linux-kernel - Re: [PATCH 2/7] ftrace: Add enable/disable ftrace

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Message-ID: <20120117014209.GB24200@somewhere.redhat.com>
Date:	Tue, 17 Jan 2012 02:42:12 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	rostedt@...dmis.org, mingo@...hat.com, paulus@...ba.org,
	acme@...stprotocols.net, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, aarapov@...hat.com
Subject: Re: [PATCH 2/7] ftrace: Add enable/disable ftrace_ops control
 interface

On Mon, Jan 02, 2012 at 10:04:15AM +0100, Jiri Olsa wrote:
> Adding a way to temporarily enable/disable ftrace_ops. The change
> follows the same way as 'global' ftrace_ops are done.
> 
> Introducing 2 global ftrace_ops - control_ops and ftrace_control_list
> which take over all ftrace_ops registered with FTRACE_OPS_FL_CONTROL
> flag. In addition new per cpu flag called 'disabled' is also added to
> ftrace_ops to provide the control information for each cpu.
> 
> When ftrace_ops with FTRACE_OPS_FL_CONTROL is registered, it is
> set as disabled for all cpus.
> 
> The ftrace_control_list contains all the registered 'control' ftrace_ops.
> The control_ops provides function which iterates ftrace_control_list
> and does the check for 'disabled' flag on current cpu.
> 
> Adding 2 inline functions ftrace_function_enable/ftrace_function_disable,
> which enable/disable the ftrace_ops for given cpu.
> 
> Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> ---

So this is used to implement pmu->add() / -> del(). But perf_tp_event_match()
already takes care of that by checking PERF_HES_STOPPED.

Now what you are doing here is an interesting optimization as it doesn't even
call on ftrace_ops that have called pmu->del().

I'm not against the idea but I want to ensure this is really your purpose
and it would be nice to put some words about that in the changelog as
well as in PATCH 4/7.

>  include/linux/ftrace.h |   42 +++++++++++++++++
>  kernel/trace/ftrace.c  |  119 +++++++++++++++++++++++++++++++++++++++++++++---
>  kernel/trace/trace.h   |    2 +
>  3 files changed, 156 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 523640f..0d43a2b 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -35,12 +35,14 @@ enum {
>  	FTRACE_OPS_FL_ENABLED		= 1 << 0,
>  	FTRACE_OPS_FL_GLOBAL		= 1 << 1,
>  	FTRACE_OPS_FL_DYNAMIC		= 1 << 2,
> +	FTRACE_OPS_FL_CONTROL		= 1 << 3,

Please comment the role of this flag. In fact it would be nice to have
a comment that explains all these. GLOBAL and DYNAMIC don't actually give
much clues alone.

Thanks.
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