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1 | Kprobe-based Event Tracing |
2 | ========================== | |
d8ec9185 MH |
3 | |
4 | Documentation is written by Masami Hiramatsu | |
5 | ||
6 | ||
7 | Overview | |
8 | -------- | |
77b44d1b MH |
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. | |
d8ec9185 | 14 | |
77b44d1b | 15 | To enable this feature, build your kernel with CONFIG_KPROBE_TRACING=y. |
d8ec9185 | 16 | |
77b44d1b MH |
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. | |
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21 | |
22 | ||
23 | Synopsis of kprobe_events | |
24 | ------------------------- | |
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25 | p[:[GRP/]EVENT] SYMBOL[+offs]|MEMADDR [FETCHARGS] : Set a probe |
26 | r[:[GRP/]EVENT] SYMBOL[+0] [FETCHARGS] : Set a return probe | |
df3ab708 | 27 | -:[GRP/]EVENT : Clear a probe |
d8ec9185 | 28 | |
f52487e9 | 29 | GRP : Group name. If omitted, use "kprobes" for it. |
2fba0c88 MH |
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. | |
d8ec9185 | 34 | |
2fba0c88 | 35 | FETCHARGS : Arguments. Each probe can have up to 128 args. |
2e06ff63 MH |
36 | %REG : Fetch register REG |
37 | @ADDR : Fetch memory at ADDR (ADDR should be in kernel) | |
d8ec9185 | 38 | @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) |
2e06ff63 MH |
39 | $stackN : Fetch Nth entry of stack (N >= 0) |
40 | $stack : Fetch stack address. | |
14640106 MH |
41 | $retval : Fetch return value.(*) |
42 | +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) | |
93ccae7a MH |
43 | NAME=FETCHARG : Set NAME as the argument name of FETCHARG. |
44 | FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types | |
e09c8614 | 45 | (u8/u16/u32/u64/s8/s16/s32/s64) and string are supported. |
d8ec9185 | 46 | |
14640106 MH |
47 | (*) only for return probe. |
48 | (**) this is useful for fetching a field of data structures. | |
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49 | |
50 | ||
51 | Per-Probe Event Filtering | |
52 | ------------------------- | |
53 | Per-probe event filtering feature allows you to set different filter on each | |
54 | probe and gives you what arguments will be shown in trace buffer. If an event | |
77b44d1b MH |
55 | name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event |
56 | under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', | |
57 | 'enabled', 'format' and 'filter'. | |
d8ec9185 MH |
58 | |
59 | enabled: | |
60 | You can enable/disable the probe by writing 1 or 0 on it. | |
61 | ||
62 | format: | |
eca0d916 | 63 | This shows the format of this probe event. |
d8ec9185 MH |
64 | |
65 | filter: | |
eca0d916 | 66 | You can write filtering rules of this event. |
d8ec9185 | 67 | |
e08d1c65 MH |
68 | id: |
69 | This shows the id of this probe event. | |
d8ec9185 | 70 | |
77b44d1b | 71 | |
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72 | Event Profiling |
73 | --------------- | |
74 | You can check the total number of probe hits and probe miss-hits via | |
75 | /sys/kernel/debug/tracing/kprobe_profile. | |
76 | The first column is event name, the second is the number of probe hits, | |
77 | the third is the number of probe miss-hits. | |
78 | ||
79 | ||
d8ec9185 MH |
80 | Usage examples |
81 | -------------- | |
82 | To add a probe as a new event, write a new definition to kprobe_events | |
83 | as below. | |
84 | ||
580d9e00 | 85 | echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events |
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86 | |
87 | This sets a kprobe on the top of do_sys_open() function with recording | |
14640106 MH |
88 | 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is |
89 | assigned to each function argument depends on arch-specific ABI. If you unsure | |
90 | the ABI, please try to use probe subcommand of perf-tools (you can find it | |
91 | under tools/perf/). | |
92 | As this example shows, users can choose more familiar names for each arguments. | |
d8ec9185 | 93 | |
580d9e00 | 94 | echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events |
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95 | |
96 | This sets a kretprobe on the return point of do_sys_open() function with | |
99329c44 | 97 | recording return value as "myretprobe" event. |
d8ec9185 MH |
98 | You can see the format of these events via |
99 | /sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. | |
100 | ||
101 | cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format | |
102 | name: myprobe | |
ec3a9039 | 103 | ID: 780 |
d8ec9185 | 104 | format: |
ec3a9039 MH |
105 | field:unsigned short common_type; offset:0; size:2; signed:0; |
106 | field:unsigned char common_flags; offset:2; size:1; signed:0; | |
107 | field:unsigned char common_preempt_count; offset:3; size:1;signed:0; | |
108 | field:int common_pid; offset:4; size:4; signed:1; | |
109 | field:int common_lock_depth; offset:8; size:4; signed:1; | |
110 | ||
111 | field:unsigned long __probe_ip; offset:12; size:4; signed:0; | |
112 | field:int __probe_nargs; offset:16; size:4; signed:1; | |
113 | field:unsigned long dfd; offset:20; size:4; signed:0; | |
114 | field:unsigned long filename; offset:24; size:4; signed:0; | |
115 | field:unsigned long flags; offset:28; size:4; signed:0; | |
116 | field:unsigned long mode; offset:32; size:4; signed:0; | |
117 | ||
118 | ||
119 | print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, | |
120 | REC->dfd, REC->filename, REC->flags, REC->mode | |
d8ec9185 | 121 | |
eca0d916 | 122 | You can see that the event has 4 arguments as in the expressions you specified. |
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123 | |
124 | echo > /sys/kernel/debug/tracing/kprobe_events | |
125 | ||
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126 | This clears all probe points. |
127 | ||
df3ab708 MK |
128 | Or, |
129 | ||
130 | echo -:myprobe >> kprobe_events | |
131 | ||
132 | This clears probe points selectively. | |
133 | ||
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134 | Right after definition, each event is disabled by default. For tracing these |
135 | events, you need to enable it. | |
136 | ||
137 | echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable | |
138 | echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable | |
139 | ||
140 | And you can see the traced information via /sys/kernel/debug/tracing/trace. | |
d8ec9185 MH |
141 | |
142 | cat /sys/kernel/debug/tracing/trace | |
143 | # tracer: nop | |
144 | # | |
145 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
146 | # | | | | | | |
6e9f23d1 | 147 | <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 |
2e06ff63 | 148 | <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe |
6e9f23d1 | 149 | <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 |
2e06ff63 | 150 | <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 |
6e9f23d1 | 151 | <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 |
2e06ff63 | 152 | <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 |
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153 | |
154 | ||
6e9f23d1 | 155 | Each line shows when the kernel hits an event, and <- SYMBOL means kernel |
d8ec9185 MH |
156 | returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel |
157 | returns from do_sys_open to sys_open+0x1b). | |
158 |