gcov kernel support

This is version #3 of the gcov kernel support patch set (see further below for an in-depth explanation). Patch base is 2.6.26-rc4. Please replace the previous gcov patch set with this one.

Changes based on mailing list feedback:

* removed patch "kbuild: make source and include paths absolute". Replaced by new approach (patch 5) which creates symbolic links in the build directory instead.
* updated docs to match new approach
* removed patch "kbuild: delay object file renaming during module versioning". No longer required.
* gcov core code no longer creates symbolic links in debugfs
* updated patch "gcov: architecture specific compile flag adjustments". Contains changes for powerpc.

Patch overview:

[PATCH 1/6] kernel: call constructors
[PATCH 2/6] kernel: introduce GCC_VERSION_LOWER macro [PATCH 3/6] seq_file: add function to write binary data [PATCH 4/6] gcov: add gcov profiling infrastructure [PATCH 5/6] gcov: create links to .gcda files in build directory [PATCH 6/6] gcov: architecture specific compile flag adjustments

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gcov profiling kernel support enables the use of GCC's coverage testing tool gcov [1] with the Linux kernel. Coverage data of a running kernel is exported in gcov-compatible format via the "gcov "debugfs directory.

Example:

To get coverage data for file base.c in directory drivers/base, simply change to the kernel build directory and run gcov with the -o option (assumptions: kernel was built in /tmp/linux and debugfs is mounted at /sys/kernel/debug):

# cd /tmp/linux
# gcov -o drivers/base/ bus.c

This will create source code files annotated with execution counts in the current directory. In addition, graphical gcov front-ends such as lcov [2] can be used to automate the process of collecting data for the entire kernel and provide coverage overviews in HTML format.

Possible uses:

* debugging (has this line been executed at all?)
* test improvement (how do I change my test to cover these lines?)
* minimizing kernel configurations (do I need this option if the associated code is never executed?)

Known issues:

* some architecture specific problems: the patch has been tested successfully on s390 and i386. Known problems exist on x86_64 and arm (to be investigated)
* GCC's profiling mechanism together with optimization sometimes produces skewed data (see [1])
* GCC's profiling code assumes single-threaded execution
* gcov assumes that a program has finished when coverage data is analyzed

Despite these issues, the data which can be obtained has been proven to be sufficiently accurate for most practical uses.

History:

Hubertus Franke <[email protected]> wrote the first version of this patch around 2002. Since then it has been adapted to new versions of the kernel and GCC with contributions by several people (see file kernel/gcov/fs.c, write me if I missed anyone). Due to regular requests, I rewrote the gcov-kernel patch from scratch so that it would (hopefully) be fit for inclusion into the upstream kernel.

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[1] http://gcc.gnu.org/onlinedocs/gcc/Gcov.html
[2] http://ltp.sourceforge.net/coverage/lcov.php