S390 Debug Feature¶

files:

arch/s390/kernel/debug.c
arch/s390/include/asm/debug.h

Description:¶

The goal of this feature is to provide a kernel debug logging API where log records can be stored efficiently in memory, where each component (e.g. device drivers) can have one separate debug log. One purpose of this is to inspect the debug logs after a production system crash in order to analyze the reason for the crash.

If the system still runs but only a subcomponent which uses dbf fails, it is possible to look at the debug logs on a live system via the Linux debugfs filesystem.

The debug feature may also very useful for kernel and driver development.

Design:¶

Kernel components (e.g. device drivers) can register themselves at the debug feature with the function call debug_register(). This function initializes a debug log for the caller. For each debug log exists a number of debug areas where exactly one is active at one time. Each debug area consists of contiguous pages in memory. In the debug areas there are stored debug entries (log records) which are written by event- and exception-calls.

An event-call writes the specified debug entry to the active debug area and updates the log pointer for the active area. If the end of the active debug area is reached, a wrap around is done (ring buffer) and the next debug entry will be written at the beginning of the active debug area.

An exception-call writes the specified debug entry to the log and switches to the next debug area. This is done in order to be sure that the records which describe the origin of the exception are not overwritten when a wrap around for the current area occurs.

The debug areas themselves are also ordered in form of a ring buffer. When an exception is thrown in the last debug area, the following debug entries are then written again in the very first area.

There are four versions for the event- and exception-calls: One for logging raw data, one for text, one for numbers (unsigned int and long), and one for sprintf-like formatted strings.

Each debug entry contains the following data:

Timestamp
Cpu-Number of calling task
Level of debug entry (0…6)
Return Address to caller
Flag, if entry is an exception or not

The debug logs can be inspected in a live system through entries in the debugfs-filesystem. Under the toplevel directory “s390dbf” there is a directory for each registered component, which is named like the corresponding component. The debugfs normally should be mounted to /sys/kernel/debug therefore the debug feature can be accessed under /sys/kernel/debug/s390dbf.

The content of the directories are files which represent different views to the debug log. Each component can decide which views should be used through registering them with the function debug_register_view(). Predefined views for hex/ascii and sprintf data are provided. It is also possible to define other views. The content of a view can be inspected simply by reading the corresponding debugfs file.

All debug logs have an actual debug level (range from 0 to 6). The default level is 3. Event and Exception functions have a level parameter. Only debug entries with a level that is lower or equal than the actual level are written to the log. This means, when writing events, high priority log entries should have a low level value whereas low priority entries should have a high one. The actual debug level can be changed with the help of the debugfs-filesystem through writing a number string “x” to the level debugfs file which is provided for every debug log. Debugging can be switched off completely by using “-” on the level debugfs file.

Example:

> echo "-" > /sys/kernel/debug/s390dbf/dasd/level

It is also possible to deactivate the debug feature globally for every debug log. You can change the behavior using 2 sysctl parameters in /proc/sys/s390dbf:

There are currently 2 possible triggers, which stop the debug feature globally. The first possibility is to use the debug_active sysctl. If set to 1 the debug feature is running. If debug_active is set to 0 the debug feature is turned off.

The second trigger which stops the debug feature is a kernel oops. That prevents the debug feature from overwriting debug information that happened before the oops. After an oops you can reactivate the debug feature by piping 1 to /proc/sys/s390dbf/debug_active. Nevertheless, it’s not suggested to use an oopsed kernel in a production environment.

If you want to disallow the deactivation of the debug feature, you can use the debug_stoppable sysctl. If you set debug_stoppable to 0 the debug feature cannot be stopped. If the debug feature is already stopped, it will stay deactivated.

Kernel Interfaces:¶

debug_info_t * debug_register_mode(const char *name, int pages_per_area, int nr_areas, int buf_size, umode_t mode, uid_t uid, gid_t gid)¶: creates and initializes debug area.

