gcov: add gcov profiling infrastructure

Enable the use of GCC's coverage testing tool gcov [1] with the Linux
kernel.  gcov may be useful for:

 * debugging (has this code been reached 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 run?)

The profiling patch incorporates the following changes:

 * change kbuild to include profiling flags
 * provide functions needed by profiling code
 * present profiling data as files in debugfs

Note that on some architectures, enabling gcc's profiling option
"-fprofile-arcs" for the entire kernel may trigger compile/link/
run-time problems, some of which are caused by toolchain bugs and
others which require adjustment of architecture code.

For this reason profiling the entire kernel is initially restricted
to those architectures for which it is known to work without changes.
This restriction can be lifted once an architecture has been tested
and found compatible with gcc's profiling. Profiling of single files
or directories is still available on all platforms (see config help
text).

[1] http://gcc.gnu.org/onlinedocs/gcc/Gcov.html

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Li Wei <W.Li@Sun.COM>
Cc: Michael Ellerman <michaele@au1.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Heiko Carstens <heicars2@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <mschwid2@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 246 insertions, 0 deletions

diff --git a/Documentation/gcov.txt b/Documentation/gcov.txt
new file mode 100644
index 000000000000..e716aadb3a33
--- /dev/null
+++ b/Documentation/gcov.txt
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+Using gcov with the Linux kernel
+================================
+
+1. Introduction
+2. Preparation
+3. Customization
+4. Files
+5. Modules
+6. Separated build and test machines
+7. Troubleshooting
+Appendix A: sample script: gather_on_build.sh
+Appendix B: sample script: gather_on_test.sh
+
+
+1. Introduction
+===============
+
+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.
+To get coverage data for a specific file, change to the kernel build
+directory and use gcov with the -o option as follows (requires root):
+
+# cd /tmp/linux-out
+# gcov -o /sys/kernel/debug/gcov/tmp/linux-out/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 reached 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 run?)
+
+--
+
+[1] http://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+[2] http://ltp.sourceforge.net/coverage/lcov.php
+
+
+2. Preparation
+==============
+
+Configure the kernel with:
+
+        CONFIG_DEBUGFS=y
+        CONFIG_GCOV_KERNEL=y
+
+and to get coverage data for the entire kernel:
+
+        CONFIG_GCOV_PROFILE_ALL=y
+
+Note that kernels compiled with profiling flags will be significantly
+larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
+on all architectures.
+
+Profiling data will only become accessible once debugfs has been
+mounted:
+
+        mount -t debugfs none /sys/kernel/debug
+
+
+3. Customization
+================
+
+To enable profiling for specific files or directories, add a line
+similar to the following to the respective kernel Makefile:
+
+        For a single file (e.g. main.o):
+                GCOV_PROFILE_main.o := y
+
+        For all files in one directory:
+                GCOV_PROFILE := y
+
+To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
+is specified, use:
+
+                GCOV_PROFILE_main.o := n
+        and:
+                GCOV_PROFILE := n
+
+Only files which are linked to the main kernel image or are compiled as
+kernel modules are supported by this mechanism.
+
+
+4. Files
+========
+
+The gcov kernel support creates the following files in debugfs:
+
+        /sys/kernel/debug/gcov
+                Parent directory for all gcov-related files.
+
+        /sys/kernel/debug/gcov/reset
+                Global reset file: resets all coverage data to zero when
+                written to.
+
+        /sys/kernel/debug/gcov/path/to/compile/dir/file.gcda
+                The actual gcov data file as understood by the gcov
+                tool. Resets file coverage data to zero when written to.
+
+        /sys/kernel/debug/gcov/path/to/compile/dir/file.gcno
+                Symbolic link to a static data file required by the gcov
+                tool. This file is generated by gcc when compiling with
+                option -ftest-coverage.
+
+
+5. Modules
+==========
+
+Kernel modules may contain cleanup code which is only run during
+module unload time. The gcov mechanism provides a means to collect
+coverage data for such code by keeping a copy of the data associated
+with the unloaded module. This data remains available through debugfs.
+Once the module is loaded again, the associated coverage counters are
+initialized with the data from its previous instantiation.
+
+This behavior can be deactivated by specifying the gcov_persist kernel
+parameter:
+
+        gcov_persist=0
+
+At run-time, a user can also choose to discard data for an unloaded
+module by writing to its data file or the global reset file.
+
+
+6. Separated build and test machines
+====================================
+
+The gcov kernel profiling infrastructure is designed to work out-of-the
+box for setups where kernels are built and run on the same machine. In
+cases where the kernel runs on a separate machine, special preparations
+must be made, depending on where the gcov tool is used:
+
+a) gcov is run on the TEST machine
+
+The gcov tool version on the test machine must be compatible with the
+gcc version used for kernel build. Also the following files need to be
+copied from build to test machine:
+
+from the source tree:
+  - all C source files + headers
+
+from the build tree:
+  - all C source files + headers
+  - all .gcda and .gcno files
+  - all links to directories
+
+It is important to note that these files need to be placed into the
+exact same file system location on the test machine as on the build
+machine. If any of the path components is symbolic link, the actual
+directory needs to be used instead (due to make's CURDIR handling).
+
+b) gcov is run on the BUILD machine
+
+The following files need to be copied after each test case from test
+to build machine:
+
+from the gcov directory in sysfs:
+  - all .gcda files
+  - all links to .gcno files
+
+These files can be copied to any location on the build machine. gcov
+must then be called with the -o option pointing to that directory.
+
+Example directory setup on the build machine:
+
+  /tmp/linux:    kernel source tree
+  /tmp/out:      kernel build directory as specified by make O=
+  /tmp/coverage: location of the files copied from the test machine
+
+  [user@build] cd /tmp/out
+  [user@build] gcov -o /tmp/coverage/tmp/out/init main.c
+
+
+7. Troubleshooting
+==================
+
+Problem:  Compilation aborts during linker step.
+Cause:    Profiling flags are specified for source files which are not
+          linked to the main kernel or which are linked by a custom
+          linker procedure.
+Solution: Exclude affected source files from profiling by specifying
+          GCOV_PROFILE := n or GCOV_PROFILE_basename.o := n in the
+          corresponding Makefile.
+
+
+Appendix A: gather_on_build.sh
+==============================
+
+Sample script to gather coverage meta files on the build machine
+(see 6a):
+
+#!/bin/bash
+
+KSRC=$1
+KOBJ=$2
+DEST=$3
+
+if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
+  echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
+  exit 1
+fi
+
+KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
+KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
+
+find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
+                 -perm /u+r,g+r | tar cfz $DEST -P -T -
+
+if [ $? -eq 0 ] ; then
+  echo "$DEST successfully created, copy to test system and unpack with:"
+  echo "  tar xfz $DEST -P"
+else
+  echo "Could not create file $DEST"
+fi
+
+
+Appendix B: gather_on_test.sh
+=============================
+
+Sample script to gather coverage data files on the test machine
+(see 6b):
+
+#!/bin/bash
+
+DEST=$1
+GCDA=/sys/kernel/debug/gcov
+
+if [ -z "$DEST" ] ; then
+  echo "Usage: $0 <output.tar.gz>" >&2
+  exit 1
+fi
+
+find $GCDA -name '*.gcno' -o -name '*.gcda' | tar cfz $DEST -T -
+
+if [ $? -eq 0 ] ; then
+  echo "$DEST successfully created, copy to build system and unpack with:"
+  echo "  tar xfz $DEST"
+else
+  echo "Could not create file $DEST"
+fi