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: https://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 .. _gcov-test:
128
129 a) gcov is run on the TEST machine
130
131 The gcov tool version on the test machine must be compatible with the
132 gcc version used for kernel build. Also the following files need to be
133 copied from build to test machine:
134
135 from the source tree:
136 - all C source files + headers
137
138 from the build tree:
139 - all C source files + headers
140 - all .gcda and .gcno files
141 - all links to directories
142
143 It is important to note that these files need to be placed into the
144 exact same file system location on the test machine as on the build
145 machine. If any of the path components is symbolic link, the actual
146 directory needs to be used instead (due to make's CURDIR handling).
147
148 .. _gcov-build:
149
150 b) 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
172 Note on compilers
173 -----------------
174
175 GCC and LLVM gcov tools are not necessarily compatible. Use gcov_ to work with
176 GCC-generated .gcno and .gcda files, and use llvm-cov_ for Clang.
177
178 .. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
179 .. _llvm-cov: https://llvm.org/docs/CommandGuide/llvm-cov.html
180
181 Build differences between GCC and Clang gcov are handled by Kconfig. It
182 automatically selects the appropriate gcov format depending on the detected
183 toolchain.
184
185
186 Troubleshooting
187 ---------------
188
189 Problem
190 Compilation aborts during linker step.
191
192 Cause
193 Profiling flags are specified for source files which are not
194 linked to the main kernel or which are linked by a custom
195 linker procedure.
196
197 Solution
198 Exclude affected source files from profiling by specifying
199 ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the
200 corresponding Makefile.
201
202 Problem
203 Files copied from sysfs appear empty or incomplete.
204
205 Cause
206 Due to the way seq_file works, some tools such as cp or tar
207 may not correctly copy files from sysfs.
208
209 Solution
210 Use ``cat`` to read ``.gcda`` files and ``cp -d`` to copy links.
211 Alternatively use the mechanism shown in Appendix B.
212
213
214 Appendix A: gather_on_build.sh
215 ------------------------------
216
217 Sample script to gather coverage meta files on the build machine
218 (see :ref:`Separated build and test machines a. <gcov-test>`):
219
220 .. code-block:: sh
221
222 #!/bin/bash
223
224 KSRC=$1
225 KOBJ=$2
226 DEST=$3
227
228 if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
229 echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
230 exit 1
231 fi
232
233 KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
234 KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
235
236 find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
237 -perm /u+r,g+r | tar cfz $DEST -P -T -
238
239 if [ $? -eq 0 ] ; then
240 echo "$DEST successfully created, copy to test system and unpack with:"
241 echo " tar xfz $DEST -P"
242 else
243 echo "Could not create file $DEST"
244 fi
245
246
247 Appendix B: gather_on_test.sh
248 -----------------------------
249
250 Sample script to gather coverage data files on the test machine
251 (see :ref:`Separated build and test machines b. <gcov-build>`):
252
253 .. code-block:: sh
254
255 #!/bin/bash -e
256
257 DEST=$1
258 GCDA=/sys/kernel/debug/gcov
259
260 if [ -z "$DEST" ] ; then
261 echo "Usage: $0 <output.tar.gz>" >&2
262 exit 1
263 fi
264
265 TEMPDIR=$(mktemp -d)
266 echo Collecting data..
267 find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
268 find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
269 find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
270 tar czf $DEST -C $TEMPDIR sys
271 rm -rf $TEMPDIR
272
273 echo "$DEST successfully created, copy to build system and unpack with:"
274 echo " tar xfz $DEST"
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