Parameters

const char *name: Name of debug log (e.g. used for debugfs entry)
int pages_per_area: Number of pages, which will be allocated per area
int nr_areas: Number of debug areas
int buf_size: Size of data area in each debug entry
umode_t mode: File mode for debugfs files. E.g. S_IRWXUGO
uid_t uid: User ID for debugfs files. Currently only 0 is supported.
gid_t gid: Group ID for debugfs files. Currently only 0 is supported.

Return

Handle for generated debug area
NULL if register failed

Description

Allocates memory for a debug log. Must not be called within an interrupt handler.

debug_info_t * debug_register(const char *name, int pages_per_area, int nr_areas, int buf_size)¶: creates and initializes debug area with default file mode.

Parameters

const char *name: Name of debug log (e.g. used for debugfs entry)
int pages_per_area: Number of pages, which will be allocated per area
int nr_areas: Number of debug areas
int buf_size: Size of data area in each debug entry

Return

Handle for generated debug area
NULL if register failed

Description

Allocates memory for a debug log. The debugfs file mode access permissions are read and write for user. Must not be called within an interrupt handler.

void debug_unregister(debug_info_t *id)¶: give back debug area.

Parameters

debug_info_t *id: handle for debug log

Return

none

void debug_set_level(debug_info_t *id, int new_level)¶: Sets new actual debug level if new_level is valid.

Parameters

debug_info_t *id: handle for debug log
int new_level: new debug level

Return

none

void debug_stop_all(void)¶: stops the debug feature if stopping is allowed.

Parameters

void: no arguments

Return

none

Description

Currently used in case of a kernel oops.

void debug_set_critical(void)¶: event/exception functions try lock instead of spin.

Parameters

void: no arguments

Return

none

Description

Currently used in case of stopping all CPUs but the current one. Once in this state, functions to write a debug entry for an event or exception no longer spin on the debug area lock, but only try to get it and fail if they do not get the lock.

int debug_register_view(debug_info_t *id, struct debug_view *view)¶: registers new debug view and creates debugfs dir entry

Parameters

debug_info_t *id: handle for debug log
struct debug_view *view: pointer to debug view struct

Return

0 : ok
< 0: Error

int debug_unregister_view(debug_info_t *id, struct debug_view *view)¶: unregisters debug view and removes debugfs dir entry

Parameters

debug_info_t *id: handle for debug log
struct debug_view *view: pointer to debug view struct

Return

0 : ok
< 0: Error

bool debug_level_enabled(debug_info_t *id, int level)¶: Returns true if debug events for the specified level would be logged. Otherwise returns false.

Parameters

debug_info_t *id: handle for debug log
int level: debug level

Return

true if level is less or equal to the current debug level.

debug_entry_t * debug_event(debug_info_t *id, int level, void *data, int length)¶: writes binary debug entry to active debug area (if level <= actual debug level)

Parameters

debug_info_t *id: handle for debug log
int level: debug level
void *data: pointer to data for debug entry
int length: length of data in bytes

Return

Address of written debug entry
NULL if error

debug_entry_t * debug_int_event(debug_info_t *id, int level, unsigned int tag)¶: writes unsigned integer debug entry to active debug area (if level <= actual debug level)

Parameters

debug_info_t *id: handle for debug log
int level: debug level
unsigned int tag: integer value for debug entry

Return

Address of written debug entry
NULL if error

debug_entry_t * debug_long_event(debug_info_t *id, int level, unsigned long tag)¶: writes unsigned long debug entry to active debug area (if level <= actual debug level)

Parameters

debug_info_t *id: handle for debug log
int level: debug level
unsigned long tag: long integer value for debug entry

Return

Address of written debug entry
NULL if error

debug_entry_t * debug_text_event(debug_info_t *id, int level, const char *txt)¶: writes string debug entry in ascii format to active debug area (if level <= actual debug level)

Parameters

debug_info_t *id: handle for debug log
int level: debug level
const char *txt: string for debug entry

Return

Address of written debug entry
NULL if error

debug_sprintf_event(_id, _level, _fmt, …)¶: writes debug entry with format string and varargs (longs) to active debug area (if level $<=$ actual debug level).

