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Re: MIPS Function Tracer question

To: Alan Cooper <alcooperx@gmail.com>
Subject: Re: MIPS Function Tracer question
From: David Daney <ddaney.cavm@gmail.com>
Date: Thu, 29 Nov 2012 15:00:44 -0800
Cc: ralf@linux-mips.org, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
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On 11/29/2012 01:04 PM, Alan Cooper wrote:
I've been doing some testing of the MIPS Function Tracer functionality
on the 3.3 kernel. I was surprised to find that the option to generate
frame pointers was required for tracing.

It is not really required for MIPS function tracing, but the Kconfigs for some reason set it.

 When I don't enable
FRAME_POINTER along with FUNCTION_TRACER, the kernel hangs on boot. I
also noticed that a checkin to the 3.4 kernel
(b732d439cb43336cd6d7e804ecb2c81193ef63b0) no longer forces on
FRAME_POINTER when FUNCTION_TRACER is selected. I was wondering how it
works in 3.4 and beyond, so I built a Malta kernel from the latest
MIPS tree with FUNCTION_TRACING enabled and tested it with QEMU. The
kernel hung the same way. I can think of 2 reasons for this:
1. Function tracing is broken for MIPS in 3.4 and beyond.
2. The 4.5.3 GNU C compiler I'm using is generating different code for
function tracing.

Function tracing works best with recent versions of GCC (those that support -mmcount-ra-address).

I was wondering if anyone has MIPS function tracing working in 3.4 or later?

Yes.  Using GCC 4.7.0 on an octeon kernel (based on 3.4.14):

# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
  1)               |  __fsnotify_parent() {
  1)   7.154 us    |  } /* __fsnotify_parent */
  1)               |  fsnotify() {
  1)               |    __srcu_read_lock() {
  1)               |      add_preempt_count() {
  1)   1.356 us    |      } /* add_preempt_count */
  1)               |      sub_preempt_count() {
  1)   1.385 us    |      } /* sub_preempt_count */
  1)   6.747 us    |    } /* __srcu_read_lock */
  1)               |    __srcu_read_unlock() {
  1)               |      add_preempt_count() {
  1)   1.383 us    |      } /* add_preempt_count */
  1)               |      sub_preempt_count() {
  1)   1.358 us    |      } /* sub_preempt_count */
  1)   6.642 us    |    } /* __srcu_read_unlock */
  1) + 17.861 us   |  } /* fsnotify */
.
.
.




I did figure out why it's hanging and I have some changes that will
allow the function tracer to run without frame pointers, but before I
proceed I want to rule out compiler differences.

Thanks
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