RFC gcov kernel support

This is resend #1 of the gcov kernel support patch set (see further below for an in-depth explanation). Patch base is 2.6.26-rc1.

Changes to previous version:

* [patch 4/8] module: add MODULE_STATE_GOING notifier call Removed - patch is part of rc1
* [patch 5/8] module: add function to map address to containing module Removed - patch has been replaced by similar logic in the gcov base patch (patch 5/6)

Comments on the validity of this approach, especially concerning the introduction of constructors and the changes to kbuild, are very much welcome.

Patch overview:

[patch 1/6] kernel: call constructors
[patch 2/6] kbuild: let addtree result in absolute paths only [patch 3/6] kbuild: convert include and source paths [patch 4/6] seq_file: add function to write binary data [patch 5/6] gcov: add gcov profiling infrastructure [patch 6/6] gcov: architecture specific compile flag adjustments

Patches 1,3-4 introduce/change functionality required for gcov's profiling mechanism to work. Patch 2 is somewhat of a bug fix. Patch 5 implements the actual gcov support and debugfs interface. Patch 6 is required for architecture specific adjustments.

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This set of patches enables the use of GCC's coverage testing tool gcov [1] with kernel 2.6.25. Coverage data for the running kernel is exported via debugfs in a gcov-compatible format. To get coverage data for a specific file, simply use gcov with the -o option:

# gcov -o /sys/kernel/debug/gcov/tmp/linux-2.6.25/kernel spinlock.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