Parameters

_id: handle for debug log
_level: debug level
_fmt: format string for debug entry
...: varargs used as in sprintf()

Return

Address of written debug entry
NULL if error

Description

floats and long long datatypes cannot be used as varargs.

debug_entry_t * debug_exception(debug_info_t *id, int level, void *data, int length)¶: writes binary debug entry to active debug area (if level <= actual debug level) and switches to next debug area

Parameters

debug_info_t *id: handle for debug log
int level: debug level
void *data: pointer to data for debug entry
int length: length of data in bytes

Return

Address of written debug entry
NULL if error

debug_entry_t * debug_int_exception(debug_info_t *id, int level, unsigned int tag)¶: writes unsigned int debug entry to active debug area (if level <= actual debug level) and switches to next debug area

Parameters

debug_info_t *id: handle for debug log
int level: debug level
unsigned int tag: integer value for debug entry

Return

Address of written debug entry
NULL if error

debug_entry_t * debug_long_exception(debug_info_t *id, int level, unsigned long tag)¶: writes long debug entry to active debug area (if level <= actual debug level) and switches to next debug area

Parameters

debug_info_t *id: handle for debug log
int level: debug level
unsigned long tag: long integer value for debug entry

Return

Address of written debug entry
NULL if error

debug_entry_t * debug_text_exception(debug_info_t *id, int level, const char *txt)¶: writes string debug entry in ascii format to active debug area (if level <= actual debug level) and switches to next debug area area

Parameters

debug_info_t *id: handle for debug log
int level: debug level
const char *txt: string for debug entry

Return

Address of written debug entry
NULL if error

debug_sprintf_exception(_id, _level, _fmt, …)¶: writes debug entry with format string and varargs (longs) to active debug area (if level <= actual debug level) and switches to next debug area.

Parameters

_id: handle for debug log
_level: debug level
_fmt: format string for debug entry
...: varargs used as in sprintf()

Return

Address of written debug entry
NULL if error

Description

floats and long long datatypes cannot be used as varargs.

Predefined views:¶

extern struct debug_view debug_hex_ascii_view;

extern struct debug_view debug_sprintf_view;

Examples¶

/*
 * hex_ascii-view Example
 */

#include <linux/init.h>
#include <asm/debug.h>

static debug_info_t *debug_info;

static int init(void)
{
    /* register 4 debug areas with one page each and 4 byte data field */

    debug_info = debug_register("test", 1, 4, 4 );
    debug_register_view(debug_info, &debug_hex_ascii_view);

    debug_text_event(debug_info, 4 , "one ");
    debug_int_exception(debug_info, 4, 4711);
    debug_event(debug_info, 3, &debug_info, 4);

    return 0;
}

static void cleanup(void)
{
    debug_unregister(debug_info);
}

module_init(init);
module_exit(cleanup);

/*
 * sprintf-view Example
 */

#include <linux/init.h>
#include <asm/debug.h>

static debug_info_t *debug_info;

static int init(void)
{
    /* register 4 debug areas with one page each and data field for */
    /* format string pointer + 2 varargs (= 3 * sizeof(long))       */

    debug_info = debug_register("test", 1, 4, sizeof(long) * 3);
    debug_register_view(debug_info, &debug_sprintf_view);

    debug_sprintf_event(debug_info, 2 , "first event in %s:%i\n",__FILE__,__LINE__);
    debug_sprintf_exception(debug_info, 1, "pointer to debug info: %p\n",&debug_info);

    return 0;
}

static void cleanup(void)
{
    debug_unregister(debug_info);
}

module_init(init);
module_exit(cleanup);

Debugfs Interface¶

Views to the debug logs can be investigated through reading the corresponding debugfs-files:

Example:

> ls /sys/kernel/debug/s390dbf/dasd
flush  hex_ascii  level pages
> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort -k2,2 -s
00974733272:680099 2 - 02 0006ad7e  07 ea 4a 90 | ....
00974733272:682210 2 - 02 0006ade6  46 52 45 45 | FREE
00974733272:682213 2 - 02 0006adf6  07 ea 4a 90 | ....
00974733272:682281 1 * 02 0006ab08  41 4c 4c 43 | EXCP
00974733272:682284 2 - 02 0006ab16  45 43 4b 44 | ECKD
00974733272:682287 2 - 02 0006ab28  00 00 00 04 | ....
00974733272:682289 2 - 02 0006ab3e  00 00 00 20 | ...
00974733272:682297 2 - 02 0006ad7e  07 ea 4a 90 | ....
00974733272:684384 2 - 00 0006ade6  46 52 45 45 | FREE
00974733272:684388 2 - 00 0006adf6  07 ea 4a 90 | ....

