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1 | Using gcov with the Linux kernel |
2 | ================================ |
3 | |
4 | 1. Introduction |
5 | 2. Preparation |
6 | 3. Customization |
7 | 4. Files |
8 | 5. Modules |
9 | 6. Separated build and test machines |
10 | 7. Troubleshooting |
11 | Appendix A: sample script: gather_on_build.sh |
12 | Appendix B: sample script: gather_on_test.sh |
13 | |
14 | |
15 | 1. Introduction |
16 | =============== |
17 | |
18 | gcov profiling kernel support enables the use of GCC's coverage testing |
19 | tool gcov [1] with the Linux kernel. Coverage data of a running kernel |
20 | is exported in gcov-compatible format via the "gcov" debugfs directory. |
21 | To get coverage data for a specific file, change to the kernel build |
22 | directory and use gcov with the -o option as follows (requires root): |
23 | |
24 | # cd /tmp/linux-out |
25 | # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c |
26 | |
27 | This will create source code files annotated with execution counts |
28 | in the current directory. In addition, graphical gcov front-ends such |
29 | as lcov [2] can be used to automate the process of collecting data |
30 | for the entire kernel and provide coverage overviews in HTML format. |
31 | |
32 | Possible uses: |
33 | |
34 | * debugging (has this line been reached at all?) |
35 | * test improvement (how do I change my test to cover these lines?) |
36 | * minimizing kernel configurations (do I need this option if the |
37 | associated code is never run?) |
38 | |
39 | -- |
40 | |
41 | [1] http://gcc.gnu.org/onlinedocs/gcc/Gcov.html |
42 | [2] http://ltp.sourceforge.net/coverage/lcov.php |
43 | |
44 | |
45 | 2. Preparation |
46 | ============== |
47 | |
48 | Configure the kernel with: |
49 | |
50 | CONFIG_DEBUG_FS=y |
51 | CONFIG_GCOV_KERNEL=y |
52 | |
53 | and to get coverage data for the entire kernel: |
54 | |
55 | CONFIG_GCOV_PROFILE_ALL=y |
56 | |
57 | Note that kernels compiled with profiling flags will be significantly |
58 | larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported |
59 | on all architectures. |
60 | |
61 | Profiling data will only become accessible once debugfs has been |
62 | mounted: |
63 | |
64 | mount -t debugfs none /sys/kernel/debug |
65 | |
66 | |
67 | 3. Customization |
68 | ================ |
69 | |
70 | To enable profiling for specific files or directories, add a line |
71 | similar to the following to the respective kernel Makefile: |
72 | |
73 | For a single file (e.g. main.o): |
74 | GCOV_PROFILE_main.o := y |
75 | |
76 | For all files in one directory: |
77 | GCOV_PROFILE := y |
78 | |
79 | To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL |
80 | is specified, use: |
81 | |
82 | GCOV_PROFILE_main.o := n |
83 | and: |
84 | GCOV_PROFILE := n |
85 | |
86 | Only files which are linked to the main kernel image or are compiled as |
87 | kernel modules are supported by this mechanism. |
88 | |
89 | |
90 | 4. Files |
91 | ======== |
92 | |
93 | The gcov kernel support creates the following files in debugfs: |
94 | |
95 | /sys/kernel/debug/gcov |
96 | Parent directory for all gcov-related files. |
97 | |
98 | /sys/kernel/debug/gcov/reset |
99 | Global reset file: resets all coverage data to zero when |
100 | written to. |
101 | |
102 | /sys/kernel/debug/gcov/path/to/compile/dir/file.gcda |
103 | The actual gcov data file as understood by the gcov |
104 | tool. Resets file coverage data to zero when written to. |
105 | |
106 | /sys/kernel/debug/gcov/path/to/compile/dir/file.gcno |
107 | Symbolic link to a static data file required by the gcov |
108 | tool. This file is generated by gcc when compiling with |
109 | option -ftest-coverage. |
110 | |
111 | |
112 | 5. Modules |
113 | ========== |
114 | |
115 | Kernel modules may contain cleanup code which is only run during |
116 | module unload time. The gcov mechanism provides a means to collect |
117 | coverage data for such code by keeping a copy of the data associated |
118 | with the unloaded module. This data remains available through debugfs. |
119 | Once the module is loaded again, the associated coverage counters are |
120 | initialized with the data from its previous instantiation. |
121 | |
122 | This behavior can be deactivated by specifying the gcov_persist kernel |
123 | parameter: |
124 | |
125 | gcov_persist=0 |
126 | |
127 | At run-time, a user can also choose to discard data for an unloaded |
128 | module by writing to its data file or the global reset file. |
129 | |
130 | |
131 | 6. Separated build and test machines |
