blob: 19eedfea8800db38adb5382c571bb951e3206c4a [file] [log] [blame]
Jonathan Corbet2584bab2016-08-07 15:26:20 -06001Using 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"