See section about predefined views for explanation of the above output!

Changing the debug level¶

Example:

> cat /sys/kernel/debug/s390dbf/dasd/level
3
> echo "5" > /sys/kernel/debug/s390dbf/dasd/level
> cat /sys/kernel/debug/s390dbf/dasd/level
5

Flushing debug areas¶

Debug areas can be flushed with piping the number of the desired area (0…n) to the debugfs file “flush”. When using “-” all debug areas are flushed.

Examples:

Flush debug area 0:

> echo "0" > /sys/kernel/debug/s390dbf/dasd/flush

Flush all debug areas:

> echo "-" > /sys/kernel/debug/s390dbf/dasd/flush

Changing the size of debug areas¶

It is possible the change the size of debug areas through piping the number of pages to the debugfs file “pages”. The resize request will also flush the debug areas.

Example:

Define 4 pages for the debug areas of debug feature “dasd”:

> echo "4" > /sys/kernel/debug/s390dbf/dasd/pages

Stopping the debug feature¶

Example:

Check if stopping is allowed:

> cat /proc/sys/s390dbf/debug_stoppable

Stop debug feature:

> echo 0 > /proc/sys/s390dbf/debug_active

crash Interface¶

The crash tool since v5.1.0 has a built-in command s390dbf to display all the debug logs or export them to the file system. With this tool it is possible to investigate the debug logs on a live system and with a memory dump after a system crash.

Investigating raw memory¶

One last possibility to investigate the debug logs at a live system and after a system crash is to look at the raw memory under VM or at the Service Element. It is possible to find the anchor of the debug-logs through the debug_area_first symbol in the System map. Then one has to follow the correct pointers of the data-structures defined in debug.h and find the debug-areas in memory. Normally modules which use the debug feature will also have a global variable with the pointer to the debug-logs. Following this pointer it will also be possible to find the debug logs in memory.

For this method it is recommended to use ‘16 * x + 4’ byte (x = 0..n) for the length of the data field in debug_register() in order to see the debug entries well formatted.

Predefined Views¶

There are two predefined views: hex_ascii and sprintf. The hex_ascii view shows the data field in hex and ascii representation (e.g. 45 43 4b 44 | ECKD).

The sprintf view formats the debug entries in the same way as the sprintf function would do. The sprintf event/exception functions write to the debug entry a pointer to the format string (size = sizeof(long)) and for each vararg a long value. So e.g. for a debug entry with a format string plus two varargs one would need to allocate a (3 * sizeof(long)) byte data area in the debug_register() function.

IMPORTANT:: Using “%s” in sprintf event functions is dangerous. You can only use “%s” in the sprintf event functions, if the memory for the passed string is available as long as the debug feature exists. The reason behind this is that due to performance considerations only a pointer to the string is stored in the debug feature. If you log a string that is freed afterwards, you will get an OOPS when inspecting the debug feature, because then the debug feature will access the already freed memory.
NOTE:: If using the sprintf view do NOT use other event/exception functions than the sprintf-event and -exception functions.

