- Kprobe-based Event Tracer
- =========================
+ Kprobe-based Event Tracing
+ ==========================
Documentation is written by Masami Hiramatsu
Overview
--------
-This tracer is similar to the events tracer which is based on Tracepoint
-infrastructure. Instead of Tracepoint, this tracer is based on kprobes(kprobe
-and kretprobe). It probes anywhere where kprobes can probe(this means, all
-functions body except for __kprobes functions).
+These events are similar to tracepoint based events. Instead of Tracepoint,
+this is based on kprobes (kprobe and kretprobe). So it can probe wherever
+kprobes can probe (this means, all functions body except for __kprobes
+functions). Unlike the Tracepoint based event, this can be added and removed
+dynamically, on the fly.
-Unlike the function tracer, this tracer can probe instructions inside of
-kernel functions. It allows you to check which instruction has been executed.
+To enable this feature, build your kernel with CONFIG_KPROBE_TRACING=y.
-Unlike the Tracepoint based events tracer, this tracer can add and remove
-probe points on the fly.
-
-Similar to the events tracer, this tracer doesn't need to be activated via
-current_tracer, instead of that, just set probe points via
-/sys/kernel/debug/tracing/kprobe_events. And you can set filters on each
-probe events via /sys/kernel/debug/tracing/events/kprobes/<EVENT>/filter.
+Similar to the events tracer, this doesn't need to be activated via
+current_tracer. Instead of that, add probe points via
+/sys/kernel/debug/tracing/kprobe_events, and enable it via
+/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled.
Synopsis of kprobe_events
-------------------------
p[:[GRP/]EVENT] SYMBOL[+offs]|MEMADDR [FETCHARGS] : Set a probe
r[:[GRP/]EVENT] SYMBOL[+0] [FETCHARGS] : Set a return probe
+ -:[GRP/]EVENT : Clear a probe
GRP : Group name. If omitted, use "kprobes" for it.
EVENT : Event name. If omitted, the event name is generated
MEMADDR : Address where the probe is inserted.
FETCHARGS : Arguments. Each probe can have up to 128 args.
- %REG : Fetch register REG
- sN : Fetch Nth entry of stack (N >= 0)
- sa : Fetch stack address.
- @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
+ %REG : Fetch register REG
+ @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
@SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
- aN : Fetch function argument. (N >= 0)(*)
- rv : Fetch return value.(**)
- ra : Fetch return address.(**)
- +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(***)
+ $stackN : Fetch Nth entry of stack (N >= 0)
+ $stack : Fetch stack address.
+ $retval : Fetch return value.(*)
+ +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
NAME=FETCHARG: Set NAME as the argument name of FETCHARG.
- (*) aN may not correct on asmlinkaged functions and at the middle of
- function body.
- (**) only for return probe.
- (***) this is useful for fetching a field of data structures.
+ (*) only for return probe.
+ (**) this is useful for fetching a field of data structures.
Per-Probe Event Filtering
-------------------------
Per-probe event filtering feature allows you to set different filter on each
probe and gives you what arguments will be shown in trace buffer. If an event
-name is specified right after 'p:' or 'r:' in kprobe_events, the tracer adds
-an event under tracing/events/kprobes/<EVENT>, at the directory you can see
-'id', 'enabled', 'format' and 'filter'.
+name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
+under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
+'enabled', 'format' and 'filter'.
enabled:
You can enable/disable the probe by writing 1 or 0 on it.
id:
This shows the id of this probe event.
+
Event Profiling
---------------
You can check the total number of probe hits and probe miss-hits via
To add a probe as a new event, write a new definition to kprobe_events
as below.
- echo p:myprobe do_sys_open dfd=a0 filename=a1 flags=a2 mode=a3 > /sys/kernel/debug/tracing/kprobe_events
+ echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
This sets a kprobe on the top of do_sys_open() function with recording
-1st to 4th arguments as "myprobe" event. As this example shows, users can
-choose more familiar names for each arguments.
+1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
+assigned to each function argument depends on arch-specific ABI. If you unsure
+the ABI, please try to use probe subcommand of perf-tools (you can find it
+under tools/perf/).
+As this example shows, users can choose more familiar names for each arguments.
- echo r:myretprobe do_sys_open rv ra >> /sys/kernel/debug/tracing/kprobe_events
+ echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
This sets a kretprobe on the return point of do_sys_open() function with
-recording return value and return address as "myretprobe" event.
+recording return value as "myretprobe" event.
You can see the format of these events via
/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
name: myprobe
-ID: 75
+ID: 780
format:
- field:unsigned short common_type; offset:0; size:2;
- field:unsigned char common_flags; offset:2; size:1;
- field:unsigned char common_preempt_count; offset:3; size:1;
- field:int common_pid; offset:4; size:4;
- field:int common_tgid; offset:8; size:4;
+ field:unsigned short common_type; offset:0; size:2; signed:0;
+ field:unsigned char common_flags; offset:2; size:1; signed:0;
+ field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
+ field:int common_pid; offset:4; size:4; signed:1;
+ field:int common_lock_depth; offset:8; size:4; signed:1;
- field: unsigned long ip; offset:16;tsize:8;
- field: int nargs; offset:24;tsize:4;
- field: unsigned long dfd; offset:32;tsize:8;
- field: unsigned long filename; offset:40;tsize:8;
- field: unsigned long flags; offset:48;tsize:8;
- field: unsigned long mode; offset:56;tsize:8;
+ field:unsigned long __probe_ip; offset:12; size:4; signed:0;
+ field:int __probe_nargs; offset:16; size:4; signed:1;
+ field:unsigned long dfd; offset:20; size:4; signed:0;
+ field:unsigned long filename; offset:24; size:4; signed:0;
+ field:unsigned long flags; offset:28; size:4; signed:0;
+ field:unsigned long mode; offset:32; size:4; signed:0;
-print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->ip, REC->dfd, REC->filename, REC->flags, REC->mode
+print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
+REC->dfd, REC->filename, REC->flags, REC->mode
You can see that the event has 4 arguments as in the expressions you specified.
echo > /sys/kernel/debug/tracing/kprobe_events
- This clears all probe points. and you can see the traced information via
-/sys/kernel/debug/tracing/trace.
+ This clears all probe points.
+
+ Or,
+
+ echo -:myprobe >> kprobe_events
+
+ This clears probe points selectively.
+
+ Right after definition, each event is disabled by default. For tracing these
+events, you need to enable it.
+
+ echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
+ echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
+
+ And you can see the traced information via /sys/kernel/debug/tracing/trace.
cat /sys/kernel/debug/tracing/trace
# tracer: nop
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
<...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
- <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) rv=fffffffffffffffe ra=ffffffff81367a3a
+ <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
<...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
- <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) rv=3 ra=ffffffff81367a3a
+ <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
<...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
- <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) rv=3 ra=ffffffff81367a3a
+ <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
Each line shows when the kernel hits an event, and <- SYMBOL means kernel
returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
returns from do_sys_open to sys_open+0x1b).
-
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff