1 ==================
2 S390 Debug Feature
3 ==================
4
5 files:
6 - arch/s390/kernel/debug.c
7 - arch/s390/include/asm/debug.h
8
9 Description:
10 ------------
11 The goal of this feature is to provide a kernel debug logging API
12 where log records can be stored efficiently in memory, where each component
13 (e.g. device drivers) can have one separate debug log.
14 One purpose of this is to inspect the debug logs after a production system crash
15 in order to analyze the reason for the crash.
16
17 If the system still runs but only a subcomponent which uses dbf fails,
18 it is possible to look at the debug logs on a live system via the Linux
19 debugfs filesystem.
20
21 The debug feature may also very useful for kernel and driver development.
22
23 Design:
24 -------
25 Kernel components (e.g. device drivers) can register themselves at the debug
26 feature with the function call :c:func:`debug_register()`.
27 This function initializes a
28 debug log for the caller. For each debug log exists a number of debug areas
29 where exactly one is active at one time. Each debug area consists of contiguous
30 pages in memory. In the debug areas there are stored debug entries (log records)
31 which are written by event- and exception-calls.
32
33 An event-call writes the specified debug entry to the active debug
34 area and updates the log pointer for the active area. If the end
35 of the active debug area is reached, a wrap around is done (ring buffer)
36 and the next debug entry will be written at the beginning of the active
37 debug area.
38
39 An exception-call writes the specified debug entry to the log and
40 switches to the next debug area. This is done in order to be sure
41 that the records which describe the origin of the exception are not
42 overwritten when a wrap around for the current area occurs.
43
44 The debug areas themselves are also ordered in form of a ring buffer.
45 When an exception is thrown in the last debug area, the following debug
46 entries are then written again in the very first area.
47
48 There are four versions for the event- and exception-calls: One for
49 logging raw data, one for text, one for numbers (unsigned int and long),
50 and one for sprintf-like formatted strings.
51
52 Each debug entry contains the following data:
53
54 - Timestamp
55 - Cpu-Number of calling task
56 - Level of debug entry (0...6)
57 - Return Address to caller
58 - Flag, if entry is an exception or not
59
60 The debug logs can be inspected in a live system through entries in
61 the debugfs-filesystem. Under the toplevel directory "``s390dbf``" there is
62 a directory for each registered component, which is named like the
63 corresponding component. The debugfs normally should be mounted to
64 ``/sys/kernel/debug`` therefore the debug feature can be accessed under
65 ``/sys/kernel/debug/s390dbf``.
66
67 The content of the directories are files which represent different views
68 to the debug log. Each component can decide which views should be
69 used through registering them with the function :c:func:`debug_register_view()`.
70 Predefined views for hex/ascii and sprintf data are provided.
71 It is also possible to define other views. The content of
72 a view can be inspected simply by reading the corresponding debugfs file.
73
74 All debug logs have an actual debug level (range from 0 to 6).
75 The default level is 3. Event and Exception functions have a :c:data:`level`
76 parameter. Only debug entries with a level that is lower or equal
77 than the actual level are written to the log. This means, when
78 writing events, high priority log entries should have a low level
79 value whereas low priority entries should have a high one.
80 The actual debug level can be changed with the help of the debugfs-filesystem
81 through writing a number string "x" to the ``level`` debugfs file which is
82 provided for every debug log. Debugging can be switched off completely
83 by using "-" on the ``level`` debugfs file.
84
85 Example::
86
87 > echo "-" > /sys/kernel/debug/s390dbf/dasd/level
88
89 It is also possible to deactivate the debug feature globally for every
90 debug log. You can change the behavior using 2 sysctl parameters in
91 ``/proc/sys/s390dbf``:
92
93 There are currently 2 possible triggers, which stop the debug feature
94 globally. The first possibility is to use the ``debug_active`` sysctl. If
95 set to 1 the debug feature is running. If ``debug_active`` is set to 0 the
96 debug feature is turned off.
97
98 The second trigger which stops the debug feature is a kernel oops.
99 That prevents the debug feature from overwriting debug information that
100 happened before the oops. After an oops you can reactivate the debug feature
101 by piping 1 to ``/proc/sys/s390dbf/debug_active``. Nevertheless, it's not
102 suggested to use an oopsed kernel in a production environment.
