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Subjectfix callgraphs of 32-bit processes on 64-bit kernels.
Date
Hi,

Callgraph profiling 32-bit apps on a 64-bit kernel doesn't work.
The reason is that perf_callchain_user tries to read a stackframe with 64-bit
pointers, which is wrong for a 32-bit process.

This patch fixes that, and I am almost able to get nice callgraph profiles
from 32-bit apps now! (except for some problems with perf itself when tracing
kernel modules, see [1])

Page-faults can be traced nicely (sid-ia32 is a 32-bit chroot):

$ sudo perf record -e page-faults -f -g /home/edwin/sid-ia32/usr/bin/glxgears
$ sudo perf report
...
45.33% libc-2.10.2.so [.] __GI_memcpy
|
--- __GI_memcpy
_mesa_BufferDataARB
_mesa_meta_Clear
radeonUserClear
r700Clear
_mesa_Clear
0x8049367
0x804a6ba
__libc_start_main
0x8049111

16.96% libc-2.10.2.so [.] __GI_memset
|
--- __GI_memset
_tnl_init_vertices
_swsetup_CreateContext
r600CreateContext
driCreateNewContext
dri2CreateNewContext
0xf77ab7dd
0xf7783c67
0xf778514c
0x804974f
0x804a33d
__libc_start_main
0x8049111

And CPU cycles can be traced too in userspace:
$ sudo perf record -f -g /home/edwin/sid-ia32/usr/bin/glxgears
$ sudo perf report --sort comm,dso
[...]
44.97% glxgears r600_dri.so
|
|--5.85%-- r700SendSPIState
| radeonEmitState
| r700DrawPrims
| |
| |--95.45%-- vbo_save_playback_vertex_list
| | execute_list
| | _mesa_CallList
| | neutral_CallList
| | |
| | |--38.10%-- 0x80494a8
| | | 0x804a6ba
| | | __libc_start_main
| | | 0x8049111
[....]
40.00% glxgears [kernel]
|
|--3.14%-- copy_user_generic_string
| |
| |--71.70%-- 0xffffffffa01b4493
| | 0xffffffffa01b7c0b
| | 0xffffffffa018b45b
| | 0xffffffffa00ca927
| | 0xffffffffa01c524e
| | compat_sys_ioctl
| | sysenter_dispatch
| | 0xf77ca430
| | drmCommandWriteRead
| | 0xf74d7ab5
| | 0xf74d89a4
| | rcommonFlushCmdBufLocked
| | rcommonFlushCmdBuf
| | radeonFlush
| | _mesa_flush
| | _mesa_Flush
| | 0xf775f270
| | 0x804a6d5
| | __libc_start_main
| | 0x8049111
| |
| |--15.09%-- 0xffffffffa01c524e
| | compat_sys_ioctl
| | sysenter_dispatch
| | 0xf77ca430
| | drmCommandWriteRead

[1] But there is a problem with the perf tool: it can't trace addresses in
kernel modules. This is a problem regardless if the traced app is 32-bit or
64-bit; and regardless if I do callgraph profiling or not.
See the above trace, where the kernel addresses have all ffffffffa0* without a
symbol name.

If I look at /proc/kallsyms I can guess the symbols, for example
0xffffffffa01b4493 is probably this one:
ffffffffa01b4411 t r600_cs_packet_parse [radeon]

If I record/report without callgraph its the same problem:
[...]
24.01% glxgears [kernel] [k] 0xffffffffa01b4ee9
3.96% glxgears libdrm_radeon.so.1.0.0 [.] cs_gem_write_reloc
3.53% glxgears r600_dri.so [.] r700SendSPIState
2.77% glxgears r600_dri.so [.] r700DrawPrims
1.99% glxgears r600_dri.so [.] r700SendVSConsts

Kernel symbol for 0xffffffffa01b4ee9 is not shown, I can guess it is this one
(hey it was an exact match!):
ffffffffa01b4ee9 t r600_packet3_check [radeon]

It would be good if perf knew how to lookup symbols in kernel modules!

Best regards,
--Edwin


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