1 =========================================
2 user_events: User-based Event Tracing
3 =========================================
4
5 :Author: Beau Belgrave
6
7 Overview
8 --------
9 User based trace events allow user processes to create events and trace data
10 that can be viewed via existing tools, such as ftrace and perf.
11 To enable this feature, build your kernel with CONFIG_USER_EVENTS=y.
12
13 Programs can view status of the events via
14 /sys/kernel/tracing/user_events_status and can both register and write
15 data out via /sys/kernel/tracing/user_events_data.
16
17 Programs can also use /sys/kernel/tracing/dynamic_events to register and
18 delete user based events via the u: prefix. The format of the command to
19 dynamic_events is the same as the ioctl with the u: prefix applied. This
20 requires CAP_PERFMON due to the event persisting, otherwise -EPERM is returned.
21
22 Typically programs will register a set of events that they wish to expose to
23 tools that can read trace_events (such as ftrace and perf). The registration
24 process tells the kernel which address and bit to reflect if any tool has
25 enabled the event and data should be written. The registration will give back
26 a write index which describes the data when a write() or writev() is called
27 on the /sys/kernel/tracing/user_events_data file.
28
29 The structures referenced in this document are contained within the
30 /include/uapi/linux/user_events.h file in the source tree.
31
32 **NOTE:** *Both user_events_status and user_events_data are under the tracefs
33 filesystem and may be mounted at different paths than above.*
34
35 Registering
36 -----------
37 Registering within a user process is done via ioctl() out to the
38 /sys/kernel/tracing/user_events_data file. The command to issue is
39 DIAG_IOCSREG.
40
41 This command takes a packed struct user_reg as an argument::
42
43 struct user_reg {
44 /* Input: Size of the user_reg structure being used */
45 __u32 size;
46
47 /* Input: Bit in enable address to use */
48 __u8 enable_bit;
49
50 /* Input: Enable size in bytes at address */
51 __u8 enable_size;
52
53 /* Input: Flags to use, if any */
54 __u16 flags;
55
56 /* Input: Address to update when enabled */
57 __u64 enable_addr;
58
59 /* Input: Pointer to string with event name, description and flags */
60 __u64 name_args;
61
62 /* Output: Index of the event to use when writing data */
63 __u32 write_index;
64 } __attribute__((__packed__));
65
66 The struct user_reg requires all the above inputs to be set appropriately.
67
68 + size: This must be set to sizeof(struct user_reg).
69
70 + enable_bit: The bit to reflect the event status at the address specified by
71 enable_addr.
72
73 + enable_size: The size of the value specified by enable_addr.
74 This must be 4 (32-bit) or 8 (64-bit). 64-bit values are only allowed to be
75 used on 64-bit kernels, however, 32-bit can be used on all kernels.
76
77 + flags: The flags to use, if any.
78 Callers should first attempt to use flags and retry without flags to ensure
79 support for lower versions of the kernel. If a flag is not supported -EINVAL
80 is returned.
81
82 + enable_addr: The address of the value to use to reflect event status. This
83 must be naturally aligned and write accessible within the user program.
84
85 + name_args: The name and arguments to describe the event, see command format
86 for details.
87
88 The following flags are currently supported.
89
90 + USER_EVENT_REG_PERSIST: The event will not delete upon the last reference
91 closing. Callers may use this if an event should exist even after the
92 process closes or unregisters the event. Requires CAP_PERFMON otherwise
93 -EPERM is returned.
94
95 + USER_EVENT_REG_MULTI_FORMAT: The event can contain multiple formats. This
96 allows programs to prevent themselves from being blocked when their event
97 format changes and they wish to use the same name. When this flag is used the
98 tracepoint name will be in the new format of "name.unique_id" vs the older
99 format of "name". A tracepoint will be created for each unique pair of name
100 and format. This means if several processes use the same name and format,
101 they will use the same tracepoint. If yet another process uses the same name,
102 but a different format than the other processes, it will use a different
103 tracepoint with a new unique id. Recording programs need to scan tracefs for
104 the various different formats of the event name they are interested in
105 recording. The system name of the tracepoint will also use "user_events_multi"
106 instead of "user_events". This prevents single-format event names conflicting
107 with any multi-format event names within tracefs. The unique_id is output as
108 a hex string. Recording programs should ensure the tracepoint name starts with
109 the event name they registered and has a suffix that starts with . and only
110 has hex characters. For example to find all versions of the event "test" you
111 can use the regex "^test\.[0-9a-fA-F]+$".
112
113 Upon successful registration the following is set.
114
115 + write_index: The index to use for this file descriptor that represents this
116 event when writing out data. The index is unique to this instance of the file
117 descriptor that was used for the registration. See writing data for details.
118
119 User based events show up under tracefs like any other event under the
120 subsystem named "user_events". This means tools that wish to attach to the
121 events need to use /sys/kernel/tracing/events/user_events/[name]/enable
122 or perf record -e user_events:[name] when attaching/recording.
123
124 **NOTE:** The event subsystem name by default is "user_events". Callers should
125 not assume it will always be "user_events". Operators reserve the right in the
126 future to change the subsystem name per-process to accommodate event isolation.
127 In addition if the USER_EVENT_REG_MULTI_FORMAT flag is used the tracepoint name
128 will have a unique id appended to it and the system name will be
129 "user_events_multi" as described above.
130
131 Command Format
132 ^^^^^^^^^^^^^^
133 The command string format is as follows::
134
135 name[:FLAG1[,FLAG2...]] [Field1[;Field2...]]