103
104 If you want to disallow the deactivation of the debug feature, you can use
105 the ``debug_stoppable`` sysctl. If you set ``debug_stoppable`` to 0 the debug
106 feature cannot be stopped. If the debug feature is already stopped, it
107 will stay deactivated.
108
109 Kernel Interfaces:
110 ------------------
111
112 .. kernel-doc:: arch/s390/kernel/debug.c
113 .. kernel-doc:: arch/s390/include/asm/debug.h
114
115 Predefined views:
116 -----------------
117
118 .. code-block:: c
119
120 extern struct debug_view debug_hex_ascii_view;
121
122 extern struct debug_view debug_sprintf_view;
123
124 Examples
125 --------
126
127 .. code-block:: c
128
129 /*
130 * hex_ascii-view Example
131 */
132
133 #include <linux/init.h>
134 #include <asm/debug.h>
135
136 static debug_info_t *debug_info;
137
138 static int init(void)
139 {
140 /* register 4 debug areas with one page each and 4 byte data field */
141
142 debug_info = debug_register("test", 1, 4, 4 );
143 debug_register_view(debug_info, &debug_hex_ascii_view);
144
145 debug_text_event(debug_info, 4 , "one ");
146 debug_int_exception(debug_info, 4, 4711);
147 debug_event(debug_info, 3, &debug_info, 4);
148
149 return 0;
150 }
151
152 static void cleanup(void)
153 {
154 debug_unregister(debug_info);
155 }
156
157 module_init(init);
158 module_exit(cleanup);
159
160 .. code-block:: c
161
162 /*
163 * sprintf-view Example
164 */
165
166 #include <linux/init.h>
167 #include <asm/debug.h>
168
169 static debug_info_t *debug_info;
170
171 static int init(void)
172 {
173 /* register 4 debug areas with one page each and data field for */
174 /* format string pointer + 2 varargs (= 3 * sizeof(long)) */
175
176 debug_info = debug_register("test", 1, 4, sizeof(long) * 3);
177 debug_register_view(debug_info, &debug_sprintf_view);
178
179 debug_sprintf_event(debug_info, 2 , "first event in %s:%i\n",__FILE__,__LINE__);
180 debug_sprintf_exception(debug_info, 1, "pointer to debug info: %p\n",&debug_info);
181
182 return 0;
183 }
184
185 static void cleanup(void)
186 {
187 debug_unregister(debug_info);
188 }
189
190 module_init(init);
191 module_exit(cleanup);
192
193 Debugfs Interface
194 -----------------
195 Views to the debug logs can be investigated through reading the corresponding
196 debugfs-files:
197
198 Example::
199
200 > ls /sys/kernel/debug/s390dbf/dasd
201 flush hex_ascii level pages
202 > cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort -k2,2 -s
203 00 00974733272:680099 2 - 02 0006ad7e 07 ea 4a 90 | ....
204 00 00974733272:682210 2 - 02 0006ade6 46 52 45 45 | FREE
205 00 00974733272:682213 2 - 02 0006adf6 07 ea 4a 90 | ....
206 00 00974733272:682281 1 * 02 0006ab08 41 4c 4c 43 | EXCP
207 01 00974733272:682284 2 - 02 0006ab16 45 43 4b 44 | ECKD
208 01 00974733272:682287 2 - 02 0006ab28 00 00 00 04 | ....
209 01 00974733272:682289 2 - 02 0006ab3e 00 00 00 20 | ...
210 01 00974733272:682297 2 - 02 0006ad7e 07 ea 4a 90 | ....
211 01 00974733272:684384 2 - 00 0006ade6 46 52 45 45 | FREE
212 01 00974733272:684388 2 - 00 0006adf6 07 ea 4a 90 | ....
213
214 See section about predefined views for explanation of the above output!
215
216 Changing the debug level
217 ------------------------
218
219 Example::
220
221
222 > cat /sys/kernel/debug/s390dbf/dasd/level
223 3
224 > echo "5" > /sys/kernel/debug/s390dbf/dasd/level
225 > cat /sys/kernel/debug/s390dbf/dasd/level
226 5
227
228 Flushing debug areas
229 --------------------
230 Debug areas can be flushed with piping the number of the desired
231 area (0...n) to the debugfs file "flush". When using "-" all debug areas
232 are flushed.
