linux-kernel - Re: [PATCH 0/18] Allow different tracers to be compiled independently

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Date:	Tue, 23 Mar 2010 02:04:21 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jan Kara <jack@...e.cz>, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Li Zefan <lizf@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/18] Allow different tracers to be compiled
	independently

On Tue, Mar 23, 2010 at 01:32:02AM +0100, Jan Kara wrote:
> 
>   Hi,
> 
>   currently, when one tracer is selected, most of tracepoints for other
> tracers also gets pulled into the kernel. So for example it's not possible
> to enable BLK_DEV_IO_TRACE without polluting slab allocation paths with
> tracepoint checks (see changelog of patch 01). This patch set adds a
> possibility for each set of trace points to be compile-enabled separately.
>   The first patch contains the necessary magic in linux/tracepoint.h. Other
> patches just tell tracing framework about correspoding config options
> and possibly introduce them if they did not exist before.
>   The patches in this patch set are actually completely independent so 
> they can be merged via respective subsystem trees. But changes are rather
> tiny so I don't expect much conflicts...
> 
> 								Honza


(Adding more people in Cc)

I don't know. Yeah this first looks like a good idea but once
CONFIG_EVENT_TRACING is enabled, each tracepoint is a lightweight
thing and induce a tiny overhead, probably hard to notice, and
this is going to be even more the case after the jmp label
optimization patches.

I liked the fact we had a general tracing kernel once the above
config is selected. And we don't bother telling people that to
use tool X you need CONFIG_EVENT_Y, and you need to rebuild your
kernel, etc...

I wonder if this is going to be a win.

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