linux-kernel - [PATCH 0/2 v2] tracing: Enable tracepoints early and allow printk to use them

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Date:	Sun, 14 Dec 2014 15:16:09 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH 0/2 v2] tracing: Enable tracepoints early and allow printk to use them

Version 2:

  Removed the update to tracepoint.c code and just call trace_init()
  after rcu_init() where call_rcu_sched() can be used.

Version 1 at: http://lkml.kernel.org/r/20141214164104.307127356@goodmis.org

This adds two new features:

1) Allow traceopoints to be enabled right after mm_init(). By passing
in the trace_event= kernel command line parameter, tracepoints can be
enabled at boot up. For debugging things like the initialization of
interrupts, it is needed to have tracepoints enabled very early. People
have asked about this before and this has been on my todo list. As it
can be helpful for Thomas to debug his upcoming 3.20 IRQ work, I'm
pushing this now. This way he can add tracepoints into the IRQ set up
and have have users enable them when things go wrong.

2) Have the tracepoints printed via printk() (the console) when they
are triggered. If the irq code locks up or reboots the box, having the
tracepoint output go into the kernel ring buffer is useless for
debugging. But being able to add the tp_printk kernel command line
option along with the trace_event= option will have these tracepoints
printed as they occur, and that can be really useful for debugging
early lock up or reboot problems.

As the merge window is still open, and this code was not as complex
as I thought it might be. I'm thinking of pushing this in now.

This will allow Thomas to debug his irq work for 3.20.

This code is not that intrusive and I'm currently running it through
all my tests. But I like to have some more eyes on this, as I don't
like to push something into mainline this quickly. I'm doing this only
because the code is simple enough and is useful for work coming in 3.20.

I also feel that I could handle the fall out of this work over the
holidays if need be.

Steven Rostedt (Red Hat) (2):
      tracing: Move enabling tracepoints to just after rcu_init()
      tracing: Add tp_printk cmdline to have tracepoints go to printk()

----
 Documentation/kernel-parameters.txt | 18 ++++++++++++++++
 include/linux/ftrace.h              |  7 +++++++
 init/main.c                         |  4 ++++
 kernel/sysctl.c                     |  7 +++++++
 kernel/trace/trace.c                | 25 +++++++++++++++++++++-
 kernel/trace/trace.h                | 14 +++++++++++++
 kernel/trace/trace_events.c         | 42 +++++++++++++++++++++++++++++++++++--
 kernel/trace/trace_syscalls.c       |  7 ++-----
 8 files changed, 116 insertions(+), 8 deletions(-)
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