Analyzing Kernel Modules Data in OProfile in Fedora Core 3/4 and RHEL4

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 AMD64 machine the sampling is set up to record "Data
cache accesses" and "Data cache misses" and you would like to see the
data for the ext3 module:


$ opreport /ext3
CPU: AMD64 processors, speed 797.948 MHz (estimated)
Counted DATA_CACHE_ACCESSES events (Data cache accesses) with a unit mask of 0x00 (No unit mask) count 500000
Counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (No unit mask) count 500000
DATA_CACHE_ACC...|DATA_CACHE_MIS...|
  samples|      %|  samples|      %|
------------------------------------
   148721 100.000      1493 100.000 ext3

To get a more detailed view of the actions of the module, you will
need to ether have the module be unstripped (e.g. installed from a
custom build) or have the debuginfo RPM installed for the kernel.

Find out which kernel is running, "uname -a", get the appropriate
debuginfo rpm, and install on the machine.

Then make a symbolic link so oprofile finds the code for the module in the
correct place:

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

Then the detailed information can be obtained with:

# opreport image:/ext3 -l|more
warning: could not check that the binary file /ext3 has not been modified since the profile was taken. Results may be inaccurate.
CPU: AMD64 processors, speed 797.948 MHz (estimated)
Counted DATA_CACHE_ACCESSES events (Data cache accesses) with a unit mask of 0x00 (No unit mask) count 500000
Counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (No unit mask) count 500000
samples  %        samples  %        symbol name
16728    11.2479  7         0.4689  ext3_group_sparse
16454    11.0637  4         0.2679  ext3_count_free_blocks
14583     9.8056  51        3.4159  ext3_fill_super
8281      5.5681  129       8.6403  ext3_ioctl
7810      5.2514  62        4.1527  ext3_write_info
7286      4.8991  67        4.4876  ext3_ordered_writepage
6509      4.3767  130       8.7073  ext3_new_inode
6378      4.2886  156      10.4488  ext3_new_block
5932      3.9887  87        5.8272  ext3_xattr_block_list
...