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+                         Kprobe-based Event Tracer
+                         =========================
+
+                 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).
+
+Unlike the function tracer, this tracer can probe instructions inside of
+kernel functions. It allows you to check which instruction has been executed.
+
+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.
+
+
+Synopsis of kprobe_events
+-------------------------
+  p[:EVENT] SYMBOL[+offs|-offs]|MEMADDR [FETCHARGS]     : Set a probe
+  r[:EVENT] SYMBOL[+0] [FETCHARGS]                      : Set a return probe
+
+ EVENT                  : Event name. If omitted, the event name is generated
+                          based on SYMBOL+offs or MEMADDR.
+ SYMBOL[+offs|-offs]    : Symbol+offset where the probe is inserted.
+ 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)
+  @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.(***)
+
+  (*) 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.
+
+
+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'.
+
+enabled:
+  You can enable/disable the probe by writing 1 or 0 on it.
+
+format:
+  It shows the format of this probe event. It also shows aliases of arguments
+ which you specified to kprobe_events.
+
+filter:
+  You can write filtering rules of this event. And you can use both of aliase
+ names and field names for describing filters.
+
+
+Event Profiling
+---------------
+ You can check the total number of probe hits and probe miss-hits via
+/sys/kernel/debug/tracing/kprobe_profile.
+ The first column is event name, the second is the number of probe hits,
+the third is the number of probe miss-hits.
+
+
+Usage examples
+--------------
+To add a probe as a new event, write a new definition to kprobe_events
+as below.
+
+  echo p:myprobe do_sys_open a0 a1 a2 a3 > /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.
+
+  echo r:myretprobe do_sys_open rv ra >> /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.
+ 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: 23
+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 long ip;        offset:16;tsize:8;
+        field: int nargs;       offset:24;tsize:4;
+        field: unsigned long arg0;      offset:32;tsize:8;
+        field: unsigned long arg1;      offset:40;tsize:8;
+        field: unsigned long arg2;      offset:48;tsize:8;
+        field: unsigned long arg3;      offset:56;tsize:8;
+
+        alias: a0;      original: arg0;
+        alias: a1;      original: arg1;
+        alias: a2;      original: arg2;
+        alias: a3;      original: arg3;
+
+print fmt: "%lx: 0x%lx 0x%lx 0x%lx 0x%lx", ip, arg0, arg1, arg2, arg3
+
+
+ You can see that the event has 4 arguments and alias expressions
+corresponding to it.
+
+  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.
+
+  cat /sys/kernel/debug/tracing/trace
+# tracer: nop
+#
+#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
+#              | |       |          |         |
+           <...>-1447  [001] 1038282.286875: do_sys_open+0x0/0xd6: 0x3 0x7fffd1ec4440 0x8000 0x0
+           <...>-1447  [001] 1038282.286878: sys_openat+0xc/0xe <- do_sys_open: 0xfffffffffffffffe 0xffffffff81367a3a
+           <...>-1447  [001] 1038282.286885: do_sys_open+0x0/0xd6: 0xffffff9c 0x40413c 0x8000 0x1b6
+           <...>-1447  [001] 1038282.286915: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
+           <...>-1447  [001] 1038282.286969: do_sys_open+0x0/0xd6: 0xffffff9c 0x4041c6 0x98800 0x10
+           <...>-1447  [001] 1038282.286976: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
+
+
+ Each line shows when the kernel hits a probe, 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).
+
+