233
234 Examples:
235
236 1. Flush debug area 0::
237
238 > echo "0" > /sys/kernel/debug/s390dbf/dasd/flush
239
240 2. Flush all debug areas::
241
242 > echo "-" > /sys/kernel/debug/s390dbf/dasd/flush
243
244 Changing the size of debug areas
245 ------------------------------------
246 It is possible the change the size of debug areas through piping
247 the number of pages to the debugfs file "pages". The resize request will
248 also flush the debug areas.
249
250 Example:
251
252 Define 4 pages for the debug areas of debug feature "dasd"::
253
254 > echo "4" > /sys/kernel/debug/s390dbf/dasd/pages
255
256 Stopping the debug feature
257 --------------------------
258 Example:
259
260 1. Check if stopping is allowed::
261
262 > cat /proc/sys/s390dbf/debug_stoppable
263
264 2. Stop debug feature::
265
266 > echo 0 > /proc/sys/s390dbf/debug_active
267
268 crash Interface
269 ----------------
270 The ``crash`` tool since v5.1.0 has a built-in command
271 ``s390dbf`` to display all the debug logs or export them to the file system.
272 With this tool it is possible
273 to investigate the debug logs on a live system and with a memory dump after
274 a system crash.
275
276 Investigating raw memory
277 ------------------------
278 One last possibility to investigate the debug logs at a live
279 system and after a system crash is to look at the raw memory
280 under VM or at the Service Element.
281 It is possible to find the anchor of the debug-logs through
282 the ``debug_area_first`` symbol in the System map. Then one has
283 to follow the correct pointers of the data-structures defined
284 in debug.h and find the debug-areas in memory.
285 Normally modules which use the debug feature will also have
286 a global variable with the pointer to the debug-logs. Following
287 this pointer it will also be possible to find the debug logs in
288 memory.
289
290 For this method it is recommended to use '16 * x + 4' byte (x = 0..n)
291 for the length of the data field in :c:func:`debug_register()` in
292 order to see the debug entries well formatted.
293
294
295 Predefined Views
296 ----------------
297
298 There are two predefined views: hex_ascii and sprintf.
299 The hex_ascii view shows the data field in hex and ascii representation
300 (e.g. ``45 43 4b 44 | ECKD``).
301
302 The sprintf view formats the debug entries in the same way as the sprintf
303 function would do. The sprintf event/exception functions write to the
304 debug entry a pointer to the format string (size = sizeof(long))
305 and for each vararg a long value. So e.g. for a debug entry with a format
306 string plus two varargs one would need to allocate a (3 * sizeof(long))
307 byte data area in the debug_register() function.
308
309 IMPORTANT:
310 Using "%s" in sprintf event functions is dangerous. You can only
311 use "%s" in the sprintf event functions, if the memory for the passed string
312 is available as long as the debug feature exists. The reason behind this is
313 that due to performance considerations only a pointer to the string is stored
314 in the debug feature. If you log a string that is freed afterwards, you will
315 get an OOPS when inspecting the debug feature, because then the debug feature
316 will access the already freed memory.
317
318 NOTE:
319 If using the sprintf view do NOT use other event/exception functions
320 than the sprintf-event and -exception functions.
321
322 The format of the hex_ascii and sprintf view is as follows:
323
324 - Number of area
325 - Timestamp (formatted as seconds and microseconds since 00:00:00 Coordinated
326 Universal Time (UTC), January 1, 1970)
327 - level of debug entry
328 - Exception flag (* = Exception)
329 - Cpu-Number of calling task
330 - Return Address to caller
331 - data field
332
333 A typical line of the hex_ascii view will look like the following (first line
334 is only for explanation and will not be displayed when 'cating' the view)::
335
336 area time level exception cpu caller data (hex + ascii)
337 --------------------------------------------------------------------------
338 00 00964419409:440690 1 - 00 88023fe
339
340
341 Defining views
342 --------------
343
344 Views are specified with the 'debug_view' structure. There are defined
345 callback functions which are used for reading and writing the debugfs files:
346
347 .. code-block:: c
348
349 struct debug_view {
350 char name[DEBUG_MAX_PROCF_LEN];
351 debug_prolog_proc_t* prolog_proc;
352 debug_header_proc_t* header_proc;
353 debug_format_proc_t* format_proc;
354 debug_input_proc_t* input_proc;
355 void* private_data;
356 };
357
358 where:
359
360 .. code-block:: c
361
362 typedef int (debug_header_proc_t) (debug_info_t* id,
363 struct debug_view* view,
364 int area,
365 debug_entry_t* entry,
366 char* out_buf);
367
368 typedef int (debug_format_proc_t) (debug_info_t* id,
369 struct debug_view* view, char* out_buf,
370 const char* in_buf);
371 typedef int (debug_prolog_proc_t) (debug_info_t* id,
372 struct debug_view* view,
373 char* out_buf);
374 typedef int (debug_input_proc_t) (debug_info_t* id,
375 struct debug_view* view,
376 struct file* file, const char* user_buf,
377 size_t in_buf_size, loff_t* offset);
