Documentation/dev-tools/gcov.rst

   1 Using gcov with the Linux kernel
   2 ================================
   3
   4 gcov profiling kernel support enables the use of GCC's coverage testing
   5 tool gcov_ with the Linux kernel. Coverage data of a running kernel
   6 is exported in gcov-compatible format via the "gcov" debugfs directory.
   7 To get coverage data for a specific file, change to the kernel build
   8 directory 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
  13 This will create source code files annotated with execution counts
  14 in the current directory. In addition, graphical gcov front-ends such
  15 as lcov_ can be used to automate the process of collecting data
  16 for the entire kernel and provide coverage overviews in HTML format.
  17
  18 Possible 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
  29 Preparation
  30 -----------
  31
  32 Configure the kernel with::
  33
  34         CONFIG_DEBUG_FS=y
  35         CONFIG_GCOV_KERNEL=y
  36
  37 and to get coverage data for the entire kernel::
  38
  39         CONFIG_GCOV_PROFILE_ALL=y
  40
  41 Note that kernels compiled with profiling flags will be significantly
  42 larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
  43 on all architectures.
  44
  45 Profiling data will only become accessible once debugfs has been
  46 mounted::
  47
  48         mount -t debugfs none /sys/kernel/debug
  49
  50
  51 Customization
  52 -------------
  53
  54 To enable profiling for specific files or directories, add a line
  55 similar to the following to the respective kernel Makefile:
  56
  57 - For a single file (e.g. main.o)::
  58
  59         GCOV_PROFILE_main.o := y
  60
  61 - For all files in one directory::
  62
  63         GCOV_PROFILE := y
  64
  65 To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
  66 is specified, use::
  67
  68         GCOV_PROFILE_main.o := n
  69
  70 and::
  71
  72         GCOV_PROFILE := n
  73
  74 Only files which are linked to the main kernel image or are compiled as
  75 kernel modules are supported by this mechanism.
  76
  77
  78 Files
  79 -----
  80
  81 The gcov kernel support creates the following files in debugfs:
  82
  83 ``/sys/kernel/debug/gcov``
  84         Parent directory for all gcov-related files.
  85
  86 ``/sys/kernel/debug/gcov/reset``
  87         Global reset file: resets all coverage data to zero when
  88         written to.
  89
  90 ``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcda``
  91         The actual gcov data file as understood by the gcov
  92         tool. Resets file coverage data to zero when written to.
  93
  94 ``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcno``
  95         Symbolic link to a static data file required by the gcov
  96         tool. This file is generated by gcc when compiling with
  97         option ``-ftest-coverage``.
  98
  99
 100 Modules
 101 -------
 102
 103 Kernel modules may contain cleanup code which is only run during
 104 module unload time. The gcov mechanism provides a means to collect
 105 coverage data for such code by keeping a copy of the data associated
 106 with the unloaded module. This data remains available through debugfs.
 107 Once the module is loaded again, the associated coverage counters are
 108 initialized with the data from its previous instantiation.
 109
 110 This behavior can be deactivated by specifying the gcov_persist kernel
 111 parameter::
 112
 113         gcov_persist=0
 114
 115 At run-time, a user can also choose to discard data for an unloaded
 116 module by writing to its data file or the global reset file.
 117
 118
 119 Separated build and test machines
 120 ---------------------------------
 121
 122 The gcov kernel profiling infrastructure is designed to work out-of-the
 123 box for setups where kernels are built and run on the same machine. In
 124 cases where the kernel runs on a separate machine, special preparations
 125 must be made, depending on where the gcov tool is used:
 126
 127 a) gcov is run on the TEST machine
 128
 129     The gcov tool version on the test machine must be compatible with the
 130     gcc version used for kernel build. Also the following files need to be
 131     copied from build to test machine:
 132
 133     from the source tree:
 134       - all C source files + headers
 135
 136     from the build tree:
 137       - all C source files + headers
 138       - all .gcda and .gcno files
 139       - all links to directories
 140
 141     It is important to note that these files need to be placed into the
 142     exact same file system location on the test machine as on the build
 143     machine. If any of the path components is symbolic link, the actual
 144     directory needs to be used instead (due to make's CURDIR handling).
