mirror of
https://github.com/torvalds/linux.git
synced 2024-11-25 21:51:40 +00:00
dd29dfe78b
Fix spelling mistakes in the documentation. Signed-off-by: Saurav Shah <sauravshah.31@gmail.com> Link: https://lore.kernel.org/r/20240501233659.25441-1-sauravshah.31@gmail.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
252 lines
14 KiB
ReStructuredText
252 lines
14 KiB
ReStructuredText
.. 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/fprobe.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). Note, this argument access
|
|
is best effort, because depending on the argument type, it may be passed on
|
|
the stack. But this only support the arguments via registers.
|
|
(\*3) only for return probe. Note that this is also best effort. Depending on the
|
|
return value type, it might be passed via a pair of registers. But this only
|
|
accesses one register.
|
|
(\*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>`.
|
|
|
|
Function arguments at exit
|
|
--------------------------
|
|
Function arguments can be accessed at exit probe using $arg<N> fetcharg. This
|
|
is useful to record the function parameter and return value at once, and
|
|
trace the difference of structure fields (for debugging a function whether it
|
|
correctly updates the given data structure or not)
|
|
See the :ref:`sample<fprobetrace_exit_args_sample>` below for how it works.
|
|
|
|
BTF arguments
|
|
-------------
|
|
BTF (BPF Type Format) argument allows user to trace function and tracepoint
|
|
parameters by its name instead of ``$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``,
|
|
``$retval`` is rejected.
|
|
|
|
You can access the data fields of a data structure using allow operator ``->``
|
|
(for pointer type) and dot operator ``.`` (for data structure type.)::
|
|
|
|
# echo 't sched_switch preempt prev_pid=prev->pid next_pid=next->pid' >> dynamic_events
|
|
|
|
The field access operators, ``->`` and ``.`` can be combined for accessing deeper
|
|
members and other structure members pointed by the member. e.g. ``foo->bar.baz->qux``
|
|
If there is non-name union member, you can directly access it as the C code does.
|
|
For example::
|
|
|
|
struct {
|
|
union {
|
|
int a;
|
|
int b;
|
|
};
|
|
} *foo;
|
|
|
|
To access ``a`` and ``b``, use ``foo->a`` and ``foo->b`` in this case.
|
|
|
|
This data field access is available for the return value via ``$retval``,
|
|
e.g. ``$retval->name``.
|
|
|
|
For these BTF arguments and fields, ``:string`` and ``:ustring`` change the
|
|
behavior. If these are used for BTF argument or field, it checks whether
|
|
the BTF type of the argument or the data field is ``char *`` or ``char []``,
|
|
or not. If not, it rejects applying the string types. Also, with the BTF
|
|
support, you don't need a memory dereference operator (``+0(PTR)``) for
|
|
accessing the string pointed by a ``PTR``. It automatically adds the memory
|
|
dereference operator according to the BTF type. e.g. ::
|
|
|
|
# echo 't sched_switch prev->comm:string' >> dynamic_events
|
|
# echo 'f getname_flags%return $retval->name:string' >> dynamic_events
|
|
|
|
The ``prev->comm`` is an embedded char array in the data structure, and
|
|
``$retval->name`` is a char pointer in the data structure. But in both
|
|
cases, you can use ``:string`` type to get the string.
|
|
|
|
|
|
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 that field as below.
|
|
::
|
|
|
|
# echo 't sched_switch comm=next->comm:string next->start_time' > 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
|
|
|
|
.. _fprobetrace_exit_args_sample:
|
|
|
|
The return probe allows us to access the results of some functions, which returns
|
|
the error code and its results are passed via function parameter, such as an
|
|
structure-initialization function.
|
|
|
|
For example, vfs_open() will link the file structure to the inode and update
|
|
mode. You can trace that changes with return probe.
|
|
::
|
|
|
|
# echo 'f vfs_open mode=file->f_mode:x32 inode=file->f_inode:x64' >> dynamic_events
|
|
# echo 'f vfs_open%%return mode=file->f_mode:x32 inode=file->f_inode:x64' >> dynamic_events
|
|
# echo 1 > events/fprobes/enable
|
|
# cat trace
|
|
sh-131 [006] ...1. 1945.714346: vfs_open__entry: (vfs_open+0x4/0x40) mode=0x2 inode=0x0
|
|
sh-131 [006] ...1. 1945.714358: vfs_open__exit: (do_open+0x274/0x3d0 <- vfs_open) mode=0x4d801e inode=0xffff888008470168
|
|
cat-143 [007] ...1. 1945.717949: vfs_open__entry: (vfs_open+0x4/0x40) mode=0x1 inode=0x0
|
|
cat-143 [007] ...1. 1945.717956: vfs_open__exit: (do_open+0x274/0x3d0 <- vfs_open) mode=0x4a801d inode=0xffff888005f78d28
|
|
cat-143 [007] ...1. 1945.720616: vfs_open__entry: (vfs_open+0x4/0x40) mode=0x1 inode=0x0
|
|
cat-143 [007] ...1. 1945.728263: vfs_open__exit: (do_open+0x274/0x3d0 <- vfs_open) mode=0xa800d inode=0xffff888004ada8d8
|
|
|
|
You can see the `file::f_mode` and `file::f_inode` are updated in `vfs_open()`.
|