[PATCH v6 10/10] Documentation: tracing/probes: Add fprobe event tracing document

From: Masami Hiramatsu (Google)
Date: Mon Apr 24 2023 - 10:48:50 EST

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From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Add a documentation about fprobe event tracing including
tracepoint probe event and BTF argument.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
Documentation/trace/fprobetrace.rst | 184 +++++++++++++++++++++++++++++++++++
Documentation/trace/index.rst | 1
2 files changed, 185 insertions(+)
create mode 100644 Documentation/trace/fprobetrace.rst

diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
new file mode 100644
index 000000000000..e3304d24078f
--- /dev/null
+++ b/Documentation/trace/fprobetrace.rst
@@ -0,0 +1,184 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+Fprobe-based Event Tracing
+==========================
+
+.. Author: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
+
+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)
+ $comm : Fetch current task comm.
+ +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*4)(\*5)
+ \IMM : Store an immediate value to the argument.
+ NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
+ FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
+ (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
+ (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
+ and bitfield are supported.
+
+ (\*1) This is available only when BTF is enabled.
+ (\*2) only for the probe on function entry (offs == 0).
+ (\*3) only for return probe.
+ (\*4) this is useful for fetching a field of data structures.
+ (\*5) "u" means user-space dereference.
+
+For the details of TYPE, see :file:`Documentation/trace/kprobetrace.rst`.
+
+BTF arguments
+-------------
+BTF (BPF Type Format) argument allows user to trace function and tracepoint
+parameters by its name instead of `$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 ::
+
+ # 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, `$$args` will be helpful. The `$$args`
+is expanded to all function arguments of the function or the tracepoint. ::
+
+ # echo 'f:myprobe vfs_read $$args' >> dynamic_events
+ # cat dynamic_events
+ f:fprobes/myprobe vfs_read file=file buf=buf count=count pos=pos
+
+
+Usage examples
+--------------
+Here is an example to add fprobe events on `vfs_read()` function entry
+and exit, with BTF arguments.
+::
+
+ # echo 'f vfs_read $$args' >> 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. 3627.602898: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9e00 buf=0x7ffe3c8bd429 count=1 pos=0xffffc900005aff08
+ sh-70 [000] ..... 3627.602910: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=0x1
+ sh-70 [000] ...1. 3627.727129: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9e00 buf=0x7ffe3c8bd429 count=1 pos=0xffffc900005aff08
+ sh-70 [000] ..... 3627.727175: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=0x1
+ sh-70 [000] ...1. 3627.816332: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9e00 buf=0x7ffe3c8bd429 count=1 pos=0xffffc900005aff08
+ sh-70 [000] ..... 3627.816344: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=0x1
+ sh-70 [000] ...1. 3627.976995: vfs_read__entry: (vfs_read+0x4/0x340) file=0xffff888005cf9e00 buf=0x7ffe3c8bd429 count=1 pos=0xffffc900005aff08
+ sh-70 [000] ..... 3627.977009: vfs_read__exit: (ksys_read+0x75/0x100 <- vfs_read) arg1=0x1
+
+You can see all function arguments and return values are recorded.
+
+Also, here is tracepoint events on `sched_switch` tracepoint. To compare the
+result, this also enables the `sched_switch` traceevent too.
+::
+
+ # echo 't sched_switch $$args' >> 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 dereference 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`.
diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index ea25a9220f92..5092d6c13af5 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -13,6 +13,7 @@ Linux Tracing Technologies
kprobes
kprobetrace
uprobetracer
+ fprobetrace
tracepoints
events
events-kmem

Next message: Nishanth Menon: "[PATCH 5/7] arm64: dts: ti: k3-am65-main: Drop deprecated ti,otap-del-sel property"
Previous message: Michal Hocko: "Re: [PATCH v3 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page()"
In reply to: Masami Hiramatsu (Google): "[PATCH v6 07/10] tracing/probes: Add $$args meta argument for all function args"
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