linux-kernel - Re: MIPS Function Tracer question

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Date:	Thu, 29 Nov 2012 15:00:44 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	Alan Cooper <alcooperx@...il.com>
CC:	ralf@...ux-mips.org, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: MIPS Function Tracer question

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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