132 | ==================================== |
133 | |
134 | The gcov kernel profiling infrastructure is designed to work out-of-the |
135 | box for setups where kernels are built and run on the same machine. In |
136 | cases where the kernel runs on a separate machine, special preparations |
137 | must be made, depending on where the gcov tool is used: |
138 | |
139 | a) gcov is run on the TEST machine |
140 | |
141 | The gcov tool version on the test machine must be compatible with the |
142 | gcc version used for kernel build. Also the following files need to be |
143 | copied from build to test machine: |
144 | |
145 | from the source tree: |
146 | - all C source files + headers |
147 | |
148 | from the build tree: |
149 | - all C source files + headers |
150 | - all .gcda and .gcno files |
151 | - all links to directories |
152 | |
153 | It is important to note that these files need to be placed into the |
154 | exact same file system location on the test machine as on the build |
155 | machine. If any of the path components is symbolic link, the actual |
156 | directory needs to be used instead (due to make's CURDIR handling). |
157 | |
158 | b) gcov is run on the BUILD machine |
159 | |
160 | The following files need to be copied after each test case from test |
161 | to build machine: |
162 | |
163 | from the gcov directory in sysfs: |
164 | - all .gcda files |
165 | - all links to .gcno files |
166 | |
167 | These files can be copied to any location on the build machine. gcov |
168 | must then be called with the -o option pointing to that directory. |
169 | |
170 | Example directory setup on the build machine: |
171 | |
172 | /tmp/linux: kernel source tree |
173 | /tmp/out: kernel build directory as specified by make O= |
174 | /tmp/coverage: location of the files copied from the test machine |
175 | |
176 | [user@build] cd /tmp/out |
177 | [user@build] gcov -o /tmp/coverage/tmp/out/init main.c |
178 | |
179 | |
180 | 7. Troubleshooting |
181 | ================== |
182 | |
183 | Problem: Compilation aborts during linker step. |
184 | Cause: Profiling flags are specified for source files which are not |
185 | linked to the main kernel or which are linked by a custom |
186 | linker procedure. |
187 | Solution: Exclude affected source files from profiling by specifying |
188 | GCOV_PROFILE := n or GCOV_PROFILE_basename.o := n in the |
189 | corresponding Makefile. |
190 | |
191 | Problem: Files copied from sysfs appear empty or incomplete. |
192 | Cause: Due to the way seq_file works, some tools such as cp or tar |
193 | may not correctly copy files from sysfs. |
194 | Solution: Use 'cat' to read .gcda files and 'cp -d' to copy links. |
195 | Alternatively use the mechanism shown in Appendix B. |
196 | |
197 | |
198 | Appendix A: gather_on_build.sh |
199 | ============================== |
200 | |
201 | Sample script to gather coverage meta files on the build machine |
202 | (see 6a): |
203 | #!/bin/bash |
204 | |
205 | KSRC=$1 |
206 | KOBJ=$2 |
207 | DEST=$3 |
208 | |
209 | if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then |
210 | echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2 |
211 | exit 1 |
212 | fi |
213 | |
214 | KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) |
215 | KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -) |
216 | |
217 | find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \ |
218 | -perm /u+r,g+r | tar cfz $DEST -P -T - |
219 | |
220 | if [ $? -eq 0 ] ; then |
221 | echo "$DEST successfully created, copy to test system and unpack with:" |
222 | echo " tar xfz $DEST -P" |
223 | else |
224 | echo "Could not create file $DEST" |
225 | fi |
226 | |
227 | |
228 | Appendix B: gather_on_test.sh |
229 | ============================= |
230 | |
231 | Sample script to gather coverage data files on the test machine |
232 | (see 6b): |
233 | |
234 | #!/bin/bash -e |
235 | |
236 | DEST=$1 |
237 | GCDA=/sys/kernel/debug/gcov |
238 | |
239 | if [ -z "$DEST" ] ; then |
240 | echo "Usage: $0 <output.tar.gz>" >&2 |
241 | exit 1 |
242 | fi |
243 | |
244 | TEMPDIR=$(mktemp -d) |
245 | echo Collecting data.. |
246 | find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \; |
247 | find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \; |
248 | find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \; |
249 | tar czf $DEST -C $TEMPDIR sys |
250 | rm -rf $TEMPDIR |
251 | |
252 | echo "$DEST successfully created, copy to build system and unpack with:" |
253 | echo " tar xfz $DEST" |
254 |
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