Documentation: tracing/probes: Add fprobe event tracing document
Add a documentation about fprobe event tracing including tracepoint probe event and BTF argument. Link: https://lore.kernel.org/all/168507479345.913472.2804569685436422001.stgit@mhiramat.roam.corp.google.com/ Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
This commit is contained in:
Masami Hiramatsu (Google)
parent
4231f30fcc
commit
590e7b2804
|
|
@ -0,0 +1,188 @@
|
|||
.. SPDX-License-Identifier: GPL-2.0
|
||||
|
||||
==========================
|
||||
Fprobe-based Event Tracing
|
||||
==========================
|
||||
|
||||
.. Author: Masami Hiramatsu <mhiramat@kernel.org>
|
||||
|
||||
Overview
|
||||
--------
|
||||
|
||||
Fprobe event is similar to the kprobe event, but limited to probe on
|
||||
the function entry and exit only. It is good enough for many use cases
|
||||
which only traces some specific functions.
|
||||
|
||||
This document also covers tracepoint probe events (tprobe) since this
|
||||
is also works only on the tracepoint entry. User can trace a part of
|
||||
tracepoint argument, or the tracepoint without trace-event, which is
|
||||
not exposed on tracefs.
|
||||
|
||||
As same as other dynamic events, fprobe events and tracepoint probe
|
||||
events are defined via `dynamic_events` interface file on tracefs.
|
||||
|
||||
Synopsis of fprobe-events
|
||||
-------------------------
|
||||
::
|
||||
|
||||
f[:[GRP1/][EVENT1]] SYM [FETCHARGS] : Probe on function entry
|
||||
f[MAXACTIVE][:[GRP1/][EVENT1]] SYM%return [FETCHARGS] : Probe on function exit
|
||||
t[:[GRP2/][EVENT2]] TRACEPOINT [FETCHARGS] : Probe on tracepoint
|
||||
|
||||
GRP1 : Group name for fprobe. If omitted, use "fprobes" for it.
|
||||
GRP2 : Group name for tprobe. If omitted, use "tracepoints" for it.
|
||||
EVENT1 : Event name for fprobe. If omitted, the event name is
|
||||
"SYM__entry" or "SYM__exit".
|
||||
EVENT2 : Event name for tprobe. If omitted, the event name is
|
||||
the same as "TRACEPOINT", but if the "TRACEPOINT" starts
|
||||
with a digit character, "_TRACEPOINT" is used.
|
||||
MAXACTIVE : Maximum number of instances of the specified function that
|
||||
can be probed simultaneously, or 0 for the default value
|
||||
as defined in Documentation/trace/fprobes.rst
|
||||
|
||||
FETCHARGS : Arguments. Each probe can have up to 128 args.
|
||||
ARG : Fetch "ARG" function argument using BTF (only for function
|
||||
entry or tracepoint.) (\*1)
|
||||
@ADDR : Fetch memory at ADDR (ADDR should be in kernel)
|
||||
@SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
|
||||
$stackN : Fetch Nth entry of stack (N >= 0)
|
||||
$stack : Fetch stack address.
|
||||
$argN : Fetch the Nth function argument. (N >= 1) (\*2)
|
||||
$retval : Fetch return value.(\*3)
|
||||
$comm : Fetch current task comm.
|
||||
+|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*4)(\*5)
|
||||
\IMM : Store an immediate value to the argument.
|
||||
NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
|
||||
FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
|
||||
(u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
|
||||
(x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
|
||||
and bitfield are supported.
|
||||
|
||||
(\*1) This is available only when BTF is enabled.
|
||||
(\*2) only for the probe on function entry (offs == 0).
|
||||
(\*3) only for return probe.
|
||||
(\*4) this is useful for fetching a field of data structures.
|
||||
(\*5) "u" means user-space dereference.
|
||||
|
||||
For the details of TYPE, see :ref:`kprobetrace documentation <kprobetrace_types>`.
|
||||
|
||||
BTF arguments
|
||||
-------------
|
||||
BTF (BPF Type Format) argument allows user to trace function and tracepoint
|
||||
parameters by its name instead of ``$argN``. This feature is available if the
|
||||
kernel is configured with CONFIG_BPF_SYSCALL and CONFIG_DEBUG_INFO_BTF.
