Peter Oberparleiter | 2521f2c | 2009-06-17 16:28:08 -0700 | [diff] [blame^] | 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_DEBUGFS=y |
| 51 | CONFIG_GCOV_KERNEL=y |
| 52 | |
| 53 | and to get coverage data for the entire kernel: |
| 54 | |
| 55 | CONFIG_GCOV_PROFILE_ALL=y |
| 56 | |
| 57 | Note that kernels compiled with profiling flags will be significantly |
| 58 | larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported |
| 59 | on all architectures. |
| 60 | |
| 61 | Profiling data will only become accessible once debugfs has been |
| 62 | mounted: |
| 63 | |
| 64 | mount -t debugfs none /sys/kernel/debug |
| 65 | |
| 66 | |
| 67 | 3. Customization |
| 68 | ================ |
| 69 | |
| 70 | To enable profiling for specific files or directories, add a line |
| 71 | similar to the following to the respective kernel Makefile: |
| 72 | |
| 73 | For a single file (e.g. main.o): |
| 74 | GCOV_PROFILE_main.o := y |
| 75 | |
| 76 | For all files in one directory: |
| 77 | GCOV_PROFILE := y |
| 78 | |
| 79 | To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL |
| 80 | is specified, use: |
| 81 | |
| 82 | GCOV_PROFILE_main.o := n |
| 83 | and: |
| 84 | GCOV_PROFILE := n |
| 85 | |
| 86 | Only files which are linked to the main kernel image or are compiled as |
| 87 | kernel modules are supported by this mechanism. |
| 88 | |
| 89 | |
| 90 | 4. Files |
| 91 | ======== |
| 92 | |
| 93 | The gcov kernel support creates the following files in debugfs: |
| 94 | |
| 95 | /sys/kernel/debug/gcov |
| 96 | Parent directory for all gcov-related files. |
| 97 | |
| 98 | /sys/kernel/debug/gcov/reset |
| 99 | Global reset file: resets all coverage data to zero when |
| 100 | written to. |
| 101 | |
| 102 | /sys/kernel/debug/gcov/path/to/compile/dir/file.gcda |
| 103 | The actual gcov data file as understood by the gcov |
| 104 | tool. Resets file coverage data to zero when written to. |
| 105 | |
| 106 | /sys/kernel/debug/gcov/path/to/compile/dir/file.gcno |
| 107 | Symbolic link to a static data file required by the gcov |
| 108 | tool. This file is generated by gcc when compiling with |
| 109 | option -ftest-coverage. |
| 110 | |
| 111 | |
| 112 | 5. Modules |
| 113 | ========== |
| 114 | |
| 115 | Kernel modules may contain cleanup code which is only run during |
| 116 | module unload time. The gcov mechanism provides a means to collect |
| 117 | coverage data for such code by keeping a copy of the data associated |
| 118 | with the unloaded module. This data remains available through debugfs. |
| 119 | Once the module is loaded again, the associated coverage counters are |
| 120 | initialized with the data from its previous instantiation. |
| 121 | |
| 122 | This behavior can be deactivated by specifying the gcov_persist kernel |
| 123 | parameter: |
| 124 | |
| 125 | gcov_persist=0 |
| 126 | |
| 127 | At run-time, a user can also choose to discard data for an unloaded |
| 128 | module by writing to its data file or the global reset file. |
| 129 | |
| 130 | |
| 131 | 6. Separated build and test machines |
| 132 | ==================================== |
| 133 | |
| 134 | The gcov kernel profiling infrastructure is designed to work out-of-the |
| 135 | box for setups where kernels are built and run on the same machine. In |
| 136 | cases where the kernel runs on a separate machine, special preparations |
| 137 | must be made, depending on where the gcov tool is used: |
| 138 | |
| 139 | a) gcov is run on the TEST machine |
| 140 | |
| 141 | The gcov tool version on the test machine must be compatible with the |
| 142 | gcc version used for kernel build. Also the following files need to be |
