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diff --git a/kernel/Documentation/gcov.txt b/kernel/Documentation/gcov.txt new file mode 100644 index 000000000..7b727783d --- /dev/null +++ b/kernel/Documentation/gcov.txt @@ -0,0 +1,257 @@ +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 + +select the gcc's gcov format, default is autodetect based on gcc version: + + CONFIG_GCOV_FORMAT_AUTODETECT=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" |