From patchwork Tue Aug 22 16:27:03 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Masami Hiramatsu (Google)" X-Patchwork-Id: 13361199 X-Patchwork-Delegate: bpf@iogearbox.net Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 134811D2E6 for ; Tue, 22 Aug 2023 16:27:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9D5C4C433C7; Tue, 22 Aug 2023 16:27:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1692721627; bh=uNEY/3JK2xrPukpXEkaIFZvYi1IrWnjA6w4voZ56Quo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=V7C99ifSdSZsU+mw7kmns/YqzdfjCfyT7lRA13nbXKCKTnx183KWApKLkds/cVWLz Hzu0y8tqZp3z+kuYGLUuCCgAW4uHHWRmQUfY4z9HuiAQ33Ubm2QCR+CU2PJoqLXYvn YjCtxaMq21QlYyBlmYqDv/CMEXgeea6/NAQx2v1EHnnt5pxuJQXs+MSafE1HahQvH7 wzRCUb1Z4JyYQDg7W7utlsbPD0KHN3Vn50xELL8gBBLSOS8oZw70HRnr14Gu7fGli2 vwmMmWGMRzxekcIN++z7dE07bF77I/QihL6+VwxsCsTWxfpZIDQXNzHKCzH3LKJMiG sjWRooUf/8ytg== From: "Masami Hiramatsu (Google)" To: linux-trace-kernel@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Steven Rostedt , mhiramat@kernel.org, Martin KaFai Lau , bpf@vger.kernel.org, Sven Schnelle , Alexei Starovoitov Subject: [PATCH v6 9/9] Documentation: tracing: Update fprobe event example with BTF field Date: Wed, 23 Aug 2023 01:27:03 +0900 Message-Id: <169272162317.160970.12282501645965083241.stgit@devnote2> X-Mailer: git-send-email 2.34.1 In-Reply-To: <169272153143.160970.15584603734373446082.stgit@devnote2> References: <169272153143.160970.15584603734373446082.stgit@devnote2> User-Agent: StGit/0.19 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Masami Hiramatsu (Google) Update fprobe event example with BTF data structure field specification. Signed-off-by: Masami Hiramatsu (Google) Reviewed-by: Alan Maguire Acked-by: Steven Rostedt (Google) --- Changes in v2: - Remove 'retval' and use '$retval'. Changes in v3: - Add description about mixture of '.' and '->' usage. --- Documentation/trace/fprobetrace.rst | 64 +++++++++++++++++++++++++---------- 1 file changed, 46 insertions(+), 18 deletions(-) diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst index 7297f9478459..8e9bebcf0a2e 100644 --- a/Documentation/trace/fprobetrace.rst +++ b/Documentation/trace/fprobetrace.rst @@ -79,9 +79,9 @@ automatically set by the given name. :: f:fprobes/myprobe vfs_read count=count pos=pos It also chooses the fetch type from BTF information. For example, in the above -example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, both -are converted to 64bit unsigned long, but only ``pos`` has "%Lx" print-format as -below :: +example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, +both are converted to 64bit unsigned long, but only ``pos`` has "%Lx" +print-format as below :: # cat events/fprobes/myprobe/format name: myprobe @@ -105,9 +105,47 @@ is expanded to all function arguments of the function or the tracepoint. :: # cat dynamic_events f:fprobes/myprobe vfs_read file=file buf=buf count=count pos=pos -BTF also affects the ``$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``, +``$retval`` is rejected. + +You can access the data fields of a data structure using allow operator ``->`` +(for pointer type) and dot operator ``.`` (for data structure type.):: + +# echo 't sched_switch preempt prev_pid=prev->pid next_pid=next->pid' >> dynamic_events + +The field access operators, ``->`` and ``.`` can be combined for accessing deeper +members and other structure members pointed by the member. e.g. ``foo->bar.baz->qux`` +If there is non-name union member, you can directly access it as the C code does. +For example:: + + struct { + union { + int a; + int b; + }; + } *foo; + +To access ``a`` and ``b``, use ``foo->a`` and ``foo->b`` in this case. + +This data field access is available for the return value via ``$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 -------------- @@ -161,10 +199,10 @@ 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 +214,3 @@ traceprobe event, you can trace it as below. -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 -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``.