On 08/11/2011 07:31 PM, Gilles Chanteperdrix wrote:
> On 08/11/2011 07:21 PM, Roland Stigge wrote:
>> Hi,
>>
>> On 08/11/2011 04:48 PM, Daniele Nicolodi wrote:
>>> I compiled linux 2.6.38.8 and xenomai-head with gcc-4.6. The obtained
>>> kernel boots fine but xenomai services do not: latency hangs right after
>>> the sched_setscheduler system call. With the same kernel I compiled user
>>> space with gcc-4.4 and xenomia services work just fine.
>>
>> I can confirm this now for Debian. Sorry for the delay.
>>
>> Will use gcc-4.4 for building the Debian package for now.
>>
The gcc 4.6 version is compiled with frame pointer disabled, whereas the
4.4 version is compiled with frame pointers. Enabling the
-fomit-frame-pointer option with gcc 4.4 seems to cause issues as well.
With gcc 4.4, the difference is very visible in rt_task_start. The
correct syscall chunk is:
11: 0d 2b 02 00 02 or $0x200022b,%eax
Computes the syscall number, put the result in eax.
16: 89 45 fc mov %eax,-0x4(%ebp)
Move the result from eax to temporary space on stack.
19: 8b 45 08 mov 0x8(%ebp),%eax
Load syscall first argument value into eax
1c: 53 push %ebx
1d: 89 c3 mov %eax,%ebx
Push ebx, move syscall first argument from eax to ebx
1f: 8b 45 fc mov -0x4(%ebp),%eax
Reload the syscall number from its temporary storage to eax.
22: 65 ff 15 10 00 00 00 call *%gs:0x10
29: 5b pop %ebx
Syscall done.
The version without frame pointer:
4e3d: 8b 00 mov (%eax),%eax
4e3f: 0d 2b 02 00 02 or $0x200022b,%eax
4e44: 89 44 24 0c mov %eax,0xc(%esp)
4e48: 8b 44 24 18 mov 0x18(%esp),%eax
4e4c: 53 push %ebx
4e4d: 89 c3 mov %eax,%ebx
4e4f: 8b 44 24 0c mov 0xc(%esp),%eax
4e53: 65 ff 15 10 00 00 00 call *%gs:0x10
4e5a: 5b pop %ebx
This is essentially the same sequence, except that the temporary storage
used to store the syscall number is obtained via a relative offset of
the stack pointer (esp) instead of a relative offset of the frame
pointer, since we are compiling without frame pointers, the problem is
that between the time this value is stored, and the time it is restored,
the stack pointer changed since we pushed %ebx.
So, I do not really know what to make of it. The simple solution seems
to me to continue compiling with frame pointers. I.e. add
-fno-omit-frame-pointer with gcc 4.6.
--
Gilles.
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