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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_DEBUG_FS=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.
-
-Problem: Files copied from sysfs appear empty or incomplete.
-Cause: Due to the way seq_file works, some tools such as cp or tar
- may not correctly copy files from sysfs.
-Solution: Use 'cat' to read .gcda files and 'cp -d' to copy links.
- Alternatively use the mechanism shown in Appendix B.
-
-
-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 -e
-
-DEST=$1
-GCDA=/sys/kernel/debug/gcov
-
-if [ -z "$DEST" ] ; then
- echo "Usage: $0 <output.tar.gz>" >&2
- exit 1
-fi
-
-TEMPDIR=$(mktemp -d)
-echo Collecting data..
-find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
-find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
-find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
-tar czf $DEST -C $TEMPDIR sys
-rm -rf $TEMPDIR
-
-echo "$DEST successfully created, copy to build system and unpack with:"
-echo " tar xfz $DEST"