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Re: MIPS Function Tracer question

To: Alan Cooper <>
Subject: Re: MIPS Function Tracer question
From: David Daney <>
Date: Fri, 30 Nov 2012 08:25:16 -0800
Cc: linux-mips <>
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On 11/30/2012 06:53 AM, Alan Cooper wrote:
I needed to be more specific. The issues I'm seeing are only on 32BIT
platforms with dynamic tracing.
Let me explain the issue. When the compiler flag "-pg" is specified
for 32BIT platforms to enable tracing, the compiler adds "addiu
sp,sp,-8" in the delay slot after every call to _mcount (some legacy
thing where 2 arguments used to be pushed on the stack). The _mcount
routine is expected to adjust the sp by 8 before returning.

If the kernel's tracer infrastructure is broken for 32-bit kernels, then it should be fixed. I think it was only really tested on 64-bit kernels.

It seems that the sp adjustment should be replaced by NOP as well if tracing is disabled.

problem is when tracing is disabled, all calls to _mcount are
dynamically converted from "jal _mcount" to "nop" but the following
"addiu sp,sp,-8" instruction is unchanged and when executed leaves the
sp off by -8 for the remainder of the function. This bug is hidden
when the compiler is told to use frame pointers because all offsets
into the stack use the fp instead of the sp and the sp is restored
from the frame pointer instead of just adding a constant to sp. When
frame pointers are not enabled the code crashes. I don't think the
toolchain version makes any difference because the _mcount assembly
language routine adjusts the stack pointer by 8 if not CONFIG_64BIT
regardless of the toolchain version.

On Thu, Nov 29, 2012 at 6:00 PM, David Daney <> wrote:
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

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

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

# tracer: function_graph
# |     |   |                     |   |   |   |
   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.

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