|
||||
If user only specify the BTF argument, the event's argument name is also
|
||||
automatically set by the given name. ::
|
||||
|
||||
# echo 'f:myprobe vfs_read count pos' >> dynamic_events
|
||||
# cat dynamic_events
|
||||
f:fprobes/myprobe vfs_read count=count pos=pos
|
||||
|
||||
It also chooses the fetch type from BTF information. For example, in the above
|
||||
example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, both
|
||||
are converted to 64bit unsigned long, but only ``pos`` has "%Lx" print-format as
|
||||
below ::
|
||||
|
||||
# cat events/fprobes/myprobe/format
|
||||
name: myprobe
|
||||
ID: 1313
|
||||
format:
|
||||
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:unsigned long __probe_ip; offset:8; size:8; signed:0;
|
||||
field:u64 count; offset:16; size:8; signed:0;
|
||||
field:u64 pos; offset:24; size:8; signed:0;
|
||||
|
||||
print fmt: "(%lx) count=%Lu pos=0x%Lx", REC->__probe_ip, REC->count, REC->pos
|
||||
|
||||
If user unsures the name of arguments, ``$arg*`` will be helpful. The ``$arg*``
|
||||
is expanded to all function arguments of the function or the tracepoint. ::
|
||||
|
||||
# echo 'f:myprobe vfs_read $arg*' >> dynamic_events
|
||||
# cat dynamic_events
|
||||
f:fprobes/myprobe vfs_read file=file buf=buf count=count pos=pos
|
||||
|
||||
BTF also affects the ``$retval``. If user doesn't set any type, the retval type is
|
||||
automatically picked from the BTF. If the function returns ``void``, ``$retval``
|
||||
is rejected.
|
||||
|
||||
Usage examples
|
||||
--------------
|
||||
Here is an example to add fprobe events on ``vfs_read()`` function entry
|
||||
and exit, with BTF arguments.
|
||||
::
|
||||
|
||||
# echo 'f vfs_read $arg*' >> dynamic_events
|
||||
# echo 'f vfs_read%return $retval' >> dynamic_events
|
||||
# cat dynamic_events
|
||||
f:fprobes/vfs_read__entry vfs_read file=file buf=buf count=count pos=pos
|
||||
f:fprobes/vfs_read__exit vfs_read%return arg1=$retval
|
||||
# echo 1 > events/fprobes/enable
|
||||
# head -n 20 trace | tail
|
||||
# TASK-PID CPU# ||||| TIMESTAMP FUNCTION
|
||||
# | | | ||||| | |
|
||||
sh-70 [000] ...1. 335.883195: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9a80 buf=0x7ffef36c6879 count=1 pos=0xffffc900005aff08
|
||||
sh-70 [000] ..... 335.883208: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=1
|
||||
sh-70 [000] ...1. 335.883220: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9a80 buf=0x7ffef36c6879 count=1 pos=0xffffc900005aff08
|
||||
sh-70 [000] ..... 335.883224: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=1
|
||||
sh-70 [000] ...1. 335.883232: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9a80 buf=0x7ffef36c687a count=1 pos=0xffffc900005aff08
|
||||
sh-70 [000] ..... 335.883237: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=1
|
||||
sh-70 [000] ...1. 336.050329: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9a80 buf=0x7ffef36c6879 count=1 pos=0xffffc900005aff08
|
||||
sh-70 [000] ..... 336.050343: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=1
|
||||
|
||||
You can see all function arguments and return values are recorded as signed int.
|
||||
|
||||
Also, here is an example of tracepoint events on ``sched_switch`` tracepoint.
|
||||
To compare the result, this also enables the ``sched_switch`` traceevent too.