| 143 | copied from build to test machine: |
| 144 | |
| 145 | from the source tree: |
| 146 | - all C source files + headers |
| 147 | |
| 148 | from the build tree: |
| 149 | - all C source files + headers |
| 150 | - all .gcda and .gcno files |
| 151 | - all links to directories |
| 152 | |
| 153 | It is important to note that these files need to be placed into the |
| 154 | exact same file system location on the test machine as on the build |
| 155 | machine. If any of the path components is symbolic link, the actual |
| 156 | directory needs to be used instead (due to make's CURDIR handling). |
| 157 | |
| 158 | b) gcov is run on the BUILD machine |
| 159 | |
| 160 | The following files need to be copied after each test case from test |
| 161 | to build machine: |
| 162 | |
| 163 | from the gcov directory in sysfs: |
| 164 | - all .gcda files |
| 165 | - all links to .gcno files |
| 166 | |
| 167 | These files can be copied to any location on the build machine. gcov |
| 168 | must then be called with the -o option pointing to that directory. |
| 169 | |
| 170 | Example directory setup on the build machine: |
| 171 | |
| 172 | /tmp/linux: kernel source tree |
| 173 | /tmp/out: kernel build directory as specified by make O= |
| 174 | /tmp/coverage: location of the files copied from the test machine |
| 175 | |
| 176 | [user@build] cd /tmp/out |
| 177 | [user@build] gcov -o /tmp/coverage/tmp/out/init main.c |
| 178 | |
| 179 | |
| 180 | 7. Troubleshooting |
| 181 | ================== |
| 182 | |
| 183 | Problem: Compilation aborts during linker step. |
| 184 | Cause: Profiling flags are specified for source files which are not |
| 185 | linked to the main kernel or which are linked by a custom |
| 186 | linker procedure. |
| 187 | Solution: Exclude affected source files from profiling by specifying |
| 188 | GCOV_PROFILE := n or GCOV_PROFILE_basename.o := n in the |
| 189 | corresponding Makefile. |
| 190 | |
| 191 | |
| 192 | Appendix A: gather_on_build.sh |
| 193 | ============================== |
| 194 | |
| 195 | Sample script to gather coverage meta files on the build machine |
| 196 | (see 6a): |
| 197 | |
| 198 | #!/bin/bash |
| 199 | |
| 200 | KSRC=$1 |
| 201 | KOBJ=$2 |
| 202 | DEST=$3 |
| 203 | |
| 204 | if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then |
| 205 | echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2 |
| 206 | exit 1 |
| 207 | fi |
| 208 | |
| 209 | KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) |
| 210 | KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) |
| 211 | |
| 212 | find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \ |
| 213 | -perm /u+r,g+r | tar cfz $DEST -P -T - |
| 214 | |
| 215 | if [ $? -eq 0 ] ; then |
| 216 | echo "$DEST successfully created, copy to test system and unpack with:" |
| 217 | echo " tar xfz $DEST -P" |
| 218 | else |
| 219 | echo "Could not create file $DEST" |
| 220 | fi |
| 221 | |
| 222 | |
| 223 | Appendix B: gather_on_test.sh |
| 224 | ============================= |
| 225 | |
| 226 | Sample script to gather coverage data files on the test machine |
| 227 | (see 6b): |
| 228 | |
| 229 | #!/bin/bash |
| 230 | |
| 231 | DEST=$1 |
| 232 | GCDA=/sys/kernel/debug/gcov |
| 233 | |
| 234 | if [ -z "$DEST" ] ; then |
| 235 | echo "Usage: $0 <output.tar.gz>" >&2 |
| 236 | exit 1 |
| 237 | fi |
| 238 | |
| 239 | find $GCDA -name '*.gcno' -o -name '*.gcda' | tar cfz $DEST -T - |
| 240 | |
| 241 | if [ $? -eq 0 ] ; then |
| 242 | echo "$DEST successfully created, copy to build system and unpack with:" |
| 243 | echo " tar xfz $DEST" |
| 244 | else |
| 245 | echo "Could not create file $DEST" |
| 246 | fi |