorrectly. It's not made for this purpose, and again the naming of
> "this_cpu" totally confuses other kernel developers. We need to change
> this name as it was agreed to at Kernel Summit.

ok, will make the changes

> 
> If the above is called with preemption enabled, it will not do what is
> expected. We could disable function tracing on one CPU and then
> re-enable it for another CPU even though it is already enabled.

It is only called inside perf reg callback within the
schedule function where the preemption is disabled.

The ftrace_function_enable is called when task is scheduled in
on respective cpu. Likewise the ftrace_function_disable is called 
when task is scheduled out on respective cpu.

thanks,
jirka
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