|
||||
::
|
||||
|
||||
# echo 't sched_switch $arg*' >> dynamic_events
|
||||
# echo 1 > events/sched/sched_switch/enable
|
||||
# echo 1 > events/tracepoints/sched_switch/enable
|
||||
# echo > trace
|
||||
# head -n 20 trace | tail
|
||||
# TASK-PID CPU# ||||| TIMESTAMP FUNCTION
|
||||
# | | | ||||| | |
|
||||
sh-70 [000] d..2. 3912.083993: sched_switch: prev_comm=sh prev_pid=70 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
|
||||
sh-70 [000] d..3. 3912.083995: sched_switch: (__probestub_sched_switch+0x4/0x10) preempt=0 prev=0xffff88800664e100 next=0xffffffff828229c0 prev_state=1
|
||||
<idle>-0 [000] d..2. 3912.084183: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=16 next_prio=120
|
||||
<idle>-0 [000] d..3. 3912.084184: sched_switch: (__probestub_sched_switch+0x4/0x10) preempt=0 prev=0xffffffff828229c0 next=0xffff888004208000 prev_state=0
|
||||
rcu_preempt-16 [000] d..2. 3912.084196: sched_switch: prev_comm=rcu_preempt prev_pid=16 prev_prio=120 prev_state=I ==> next_comm=swapper/0 next_pid=0 next_prio=120
|
||||
rcu_preempt-16 [000] d..3. 3912.084196: sched_switch: (__probestub_sched_switch+0x4/0x10) preempt=0 prev=0xffff888004208000 next=0xffffffff828229c0 prev_state=1026
|
||||
<idle>-0 [000] d..2. 3912.085191: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=16 next_prio=120
|
||||
<idle>-0 [000] d..3. 3912.085191: sched_switch: (__probestub_sched_switch+0x4/0x10) preempt=0 prev=0xffffffff828229c0 next=0xffff888004208000 prev_state=0
|
||||
|
||||
As you can see, the ``sched_switch`` trace-event shows *cooked* parameters, on
|
||||
the other hand, the ``sched_switch`` tracepoint probe event shows *raw*
|
||||
parameters. This means you can access any field values in the task
|
||||
structure pointed by the ``prev`` and ``next`` arguments.
|
||||
|
||||
For example, usually ``task_struct::start_time`` is not traced, but with this
|
||||
traceprobe event, you can trace it as below.
|
||||
::
|
||||
|
||||
# echo 't sched_switch comm=+1896(next):string start_time=+1728(next):u64' > dynamic_events
|
||||
# head -n 20 trace | tail
|
||||
# TASK-PID CPU# ||||| TIMESTAMP FUNCTION
|
||||
# | | | ||||| | |
|
||||
sh-70 [000] d..3. 5606.686577: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="rcu_preempt" usage=1 start_time=245000000
|
||||
rcu_preempt-16 [000] d..3. 5606.686602: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="sh" usage=1 start_time=1596095526
|
||||
sh-70 [000] d..3. 5606.686637: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="swapper/0" usage=2 start_time=0
|
||||
<idle>-0 [000] d..3. 5606.687190: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="rcu_preempt" usage=1 start_time=245000000
|
||||
rcu_preempt-16 [000] d..3. 5606.687202: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="swapper/0" usage=2 start_time=0
|
||||
<idle>-0 [000] d..3. 5606.690317: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="kworker/0:1" usage=1 start_time=137000000
|
||||
kworker/0:1-14 [000] d..3. 5606.690339: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="swapper/0" usage=2 start_time=0
|
||||
<idle>-0 [000] d..3. 5606.692368: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="kworker/0:1" usage=1 start_time=137000000
|
||||
|
||||
Currently, to find the offset of a specific field in the data structure,
|
||||
you need to build kernel with debuginfo and run `perf probe` command with
|
||||
`-D` option. e.g.
|
||||
::
|
||||
|
||||
# perf probe -D "__probestub_sched_switch next->comm:string next->start_time"
|
||||
p:probe/__probestub_sched_switch __probestub_sched_switch+0 comm=+1896(%cx):string start_time=+1728(%cx):u64
|
||||
|
||||
And replace the ``%cx`` with the ``next``.
|
||||
|
|
@ -13,6 +13,7 @@ Linux Tracing Technologies
|
|||
kprobes
|
||||
kprobetrace
|
||||
uprobetracer
|
||||
fprobetrace
|
||||
tracepoints
|
||||
events
|
||||
events-kmem
|
||||
|
|
|
|||
|
|
@ -66,6 +66,8 @@ Synopsis of kprobe_events
|
|||
(\*3) this is useful for fetching a field of data structures.
|
||||
(\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
|
||||
|
||||
.. _kprobetrace_types:
|
||||
|
||||
Types
|
||||
-----
|
||||
Several types are supported for fetchargs. Kprobe tracer will access memory
|
||||
|
|
|
|||
Loading…
Reference in New Issue