linux-kernel - Re: [RFC PATCH 1/1] trace: Move trace event enable from fs_initcall to early

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Date:	Fri, 17 Aug 2012 11:04:07 -0300
From:	Ezequiel Garcia <elezegarcia@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	tim.bird@...sony.com, lizefan@...wei.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH 1/1] trace: Move trace event enable from fs_initcall
 to early_initcall

On Fri, Aug 17, 2012 at 10:55 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 2012-08-17 at 08:01 -0300, Ezequiel Garcia wrote:
>
>> Regarding the 'complete solution': to be able to capture events from
>> the very beggining...
>> Have you thought about this?
>> Could you give me a hint on how could I implement it?
>
> How far in the "beginning"? Before memory is set up?

Yes.

> I wouldn't do that.

Yeah, perhaps it sounds crazy. It makes some sense for kmem events, though.

> I have in the past (set up before memory was finished being
> initialized), but things have changed since then.
>
> One thing that we could do for those that want really early tracing, is
> to add a config option to add a static temporary ring buffer, that gets

Yes, something like this would be ideal. How would this ring buffer be
allocated?
Perhaps as static and __initdata?
This way it would be released afterwards, right?

> copied into the default ring buffer after memory is set up. That may be
> the easiest way.
>
> Once memory is set up, the ring buffer can be allocated and events can
> be traced, but the ring buffer needs to be set up first. All it would
> take is some calls in init/main.c start_kernel() to the initialization.
>

Note that my main concern is on trace_events (kmem events to be precise).
However this are registered through tracepoints and in turn this tracepoints
depend on kmalloc and friends. So, right now is a chicken-egg problem.

Thanks,
Ezequiel.
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