 145
 146 b) gcov is run on the BUILD machine
 147
 148     The following files need to be copied after each test case from test
 149     to build machine:
 150
 151     from the gcov directory in sysfs:
 152       - all .gcda files
 153       - all links to .gcno files
 154
 155     These files can be copied to any location on the build machine. gcov
 156     must then be called with the -o option pointing to that directory.
 157
 158     Example directory setup on the build machine::
 159
 160       /tmp/linux:    kernel source tree
 161       /tmp/out:      kernel build directory as specified by make O=
 162       /tmp/coverage: location of the files copied from the test machine
 163
 164       [user@build] cd /tmp/out
 165       [user@build] gcov -o /tmp/coverage/tmp/out/init main.c
 166
 167
 168 Note on compilers
 169 -----------------
 170
 171 GCC and LLVM gcov tools are not necessarily compatible. Use gcov_ to work with
 172 GCC-generated .gcno and .gcda files, and use llvm-cov_ for Clang.
 173
 174 .. _gcov: http://gcc.gnu.org/onlinedocs/gcc/Gcov.html
 175 .. _llvm-cov: https://llvm.org/docs/CommandGuide/llvm-cov.html
 176
 177 Build differences between GCC and Clang gcov are handled by Kconfig. It
 178 automatically selects the appropriate gcov format depending on the detected
 179 toolchain.
 180
 181
 182 Troubleshooting
 183 ---------------
 184
 185 Problem
 186     Compilation aborts during linker step.
 187
 188 Cause
 189     Profiling flags are specified for source files which are not
 190     linked to the main kernel or which are linked by a custom
 191     linker procedure.
 192
 193 Solution
 194     Exclude affected source files from profiling by specifying
 195     ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the
 196     corresponding Makefile.
 197
 198 Problem
 199     Files copied from sysfs appear empty or incomplete.
 200
 201 Cause
 202     Due to the way seq_file works, some tools such as cp or tar
 203     may not correctly copy files from sysfs.
 204
 205 Solution
 206     Use ``cat``' to read ``.gcda`` files and ``cp -d`` to copy links.
 207     Alternatively use the mechanism shown in Appendix B.
 208
 209
 210 Appendix A: gather_on_build.sh
 211 ------------------------------
 212
 213 Sample script to gather coverage meta files on the build machine
 214 (see 6a):
 215
 216 .. code-block:: sh
 217
 218     #!/bin/bash
 219
 220     KSRC=$1
 221     KOBJ=$2
 222     DEST=$3
 223
 224     if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
 225       echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
 226       exit 1
 227     fi
 228
 229     KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
 230     KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
 231
 232     find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
 233                      -perm /u+r,g+r | tar cfz $DEST -P -T -
 234
 235     if [ $? -eq 0 ] ; then
 236       echo "$DEST successfully created, copy to test system and unpack with:"
 237       echo "  tar xfz $DEST -P"
 238     else
 239       echo "Could not create file $DEST"
 240     fi
 241
 242
 243 Appendix B: gather_on_test.sh
 244 -----------------------------
 245
 246 Sample script to gather coverage data files on the test machine
 247 (see 6b):
 248
 249 .. code-block:: sh
 250
 251     #!/bin/bash -e
 252
 253     DEST=$1
 254     GCDA=/sys/kernel/debug/gcov
 255
 256     if [ -z "$DEST" ] ; then
 257       echo "Usage: $0 <output.tar.gz>" >&2
 258       exit 1
 259     fi
 260
 261     TEMPDIR=$(mktemp -d)
 262     echo Collecting data..
 263     find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
 264     find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
 265     find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
 266     tar czf $DEST -C $TEMPDIR sys
 267     rm -rf $TEMPDIR
 268
 269     echo "$DEST successfully created, copy to build system and unpack with:"
 270     echo "  tar xfz $DEST"