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Date: Sun, 21 May 2023 16:51:48 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Bagas Sanjaya <bagasdotme@...il.com>
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>,
Florent Revest <revest@...omium.org>,
Mark Rutland <mark.rutland@....com>,
Will Deacon <will@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH v12 11/11] Documentation: tracing/probes: Add fprobe
event tracing document
On Fri, 19 May 2023 16:00:49 +0700
Bagas Sanjaya <bagasdotme@...il.com> wrote:
> On 5/18/23 12:26, Masami Hiramatsu (Google) wrote:
> > diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
> > new file mode 100644
> > index 000000000000..e949bc0cff05
> > --- /dev/null
> > +++ b/Documentation/trace/fprobetrace.rst
> > @@ -0,0 +1,188 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +==========================
> > +Fprobe-based Event Tracing
> > +==========================
> > +
> > +.. Author: Masami Hiramatsu <mhiramat@...nel.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``.
>
> The doc LGTM, thanks!
>
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
>
Thanks Bagas!
> --
> An old man doll... just what I always wanted! - Clara
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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