Analyzing Kernel Modules Data in OProfile in RHEL3

William Cohen
wcohen@redhat.com
6/20/2005

OProfile collects data on a system-wide basis for kernel- and
user-space code running on the machine. However, once a module is
loaded into the kernel, the information about the origin of the kernel
module is lost. The module could have come from the initrd file on
boot up, the directory with the various kernel modules, or a locally
created kernel module.  As a result when OProfile records sample for a
module, it just lists the samples for the modules for an executable in
the root directory, but this is unlikley to the place with the actual
code for the module.  You will need to take some steps to make sure
that analysis tools get the executable.

For example on a i686 machine the sampling is set up to record
"CPU_CLOCK_UNHALTED" and you would like to see the data for the ext3
module:

opcontrol --setup --vmlinux=/boot/vmlinux-`uname -r` \
--ctr0-event=CPU_CLK_UNHALTED --ctr0-count=366000 \
--separate=library

opcontrol --start


Find the appropriate module by doing something like the the following
(and replacing ext3 with the appropriate module name)

$ op_time |grep ext3
23         0.4061  0.0000 /lib/ext3.o

If the op_time did not list the file in the correct place, make a
symbolic link so oprofile finds the code for the module in the
location listed by op_time (for the case of ext3.o):

# ln -s /lib/modules/`uname -r`/kernel/fs/ext3/ext3.o /lib/.


Then the detailed information can be obtained with:

# oprofpp -r -l /lib/ext3.o |more
oprofpp: warning: could not check that the binary file /lib/ext3.o
has not been modified since the profile was taken. Results may be inaccurate.
Cpu type: PIII
Cpu speed was (MHz estimation) : 731.48
Counter 0 counted CPU_CLK_UNHALTED events (clocks processor is not halted) with
a unit mask of 0x00 (No unit mask) count 366000
vma      samples  %           symbol name
000106d0 5        21.7391     ext3_permission
000082d0 3        13.0435     ext3_find_entry
00007010 2        8.69565     ext3_do_update_inode
00006af0 2        8.69565     ext3_get_inode_loc
00004610 2        8.69565     ext3_getblk
00004160 2        8.69565     ext3_get_block_handle
00001db0 2        8.69565     ext3_open_file
000055e0 1        4.34783     ext3_releasepage
00004a90 1        4.34783     ext3_prepare_write
00003aa0 1        4.34783     ext3_get_branch
00003960 1        4.34783     ext3_block_to_path
00001860 1        4.34783     ext3_check_dir_entry
00011020 0        0           __constant_memcpy
...