136
137 Supported Flags
138 ^^^^^^^^^^^^^^^
139 None yet
140
141 Field Format
142 ^^^^^^^^^^^^
143 ::
144
145 type name [size]
146
147 Basic types are supported (__data_loc, u32, u64, int, char, char[20], etc).
148 User programs are encouraged to use clearly sized types like u32.
149
150 **NOTE:** *Long is not supported since size can vary between user and kernel.*
151
152 The size is only valid for types that start with a struct prefix.
153 This allows user programs to describe custom structs out to tools, if required.
154
155 For example, a struct in C that looks like this::
156
157 struct mytype {
158 char data[20];
159 };
160
161 Would be represented by the following field::
162
163 struct mytype myname 20
164
165 Deleting
166 --------
167 Deleting an event from within a user process is done via ioctl() out to the
168 /sys/kernel/tracing/user_events_data file. The command to issue is
169 DIAG_IOCSDEL.
170
171 This command only requires a single string specifying the event to delete by
172 its name. Delete will only succeed if there are no references left to the
173 event (in both user and kernel space). User programs should use a separate file
174 to request deletes than the one used for registration due to this.
175
176 **NOTE:** By default events will auto-delete when there are no references left
177 to the event. If programs do not want auto-delete, they must use the
178 USER_EVENT_REG_PERSIST flag when registering the event. Once that flag is used
179 the event exists until DIAG_IOCSDEL is invoked. Both register and delete of an
180 event that persists requires CAP_PERFMON, otherwise -EPERM is returned. When
181 there are multiple formats of the same event name, all events with the same
182 name will be attempted to be deleted. If only a specific version is wanted to
183 be deleted then the /sys/kernel/tracing/dynamic_events file should be used for
184 that specific format of the event.
185
186 Unregistering
187 -------------
188 If after registering an event it is no longer wanted to be updated then it can
189 be disabled via ioctl() out to the /sys/kernel/tracing/user_events_data file.
190 The command to issue is DIAG_IOCSUNREG. This is different than deleting, where
191 deleting actually removes the event from the system. Unregistering simply tells
192 the kernel your process is no longer interested in updates to the event.
193
194 This command takes a packed struct user_unreg as an argument::
195
196 struct user_unreg {
197 /* Input: Size of the user_unreg structure being used */
198 __u32 size;
199
200 /* Input: Bit to unregister */
201 __u8 disable_bit;
202
203 /* Input: Reserved, set to 0 */
204 __u8 __reserved;
205
206 /* Input: Reserved, set to 0 */
207 __u16 __reserved2;
208
209 /* Input: Address to unregister */
210 __u64 disable_addr;
211 } __attribute__((__packed__));
212
213 The struct user_unreg requires all the above inputs to be set appropriately.
214
215 + size: This must be set to sizeof(struct user_unreg).
216
217 + disable_bit: This must be set to the bit to disable (same bit that was
218 previously registered via enable_bit).
219
220 + disable_addr: This must be set to the address to disable (same address that was
221 previously registered via enable_addr).
222
223 **NOTE:** Events are automatically unregistered when execve() is invoked. During
224 fork() the registered events will be retained and must be unregistered manually
225 in each process if wanted.
226
227 Status
228 ------
229 When tools attach/record user based events the status of the event is updated
230 in realtime. This allows user programs to only incur the cost of the write() or
231 writev() calls when something is actively attached to the event.
232
233 The kernel will update the specified bit that was registered for the event as
234 tools attach/detach from the event. User programs simply check if the bit is set
235 to see if something is attached or not.
236
237 Administrators can easily check the status of all registered events by reading
238 the user_events_status file directly via a terminal. The output is as follows::
239
240 Name [# Comments]
241 ...
242
243 Active: ActiveCount
244 Busy: BusyCount
245
246 For example, on a system that has a single event the output looks like this::
247
248 test
249
250 Active: 1
251 Busy: 0
252
253 If a user enables the user event via ftrace, the output would change to this::
254
255 test # Used by ftrace
256
257 Active: 1
258 Busy: 1
259
260 Writing Data
261 ------------
262 After registering an event the same fd that was used to register can be used
263 to write an entry for that event. The write_index returned must be at the start
264 of the data, then the remaining data is treated as the payload of the event.
265
266 For example, if write_index returned was 1 and I wanted to write out an int
267 payload of the event. Then the data would have to be 8 bytes (2 ints) in size,
268 with the first 4 bytes being equal to 1 and the last 4 bytes being equal to the
269 value I want as the payload.
270
271 In memory this would look like this::
272
273 int index;
274 int payload;
275
276 User programs might have well known structs that they wish to use to emit out
277 as payloads. In those cases writev() can be used, with the first vector being
278 the index and the following vector(s) being the actual event payload.
279
280 For example, if I have a struct like this::
281
282 struct payload {
283 int src;
284 int dst;
285 int flags;
286 } __attribute__((__packed__));
287
288 It's advised for user programs to do the following::
289
290 struct iovec io[2];
291 struct payload e;
292
293 io[0].iov_base = &write_index;
294 io[0].iov_len = sizeof(write_index);
295 io[1].iov_base = &e;
296 io[1].iov_len = sizeof(e);
297
298 writev(fd, (const struct iovec*)io, 2);
299
300 **NOTE:** *The write_index is not emitted out into the trace being recorded.*
301
302 Example Code
303 ------------
304 See sample code in samples/user_events.
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