On 17/07/2023 16:24, Masami Hiramatsu (Google) wrote: > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> > > Update fprobe event example with BTF data structure field specification. > > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> One suggestion below, but Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx> > --- > Changes in v2: > - Remove 'retval' and use '$retval'. > --- > Documentation/trace/fprobetrace.rst | 50 ++++++++++++++++++++++------------- > 1 file changed, 32 insertions(+), 18 deletions(-) > > diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst > index 7297f9478459..e9e764fadf14 100644 > --- a/Documentation/trace/fprobetrace.rst > +++ b/Documentation/trace/fprobetrace.rst > @@ -79,9 +79,9 @@ automatically set by the given name. :: > f:fprobes/myprobe vfs_read count=count pos=pos > > It also chooses the fetch type from BTF information. For example, in the above > -example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, both > -are converted to 64bit unsigned long, but only ``pos`` has "%Lx" print-format as > -below :: > +example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, > +both are converted to 64bit unsigned long, but only ``pos`` has "%Lx" > +print-format as below :: > > # cat events/fprobes/myprobe/format > name: myprobe > @@ -105,9 +105,33 @@ is expanded to all function arguments of the function or the tracepoint. :: > # cat dynamic_events > f:fprobes/myprobe vfs_read file=file buf=buf count=count pos=pos > > -BTF also affects the ``$retval``. If user doesn't set any type, the retval type is > -automatically picked from the BTF. If the function returns ``void``, ``$retval`` > -is rejected. > +BTF also affects the ``$retval``. If user doesn't set any type, the retval > +type is automatically picked from the BTF. If the function returns ``void``, > +``$retval`` is rejected. > + > +You can access the data fields of a data structure using allow operator ``->`` > +(for pointer type) and dot operator ``.`` (for data structure type.):: > + > +# echo 't sched_switch preempt prev_pid=prev->pid next_pid=next->pid' >> dynamic_events > + Could you describe what field access combinations are supported here; i.e. foo->bar[.baz]? > +This data field access is available for the return value via ``$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 > -------------- > @@ -161,10 +185,10 @@ 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. > +traceprobe event, you can trace that field as below. > :: > > - # echo 't sched_switch comm=+1896(next):string start_time=+1728(next):u64' > dynamic_events > + # echo 't sched_switch comm=next->comm:string next->start_time' > dynamic_events > # head -n 20 trace | tail > # TASK-PID CPU# ||||| TIMESTAMP FUNCTION > # | | | ||||| | | > @@ -176,13 +200,3 @@ traceprobe event, you can trace it as below. > <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``. > >