378
379
380 The "private_data" member can be used as pointer to view specific data.
381 It is not used by the debug feature itself.
382
383 The output when reading a debugfs file is structured like this::
384
385 "prolog_proc output"
386
387 "header_proc output 1" "format_proc output 1"
388 "header_proc output 2" "format_proc output 2"
389 "header_proc output 3" "format_proc output 3"
390 ...
391
392 When a view is read from the debugfs, the Debug Feature calls the
393 'prolog_proc' once for writing the prolog.
394 Then 'header_proc' and 'format_proc' are called for each
395 existing debug entry.
396
397 The input_proc can be used to implement functionality when it is written to
398 the view (e.g. like with ``echo "0" > /sys/kernel/debug/s390dbf/dasd/level``).
399
400 For header_proc there can be used the default function
401 :c:func:`debug_dflt_header_fn()` which is defined in debug.h.
402 and which produces the same header output as the predefined views.
403 E.g::
404
405 00 00964419409:440761 2 - 00 88023ec
406
407 In order to see how to use the callback functions check the implementation
408 of the default views!
409
410 Example:
411
412 .. code-block:: c
413
414 #include <asm/debug.h>
415
416 #define UNKNOWNSTR "data: %08x"
417
418 const char* messages[] =
419 {"This error...........\n",
420 "That error...........\n",
421 "Problem..............\n",
422 "Something went wrong.\n",
423 "Everything ok........\n",
424 NULL
425 };
426
427 static int debug_test_format_fn(
428 debug_info_t *id, struct debug_view *view,
429 char *out_buf, const char *in_buf
430 )
431 {
432 int i, rc = 0;
433
434 if (id->buf_size >= 4) {
435 int msg_nr = *((int*)in_buf);
436 if (msg_nr < sizeof(messages) / sizeof(char*) - 1)
437 rc += sprintf(out_buf, "%s", messages[msg_nr]);
438 else
439 rc += sprintf(out_buf, UNKNOWNSTR, msg_nr);
440 }
441 return rc;
442 }
443
444 struct debug_view debug_test_view = {
445 "myview", /* name of view */
446 NULL, /* no prolog */
447 &debug_dflt_header_fn, /* default header for each entry */
448 &debug_test_format_fn, /* our own format function */
449 NULL, /* no input function */
450 NULL /* no private data */
451 };
452
453 test:
454 =====
455
456 .. code-block:: c
457
458 debug_info_t *debug_info;
459 int i;
460 ...
461 debug_info = debug_register("test", 0, 4, 4);
462 debug_register_view(debug_info, &debug_test_view);
463 for (i = 0; i < 10; i ++)
464 debug_int_event(debug_info, 1, i);
465
466 ::
467
468 > cat /sys/kernel/debug/s390dbf/test/myview
469 00 00964419734:611402 1 - 00 88042ca This error...........
470 00 00964419734:611405 1 - 00 88042ca That error...........
471 00 00964419734:611408 1 - 00 88042ca Problem..............
472 00 00964419734:611411 1 - 00 88042ca Something went wrong.
473 00 00964419734:611414 1 - 00 88042ca Everything ok........
474 00 00964419734:611417 1 - 00 88042ca data: 00000005
475 00 00964419734:611419 1 - 00 88042ca data: 00000006
476 00 00964419734:611422 1 - 00 88042ca data: 00000007
477 00 00964419734:611425 1 - 00 88042ca data: 00000008
478 00 00964419734:611428 1 - 00 88042ca data: 00000009
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