Based on kernel version 4.8. Page generated on 2016-10-06 23:16 EST.
1 Using gcov with the Linux kernel 2 ================================ 3 4 1. Introduction 5 2. Preparation 6 3. Customization 7 4. Files 8 5. Modules 9 6. Separated build and test machines 10 7. Troubleshooting 11 Appendix A: sample script: gather_on_build.sh 12 Appendix B: sample script: gather_on_test.sh 13 14 15 1. Introduction 16 =============== 17 18 gcov profiling kernel support enables the use of GCC's coverage testing 19 tool gcov [1] with the Linux kernel. Coverage data of a running kernel 20 is exported in gcov-compatible format via the "gcov" debugfs directory. 21 To get coverage data for a specific file, change to the kernel build 22 directory and use gcov with the -o option as follows (requires root): 23 24 # cd /tmp/linux-out 25 # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c 26 27 This will create source code files annotated with execution counts 28 in the current directory. In addition, graphical gcov front-ends such 29 as lcov [2] can be used to automate the process of collecting data 30 for the entire kernel and provide coverage overviews in HTML format. 31 32 Possible uses: 33 34 * debugging (has this line been reached at all?) 35 * test improvement (how do I change my test to cover these lines?) 36 * minimizing kernel configurations (do I need this option if the 37 associated code is never run?) 38 39 -- 40 41 [1] http://gcc.gnu.org/onlinedocs/gcc/Gcov.html 42 [2] http://ltp.sourceforge.net/coverage/lcov.php 43 44 45 2. Preparation 46 ============== 47 48 Configure the kernel with: 49 50 CONFIG_DEBUG_FS=y 51 CONFIG_GCOV_KERNEL=y 52 53 select the gcc's gcov format, default is autodetect based on gcc version: 54 55 CONFIG_GCOV_FORMAT_AUTODETECT=y 56 57 and to get coverage data for the entire kernel: 58 59 CONFIG_GCOV_PROFILE_ALL=y 60 61 Note that kernels compiled with profiling flags will be significantly 62 larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported 63 on all architectures. 64 65 Profiling data will only become accessible once debugfs has been 66 mounted: 67 68 mount -t debugfs none /sys/kernel/debug 69 70 71 3. Customization 72 ================ 73 74 To enable profiling for specific files or directories, add a line 75 similar to the following to the respective kernel Makefile: 76 77 For a single file (e.g. main.o): 78 GCOV_PROFILE_main.o := y 79 80 For all files in one directory: 81 GCOV_PROFILE := y 82 83 To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL 84 is specified, use: 85 86 GCOV_PROFILE_main.o := n 87 and: 88 GCOV_PROFILE := n 89 90 Only files which are linked to the main kernel image or are compiled as 91 kernel modules are supported by this mechanism. 92 93 94 4. Files 95 ======== 96 97 The gcov kernel support creates the following files in debugfs: 98 99 /sys/kernel/debug/gcov 100 Parent directory for all gcov-related files. 101 102 /sys/kernel/debug/gcov/reset 103 Global reset file: resets all coverage data to zero when 104 written to. 105 106 /sys/kernel/debug/gcov/path/to/compile/dir/file.gcda 107 The actual gcov data file as understood by the gcov 108 tool. Resets file coverage data to zero when written to. 109 110 /sys/kernel/debug/gcov/path/to/compile/dir/file.gcno 111 Symbolic link to a static data file required by the gcov 112 tool. This file is generated by gcc when compiling with 113 option -ftest-coverage. 114 115 116 5. Modules 117 ========== 118 119 Kernel modules may contain cleanup code which is only run during 120 module unload time. The gcov mechanism provides a means to collect 121 coverage data for such code by keeping a copy of the data associated 122 with the unloaded module. This data remains available through debugfs. 123 Once the module is loaded again, the associated coverage counters are 124 initialized with the data from its previous instantiation. 