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
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.. code-block:: sh
207
208    #!/bin/bash
209
210    KSRC=$1
211    KOBJ=$2
212    DEST=$3
213
214    if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
215      echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
216      exit 1
217    fi
218
219    KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
220    KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
221
222    find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
223                     -perm /u+r,g+r | tar cfz $DEST -P -T -
224
225    if [ $? -eq 0 ] ; then
226      echo "$DEST successfully created, copy to test system and unpack with:"
227      echo "  tar xfz $DEST -P"
228    else
229      echo "Could not create file $DEST"
230    fi
231
232
233Appendix B: gather_on_test.sh
234-----------------------------
235
236Sample script to gather coverage data files on the test machine
237(see 6b):
238
239.. code-block:: sh
240
241    #!/bin/bash -e
242
243    DEST=$1
244    GCDA=/sys/kernel/debug/gcov
245
246    if [ -z "$DEST" ] ; then
247      echo "Usage: $0 <output.tar.gz>" >&2
248      exit 1
249    fi
250
251    TEMPDIR=$(mktemp -d)
252    echo Collecting data..
253    find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
254    find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
255    find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
256    tar czf $DEST -C $TEMPDIR sys
257    rm -rf $TEMPDIR
258
259    echo "$DEST successfully created, copy to build system and unpack with:"
260    echo "  tar xfz $DEST"
261