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