125 126 This behavior can be deactivated by specifying the gcov_persist kernel 127 parameter: 128 129 gcov_persist=0 130 131 At run-time, a user can also choose to discard data for an unloaded 132 module by writing to its data file or the global reset file. 133 134 135 6. Separated build and test machines 136 ==================================== 137 138 The gcov kernel profiling infrastructure is designed to work out-of-the 139 box for setups where kernels are built and run on the same machine. In 140 cases where the kernel runs on a separate machine, special preparations 141 must be made, depending on where the gcov tool is used: 142 143 a) gcov is run on the TEST machine 144 145 The gcov tool version on the test machine must be compatible with the 146 gcc version used for kernel build. Also the following files need to be 147 copied from build to test machine: 148 149 from the source tree: 150 - all C source files + headers 151 152 from the build tree: 153 - all C source files + headers 154 - all .gcda and .gcno files 155 - all links to directories 156 157 It is important to note that these files need to be placed into the 158 exact same file system location on the test machine as on the build 159 machine. If any of the path components is symbolic link, the actual 160 directory needs to be used instead (due to make's CURDIR handling). 161 162 b) gcov is run on the BUILD machine 163 164 The following files need to be copied after each test case from test 165 to build machine: 166 167 from the gcov directory in sysfs: 168 - all .gcda files 169 - all links to .gcno files 170 171 These files can be copied to any location on the build machine. gcov 172 must then be called with the -o option pointing to that directory. 173 174 Example directory setup on the build machine: 175 176 /tmp/linux: kernel source tree 177 /tmp/out: kernel build directory as specified by make O= 178 /tmp/coverage: location of the files copied from the test machine 179 180 [user@build] cd /tmp/out 181 [user@build] gcov -o /tmp/coverage/tmp/out/init main.c 182 183 184 7. Troubleshooting 185 ================== 186 187 Problem: Compilation aborts during linker step. 188 Cause: Profiling flags are specified for source files which are not 189 linked to the main kernel or which are linked by a custom 190 linker procedure. 191 Solution: Exclude affected source files from profiling by specifying 192 GCOV_PROFILE := n or GCOV_PROFILE_basename.o := n in the 193 corresponding Makefile. 194 195 Problem: Files copied from sysfs appear empty or incomplete. 196 Cause: Due to the way seq_file works, some tools such as cp or tar 197 may not correctly copy files from sysfs. 198 Solution: Use 'cat' to read .gcda files and 'cp -d' to copy links. 199 Alternatively use the mechanism shown in Appendix B. 200 201 202 Appendix A: gather_on_build.sh 203 ============================== 204 205 Sample script to gather coverage meta files on the build machine 206 (see 6a): 207 #!/bin/bash 208 209 KSRC=$1 210 KOBJ=$2 211 DEST=$3 212 213 if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then 214 echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2 215 exit 1 216 fi 217 218 KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) 219 KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) 220 221 find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \ 222 -perm /u+r,g+r | tar cfz $DEST -P -T - 223 224 if [ $? -eq 0 ] ; then 225 echo "$DEST successfully created, copy to test system and unpack with:" 226 echo " tar xfz $DEST -P" 227 else 228 echo "Could not create file $DEST" 229 fi 230 231 232 Appendix B: gather_on_test.sh 233 ============================= 234 235 Sample script to gather coverage data files on the test machine 236 (see 6b): 237 238 #!/bin/bash -e 239 240 DEST=$1 241 GCDA=/sys/kernel/debug/gcov 242 243 if [ -z "$DEST" ] ; then 244 echo "Usage: $0 <output.tar.gz>" >&2 245 exit 1 246 fi 247 248 TEMPDIR=$(mktemp -d) 249 echo Collecting data.. 250 find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \; 251 find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \; 252 find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \; 253 tar czf $DEST -C $TEMPDIR sys 254 rm -rf $TEMPDIR 255 256 echo "$DEST successfully created, copy to build system and unpack with:" 257 echo " tar xfz $DEST"
