1Using 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
37and to get coverage data for the entire kernel::
38
39        CONFIG_GCOV_PROFILE_ALL=y
40
41Note that kernels compiled with profiling flags will be significantly
42larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
43on all architectures.
44
45Profiling data will only become accessible once debugfs has been
46mounted::
47
48        mount -t debugfs none /sys/kernel/debug
49
50
51Customization
52-------------
53
54To enable profiling for specific files or directories, add a line
55similar 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
65To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
66is specified, use::
67
68	GCOV_PROFILE_main.o := n
69
70and::
71
72	GCOV_PROFILE := n
73
74Only files which are linked to the main kernel image or are compiled as
75kernel modules are supported by this mechanism.
76
77
78Files
79-----
80
81The 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
100Modules
101-------
102
103Kernel modules may contain cleanup code which is only run during
104module unload time. The gcov mechanism provides a means to collect
105coverage data for such code by keeping a copy of the data associated
106with the unloaded module. This data remains available through debugfs.
107Once the module is loaded again, the associated coverage counters are
108initialized with the data from its previous instantiation.
109
110This behavior can be deactivated by specifying the gcov_persist kernel
111parameter::
112
113        gcov_persist=0
114
115At run-time, a user can also choose to discard data for an unloaded
116module by writing to its data file or the global reset file.
117
118
119Separated build and test machines
120---------------------------------
121
122The gcov kernel profiling infrastructure is designed to work out-of-the
123box for setups where kernels are built and run on the same machine. In
124cases where the kernel runs on a separate machine, special preparations
125must be made, depending on where the gcov tool is used:
126
127a) 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
146b) 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
168Note on compilers
169-----------------
170
171GCC and LLVM gcov tools are not necessarily compatible. Use gcov_ to work with
172GCC-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
177Build differences between GCC and Clang gcov are handled by Kconfig. It
178automatically selects the appropriate gcov format depending on the detected
179toolchain.
180
181
182Troubleshooting
183---------------
184
185Problem
186    Compilation aborts during linker step.
187
188Cause
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
193Solution
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
198Problem
199    Files copied from sysfs appear empty or incomplete.
200
201Cause
202    Due to the way seq_file works, some tools such as cp or tar
203    may not correctly copy files from sysfs.
204
205Solution
206    Use ``cat`` to read ``.gcda`` files and ``cp -d`` to copy links.
207    Alternatively use the mechanism shown in Appendix B.
208
209
210Appendix A: gather_on_build.sh
211------------------------------
212
213Sample 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
243Appendix B: gather_on_test.sh
244-----------------------------
245
246Sample 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"
271