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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	                        Kprobe-based Event Tracing
2	                        ==========================
3	
4	                 Documentation is written by Masami Hiramatsu
5	
6	
7	Overview
8	--------
9	These events are similar to tracepoint based events. Instead of Tracepoint,
10	this is based on kprobes (kprobe and kretprobe). So it can probe wherever
11	kprobes can probe (this means, all functions body except for __kprobes
12	functions). Unlike the Tracepoint based event, this can be added and removed
13	dynamically, on the fly.
14	
15	To enable this feature, build your kernel with CONFIG_KPROBE_EVENT=y.
16	
17	Similar to the events tracer, this doesn't need to be activated via
18	current_tracer. Instead of that, add probe points via
19	/sys/kernel/debug/tracing/kprobe_events, and enable it via
20	/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled.
21	
22	
23	Synopsis of kprobe_events
24	-------------------------
25	  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
26	  r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]		: Set a return probe
27	  -:[GRP/]EVENT						: Clear a probe
28	
29	 GRP		: Group name. If omitted, use "kprobes" for it.
30	 EVENT		: Event name. If omitted, the event name is generated
31			  based on SYM+offs or MEMADDR.
32	 MOD		: Module name which has given SYM.
33	 SYM[+offs]	: Symbol+offset where the probe is inserted.
34	 MEMADDR	: Address where the probe is inserted.
35	
36	 FETCHARGS	: Arguments. Each probe can have up to 128 args.
37	  %REG		: Fetch register REG
38	  @ADDR		: Fetch memory at ADDR (ADDR should be in kernel)
39	  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
40	  $stackN	: Fetch Nth entry of stack (N >= 0)
41	  $stack	: Fetch stack address.
42	  $retval	: Fetch return value.(*)
43	  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
44	  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
45	  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
46			  (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
47			  are supported.
48	
49	  (*) only for return probe.
50	  (**) this is useful for fetching a field of data structures.
51	
52	Types
53	-----
54	Several types are supported for fetch-args. Kprobe tracer will access memory
55	by given type. Prefix 's' and 'u' means those types are signed and unsigned
56	respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
57	String type is a special type, which fetches a "null-terminated" string from
58	kernel space. This means it will fail and store NULL if the string container
59	has been paged out.
60	Bitfield is another special type, which takes 3 parameters, bit-width, bit-
61	offset, and container-size (usually 32). The syntax is;
62	
63	 b<bit-width>@<bit-offset>/<container-size>
64	
65	
66	Per-Probe Event Filtering
67	-------------------------
68	 Per-probe event filtering feature allows you to set different filter on each
69	probe and gives you what arguments will be shown in trace buffer. If an event
70	name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
71	under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
72	'enabled', 'format' and 'filter'.
73	
74	enabled:
75	  You can enable/disable the probe by writing 1 or 0 on it.
76	
77	format:
78	  This shows the format of this probe event.
79	
80	filter:
81	  You can write filtering rules of this event.
82	
83	id:
84	  This shows the id of this probe event.
85	
86	
87	Event Profiling
88	---------------
89	 You can check the total number of probe hits and probe miss-hits via
90	/sys/kernel/debug/tracing/kprobe_profile.
91	 The first column is event name, the second is the number of probe hits,
92	the third is the number of probe miss-hits.
93	
94	
95	Usage examples
96	--------------
97	To add a probe as a new event, write a new definition to kprobe_events
98	as below.
99	
100	  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
101	
102	 This sets a kprobe on the top of do_sys_open() function with recording
103	1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
104	assigned to each function argument depends on arch-specific ABI. If you unsure
105	the ABI, please try to use probe subcommand of perf-tools (you can find it
106	under tools/perf/).
107	As this example shows, users can choose more familiar names for each arguments.
108	
109	  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
110	
111	 This sets a kretprobe on the return point of do_sys_open() function with
112	recording return value as "myretprobe" event.
113	 You can see the format of these events via
114	/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
115	
116	  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
117	name: myprobe
118	ID: 780
119	format:
120	        field:unsigned short common_type;       offset:0;       size:2; signed:0;
121	        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
122	        field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
123	        field:int common_pid;   offset:4;       size:4; signed:1;
124	
125	        field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
126	        field:int __probe_nargs;        offset:16;      size:4; signed:1;
127	        field:unsigned long dfd;        offset:20;      size:4; signed:0;
128	        field:unsigned long filename;   offset:24;      size:4; signed:0;
129	        field:unsigned long flags;      offset:28;      size:4; signed:0;
130	        field:unsigned long mode;       offset:32;      size:4; signed:0;
131	
132	
133	print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
134	REC->dfd, REC->filename, REC->flags, REC->mode
135	
136	 You can see that the event has 4 arguments as in the expressions you specified.
137	
138	  echo > /sys/kernel/debug/tracing/kprobe_events
139	
140	 This clears all probe points.
141	
142	 Or,
143	
144	  echo -:myprobe >> kprobe_events
145	
146	 This clears probe points selectively.
147	
148	 Right after definition, each event is disabled by default. For tracing these
149	events, you need to enable it.
150	
151	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
152	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
153	
154	 And you can see the traced information via /sys/kernel/debug/tracing/trace.
155	
156	  cat /sys/kernel/debug/tracing/trace
157	# tracer: nop
158	#
159	#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
160	#              | |       |          |         |
161	           <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
162	           <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
163	           <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
164	           <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
165	           <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
166	           <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
167	
168	
169	 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
170	returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
171	returns from do_sys_open to sys_open+0x1b).
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