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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_TRACING=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] SYMBOL[+offs]|MEMADDR [FETCHARGS] : Set a probe |
26 | r[:[GRP/]EVENT] SYMBOL[+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 SYMBOL+offs or MEMADDR. |
32 | SYMBOL[+offs] : Symbol+offset where the probe is inserted. |
33 | MEMADDR : Address where the probe is inserted. |
34 | |
35 | FETCHARGS : Arguments. Each probe can have up to 128 args. |
36 | %REG : Fetch register REG |
37 | @ADDR : Fetch memory at ADDR (ADDR should be in kernel) |
38 | @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) |
39 | $stackN : Fetch Nth entry of stack (N >= 0) |
40 | $stack : Fetch stack address. |
41 | $retval : Fetch return value.(*) |
42 | +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) |
43 | NAME=FETCHARG: Set NAME as the argument name of FETCHARG. |
44 | |
45 | (*) only for return probe. |
46 | (**) this is useful for fetching a field of data structures. |
47 | |
48 | |
49 | Per-Probe Event Filtering |
50 | ------------------------- |
51 | Per-probe event filtering feature allows you to set different filter on each |
52 | probe and gives you what arguments will be shown in trace buffer. If an event |
53 | name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event |
54 | under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', |
55 | 'enabled', 'format' and 'filter'. |
56 | |
57 | enabled: |
58 | You can enable/disable the probe by writing 1 or 0 on it. |
59 | |
60 | format: |
61 | This shows the format of this probe event. |
62 | |
63 | filter: |
64 | You can write filtering rules of this event. |
65 | |
66 | id: |
67 | This shows the id of this probe event. |
68 | |
69 | |
70 | Event Profiling |
71 | --------------- |
72 | You can check the total number of probe hits and probe miss-hits via |
73 | /sys/kernel/debug/tracing/kprobe_profile. |
74 | The first column is event name, the second is the number of probe hits, |
75 | the third is the number of probe miss-hits. |
76 | |
77 | |
78 | Usage examples |
79 | -------------- |
80 | To add a probe as a new event, write a new definition to kprobe_events |
81 | as below. |
82 | |
83 | echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events |
84 | |
85 | This sets a kprobe on the top of do_sys_open() function with recording |
86 | 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is |
87 | assigned to each function argument depends on arch-specific ABI. If you unsure |
88 | the ABI, please try to use probe subcommand of perf-tools (you can find it |
89 | under tools/perf/). |
90 | As this example shows, users can choose more familiar names for each arguments. |
91 | |
92 | echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events |
93 | |
94 | This sets a kretprobe on the return point of do_sys_open() function with |
95 | recording return value as "myretprobe" event. |
96 | You can see the format of these events via |
97 | /sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. |
98 | |
99 | cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format |
100 | name: myprobe |
101 | ID: 780 |
102 | format: |
103 | field:unsigned short common_type; offset:0; size:2; signed:0; |
104 | field:unsigned char common_flags; offset:2; size:1; signed:0; |
105 | field:unsigned char common_preempt_count; offset:3; size:1;signed:0; |
106 | field:int common_pid; offset:4; size:4; signed:1; |
107 | field:int common_lock_depth; offset:8; size:4; signed:1; |
108 | |
109 | field:unsigned long __probe_ip; offset:12; size:4; signed:0; |
110 | field:int __probe_nargs; offset:16; size:4; signed:1; |
111 | field:unsigned long dfd; offset:20; size:4; signed:0; |
112 | field:unsigned long filename; offset:24; size:4; signed:0; |
113 | field:unsigned long flags; offset:28; size:4; signed:0; |
114 | field:unsigned long mode; offset:32; size:4; signed:0; |
115 | |
116 | |
117 | print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, |
118 | REC->dfd, REC->filename, REC->flags, REC->mode |
119 | |
120 | You can see that the event has 4 arguments as in the expressions you specified. |
121 | |
122 | echo > /sys/kernel/debug/tracing/kprobe_events |
123 | |
124 | This clears all probe points. |
125 | |
126 | Or, |
127 | |
128 | echo -:myprobe >> kprobe_events |
129 | |
130 | This clears probe points selectively. |
131 | |
132 | Right after definition, each event is disabled by default. For tracing these |
133 | events, you need to enable it. |
134 | |
135 | echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable |
136 | echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable |
137 | |
138 | And you can see the traced information via /sys/kernel/debug/tracing/trace. |
139 | |
140 | cat /sys/kernel/debug/tracing/trace |
141 | # tracer: nop |
142 | # |
143 | # TASK-PID CPU# TIMESTAMP FUNCTION |
144 | # | | | | | |
145 | <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 |
146 | <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe |
147 | <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 |
148 | <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 |
149 | <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 |
150 | <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 |
151 | |
152 | |
153 | Each line shows when the kernel hits an event, and <- SYMBOL means kernel |
154 | returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel |
155 | returns from do_sys_open to sys_open+0x1b). |
156 | |
157 |
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