The format of the hex_ascii and sprintf view is as follows:

Number of area
Timestamp (formatted as seconds and microseconds since 00:00:00 Coordinated Universal Time (UTC), January 1, 1970)
level of debug entry
Exception flag (* = Exception)
Cpu-Number of calling task
Return Address to caller
data field

A typical line of the hex_ascii view will look like the following (first line is only for explanation and will not be displayed when ‘cating’ the view):

area  time           level exception cpu caller    data (hex + ascii)
--------------------------------------------------------------------------
00    00964419409:440690 1 -         00  88023fe

Defining views¶

Views are specified with the ‘debug_view’ structure. There are defined callback functions which are used for reading and writing the debugfs files:

struct debug_view {
      char name[DEBUG_MAX_PROCF_LEN];
      debug_prolog_proc_t* prolog_proc;
      debug_header_proc_t* header_proc;
      debug_format_proc_t* format_proc;
      debug_input_proc_t*  input_proc;
      void*                private_data;
};

where:

typedef int (debug_header_proc_t) (debug_info_t* id,
                                   struct debug_view* view,
                                   int area,
                                   debug_entry_t* entry,
                                   char* out_buf);

typedef int (debug_format_proc_t) (debug_info_t* id,
                                   struct debug_view* view, char* out_buf,
                                   const char* in_buf);
typedef int (debug_prolog_proc_t) (debug_info_t* id,
                                   struct debug_view* view,
                                   char* out_buf);
typedef int (debug_input_proc_t) (debug_info_t* id,
                                  struct debug_view* view,
                                  struct file* file, const char* user_buf,
                                  size_t in_buf_size, loff_t* offset);

The “private_data” member can be used as pointer to view specific data. It is not used by the debug feature itself.

The output when reading a debugfs file is structured like this:

"prolog_proc output"

"header_proc output 1"  "format_proc output 1"
"header_proc output 2"  "format_proc output 2"
"header_proc output 3"  "format_proc output 3"
...

When a view is read from the debugfs, the Debug Feature calls the ‘prolog_proc’ once for writing the prolog. Then ‘header_proc’ and ‘format_proc’ are called for each existing debug entry.

The input_proc can be used to implement functionality when it is written to the view (e.g. like with echo "0" > /sys/kernel/debug/s390dbf/dasd/level).

For header_proc there can be used the default function debug_dflt_header_fn() which is defined in debug.h. and which produces the same header output as the predefined views. E.g:

00 00964419409:440761 2 - 00 88023ec

In order to see how to use the callback functions check the implementation of the default views!

Example:

#include <asm/debug.h>

#define UNKNOWNSTR "data: %08x"

const char* messages[] =
{"This error...........\n",
 "That error...........\n",
 "Problem..............\n",
 "Something went wrong.\n",
 "Everything ok........\n",
 NULL
};

static int debug_test_format_fn(
   debug_info_t *id, struct debug_view *view,
   char *out_buf, const char *in_buf
)
{
  int i, rc = 0;

  if (id->buf_size >= 4) {
     int msg_nr = *((int*)in_buf);
     if (msg_nr < sizeof(messages) / sizeof(char*) - 1)
        rc += sprintf(out_buf, "%s", messages[msg_nr]);
     else
        rc += sprintf(out_buf, UNKNOWNSTR, msg_nr);
  }
  return rc;
}

struct debug_view debug_test_view = {
  "myview",                 /* name of view */
  NULL,                     /* no prolog */
  &debug_dflt_header_fn,    /* default header for each entry */
  &debug_test_format_fn,    /* our own format function */
  NULL,                     /* no input function */
  NULL                      /* no private data */
};

test:¶

debug_info_t *debug_info;
int i;
...
debug_info = debug_register("test", 0, 4, 4);
debug_register_view(debug_info, &debug_test_view);
for (i = 0; i < 10; i ++)
  debug_int_event(debug_info, 1, i);

> cat /sys/kernel/debug/s390dbf/test/myview
00964419734:611402 1 - 00 88042ca   This error...........
00964419734:611405 1 - 00 88042ca   That error...........
00964419734:611408 1 - 00 88042ca   Problem..............
00964419734:611411 1 - 00 88042ca   Something went wrong.
00964419734:611414 1 - 00 88042ca   Everything ok........
00964419734:611417 1 - 00 88042ca   data: 00000005
00964419734:611419 1 - 00 88042ca   data: 00000006
00964419734:611422 1 - 00 88042ca   data: 00000007
00964419734:611425 1 - 00 88042ca   data: 00000008
00964419734:611428 1 - 00 88042ca   data: 00000009