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@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) > + $comm : Fetch current task comm. > + +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*4)(\*5) > + \IMM : Store an immediate value to the argument. > + NAME=FETCHARG : Set NAME as the argument name of FETCHARG. > + FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types > + (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types > + (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr" > + and bitfield are supported. > + > + (\*1) This is available only when BTF is enabled. > + (\*2) only for the probe on function entry (offs == 0). > + (\*3) only for return probe. > + (\*4) this is useful for fetching a field of data structures. > + (\*5) "u" means user-space dereference. > + > +For the details of TYPE, see :ref:`kprobetrace documentation <kprobetrace_types>`. > + > +BTF arguments > +------------- > +BTF (BPF Type Format) argument allows user to trace function and tracepoint > +parameters by its name instead of ``$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@xxxxxxxxx> -- An old man doll... just what I always wanted! - Clara
