1Using gcov with the Linux kernel 2================================ 3 4gcov profiling kernel support enables the use of GCC's coverage testing 5tool gcov_ with the Linux kernel. Coverage data of a running kernel 6is exported in gcov-compatible format via the "gcov" debugfs directory. 7To get coverage data for a specific file, change to the kernel build 8directory and use gcov with the ``-o`` option as follows (requires root):: 9 10 # cd /tmp/linux-out 11 # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c 12 13This will create source code files annotated with execution counts 14in the current directory. In addition, graphical gcov front-ends such 15as lcov_ can be used to automate the process of collecting data 16for the entire kernel and provide coverage overviews in HTML format. 17 18Possible uses: 19 20* debugging (has this line been reached at all?) 21* test improvement (how do I change my test to cover these lines?) 22* minimizing kernel configurations (do I need this option if the 23 associated code is never run?) 24 25.. _gcov: http://gcc.gnu.org/onlinedocs/gcc/Gcov.html 26.. _lcov: http://ltp.sourceforge.net/coverage/lcov.php 27 28 29Preparation 30----------- 31 32Configure the kernel with:: 33 34 CONFIG_DEBUG_FS=y 35 CONFIG_GCOV_KERNEL=y 36 37select the gcc's gcov format, default is autodetect based on gcc version:: 38 39 CONFIG_GCOV_FORMAT_AUTODETECT=y 40 41and to get coverage data for the entire kernel:: 42 43 CONFIG_GCOV_PROFILE_ALL=y 44 45Note that kernels compiled with profiling flags will be significantly 46larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported 47on all architectures. 48 49Profiling data will only become accessible once debugfs has been 50mounted:: 51 52 mount -t debugfs none /sys/kernel/debug 53 54 55Customization 56------------- 57 58To enable profiling for specific files or directories, add a line 59similar to the following to the respective kernel Makefile: 60 61- For a single file (e.g. main.o):: 62 63 GCOV_PROFILE_main.o := y 64 65- For all files in one directory:: 66 67 GCOV_PROFILE := y 68 69To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL 70is specified, use:: 71 72 GCOV_PROFILE_main.o := n 73 74and:: 75 76 GCOV_PROFILE := n 77 78Only files which are linked to the main kernel image or are compiled as 79kernel modules are supported by this mechanism. 80 81 82Files 83----- 84 85The gcov kernel support creates the following files in debugfs: 86 87``/sys/kernel/debug/gcov`` 88 Parent directory for all gcov-related files. 89 90``/sys/kernel/debug/gcov/reset`` 91 Global reset file: resets all coverage data to zero when 92 written to. 93 94``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcda`` 95 The actual gcov data file as understood by the gcov 96 tool. Resets file coverage data to zero when written to. 97 98``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcno`` 99 Symbolic link to a static data file required by the gcov 100 tool. This file is generated by gcc when compiling with 101 option ``-ftest-coverage``. 102 103 104Modules 105------- 106 107Kernel modules may contain cleanup code which is only run during 108module unload time. The gcov mechanism provides a means to collect 109coverage data for such code by keeping a copy of the data associated 110with the unloaded module. This data remains available through debugfs. 111Once the module is loaded again, the associated coverage counters are 112initialized with the data from its previous instantiation. 113 114This behavior can be deactivated by specifying the gcov_persist kernel 115parameter:: 116 117 gcov_persist=0 118 119At run-time, a user can also choose to discard data for an unloaded 120module by writing to its data file or the global reset file. 121 122 123Separated build and test machines 124--------------------------------- 125 126The gcov kernel profiling infrastructure is designed to work out-of-the 127box for setups where kernels are built and run on the same machine. In 128cases where the kernel runs on a separate machine, special preparations 129must be made, depending on where the gcov tool is used: 130 131a) gcov is run on the TEST machine 132 133 The gcov tool version on the test machine must be compatible with the 134 gcc version used for kernel build. Also the following files need to be 135 copied from build to test machine: 136 137 from the source tree: 138 - all C source files + headers 139 140 from the build tree: 141 - all C source files + headers 142 - all .gcda and .gcno files 143 - all links to directories 144 145 It is important to note that these files need to be placed into the 146 exact same file system location on the test machine as on the build 147 machine. If any of the path components is symbolic link, the actual 148 directory needs to be used instead (due to make's CURDIR handling). 149 150b) gcov is run on the BUILD machine 151 152 The following files need to be copied after each test case from test 153 to build machine: 154 155 from the gcov directory in sysfs: 156 - all .gcda files 157 - all links to .gcno files 158 159 These files can be copied to any location on the build machine. gcov 160 must then be called with the -o option pointing to that directory. 161 162 Example directory setup on the build machine:: 163 164 /tmp/linux: kernel source tree 165 /tmp/out: kernel build directory as specified by make O= 166 /tmp/coverage: location of the files copied from the test machine 167 168 [user@build] cd /tmp/out 169 [user@build] gcov -o /tmp/coverage/tmp/out/init main.c 170 171 172Troubleshooting 173--------------- 174 175Problem 176 Compilation aborts during linker step. 177 178Cause 179 Profiling flags are specified for source files which are not 180 linked to the main kernel or which are linked by a custom 181 linker procedure. 182 183Solution 184 Exclude affected source files from profiling by specifying 185 ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the 186 corresponding Makefile. 187 188Problem 189 Files copied from sysfs appear empty or incomplete. 190 191Cause 192 Due to the way seq_file works, some tools such as cp or tar 193 may not correctly copy files from sysfs. 194 195Solution 196 Use ``cat``' to read ``.gcda`` files and ``cp -d`` to copy links. 197 Alternatively use the mechanism shown in Appendix B. 198 199 200Appendix A: gather_on_build.sh 201------------------------------ 202 203Sample script to gather coverage meta files on the build machine 204(see 6a):: 205 206 #!/bin/bash 207 208 KSRC=$1 209 KOBJ=$2 210 DEST=$3 211 212 if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then 213 echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2 214 exit 1 215 fi 216 217 KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) 218 KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) 219 220 find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \ 221 -perm /u+r,g+r | tar cfz $DEST -P -T - 222 223 if [ $? -eq 0 ] ; then 224 echo "$DEST successfully created, copy to test system and unpack with:" 225 echo " tar xfz $DEST -P" 226 else 227 echo "Could not create file $DEST" 228 fi 229 230 231Appendix B: gather_on_test.sh 232----------------------------- 233 234Sample script to gather coverage data files on the test machine 235(see 6b):: 236 237 #!/bin/bash -e 238 239 DEST=$1 240 GCDA=/sys/kernel/debug/gcov 241 242 if [ -z "$DEST" ] ; then 243 echo "Usage: $0 <output.tar.gz>" >&2 244 exit 1 245 fi 246 247 TEMPDIR=$(mktemp -d) 248 echo Collecting data.. 249 find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \; 250 find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \; 251 find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \; 252 tar czf $DEST -C $TEMPDIR sys 253 rm -rf $TEMPDIR 254 255 echo "$DEST successfully created, copy to build system and unpack with:" 256 echo " tar xfz $DEST" 257