linux-kernel - [RFC PATCH 0/1] trace: Add early event trace support

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Message-ID: <CALF0-+W4afY+sCn5KMa-4ONmr7YnCu1m6d5P+aboSKe5=mgOMQ@mail.gmail.com>
Date:	Wed, 15 Aug 2012 12:20:26 -0300
From:	Ezequiel Garcia <elezegarcia@...il.com>
To:	rostedt@...dmis.org, linux-kernel@...r.kernel.org
Cc:	Pekka Enberg <penberg@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Tim Bird <tim.bird@...sony.com>, lizefan@...wei.com,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [RFC PATCH 0/1] trace: Add early event trace support

Hi Steven,

This patch is my first attempt to address the "early tracing issue".
Right now, trace events are registered on fs_initcall, which is very late
if you want to capture boot-up events.

This feature is in developers wish list as this old (2009) thread shows [1].

By splitting the initialization, separating the ftrace enable from the
sysfs entry
creation, it is possible to move events enable to early_initcall.
(which is also late, but I wouldn't expect more from such a small patch).

Without this patch, and using
"trace_event=kmem:kmalloc,kmem:kmalloc_node,kmem:kfree"
as kernel parameter, on a bare kernel I can start capturing kmallocs
from ~0.4 secs,
and I get 186 callers.

This patch moves the capture to ~ 0.2 secs and captures 220 callers.

The caller count is inaccurate since gcc does some automatic inlining
(I have another RFC PATCH for that), but it shows a rough increase.

Also, this enable is not time-consuming:
[    0.190000] calling  event_trace_enable+0x0/0xad @ 1
[    0.190000] initcall event_trace_enable+0x0/0xad returned 0 after 0 usecs

This patch is intended to trigger this discussion: to be honest I'm
not sure how ugly
this is. So some feedback about it would be greatly appreciated.

Also, I'd like to ask for some guidelines (not solutions, just some guidelines)
on how to attack this problem.
I'm interesting on capturing kmem events, from the very first kmalloc onwards.
Though, it would be nice to extend the solution to other event classes.

Thanks,
Ezequiel.

[1] http://lkml.indiana.edu/hypermail/linux/kernel/0911.3/00036.html
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