From nobody Wed Dec 17 13:51:46 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C445FC4167B for ; Mon, 27 Nov 2023 14:18:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233239AbjK0OSY (ORCPT ); Mon, 27 Nov 2023 09:18:24 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34780 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233729AbjK0ORr (ORCPT ); Mon, 27 Nov 2023 09:17:47 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 812FF3850 for ; Mon, 27 Nov 2023 05:59:49 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id CD5DCC433A9; Mon, 27 Nov 2023 13:59:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1701093589; bh=8t7YVZmoVUR7dCk/2nFP4SmQbrkuMwJVZH30PnGw0sc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=tELN8aoX40aJ3lB0DKKykbLK1uUy9V1D77TNJxxYCctsNRWVqd4ZCfCPBYhEmziqX jOeVOFDjwIT+EhAtfa4M1NgVS6HyZ5WdhnI4APV3AC4QltCobFb3RcuUYL8vxxHo8g Yxy5A8KBGBs3lwyfQFRHcS5DQX5KuRQQK07WL9NbkSnnVATphZ1LXOShQRQBoBfz3u DcDf21AsHeMNJ95Ny8KeSlYk0vUq86dXxGoALbnlTTTWGPZHAJs/m2Nwei/1No74mm 9YOSV0/4zsemgvLdfBBJrLIZIajJxa0e7arsmg5240Zp4FmFlxtSAoVH03+uFWWtTG hMMAtrxpDYfDQ== From: "Masami Hiramatsu (Google)" To: Alexei Starovoitov , Steven Rostedt , Florent Revest Cc: linux-trace-kernel@vger.kernel.org, LKML , Martin KaFai Lau , bpf , Sven Schnelle , Alexei Starovoitov , Jiri Olsa , Arnaldo Carvalho de Melo , Daniel Borkmann , Alan Maguire , Mark Rutland , Peter Zijlstra , Thomas Gleixner , Guo Ren Subject: [PATCH v3 33/33] Documentation: probes: Update fprobe on function-graph tracer Date: Mon, 27 Nov 2023 22:59:42 +0900 Message-Id: <170109358189.343914.13273695712450878884.stgit@devnote2> X-Mailer: git-send-email 2.34.1 In-Reply-To: <170109317214.343914.4784420430328654397.stgit@devnote2> References: <170109317214.343914.4784420430328654397.stgit@devnote2> User-Agent: StGit/0.19 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Masami Hiramatsu (Google) Update fprobe documentation for the new fprobe on function-graph tracer. This includes some bahvior changes and pt_regs to ftrace_regs interface change. Signed-off-by: Masami Hiramatsu (Google) --- Changes in v2: - Update @fregs parameter explanation. --- Documentation/trace/fprobe.rst | 42 ++++++++++++++++++++++++++----------= ---- 1 file changed, 27 insertions(+), 15 deletions(-) diff --git a/Documentation/trace/fprobe.rst b/Documentation/trace/fprobe.rst index 196f52386aaa..f58bdc64504f 100644 --- a/Documentation/trace/fprobe.rst +++ b/Documentation/trace/fprobe.rst @@ -9,9 +9,10 @@ Fprobe - Function entry/exit probe Introduction =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =20 -Fprobe is a function entry/exit probe mechanism based on ftrace. -Instead of using ftrace full feature, if you only want to attach callbacks -on function entry and exit, similar to the kprobes and kretprobes, you can +Fprobe is a function entry/exit probe mechanism based on the function-graph +tracer. +Instead of tracing all functions, if you want to attach callbacks on speci= fic +function entry and exit, similar to the kprobes and kretprobes, you can use fprobe. Compared with kprobes and kretprobes, fprobe gives faster instrumentation for multiple functions with single handler. This document describes how to use fprobe. @@ -91,12 +92,14 @@ The prototype of the entry/exit callback function are a= s follows: =20 .. code-block:: c =20 - int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned lo= ng ret_ip, struct pt_regs *regs, void *entry_data); + int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned lo= ng ret_ip, struct ftrace_regs *fregs, void *entry_data); =20 - void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned lo= ng ret_ip, struct pt_regs *regs, void *entry_data); + void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned lo= ng ret_ip, struct ftrace_regs *fregs, void *entry_data); =20 -Note that the @entry_ip is saved at function entry and passed to exit hand= ler. -If the entry callback function returns !0, the corresponding exit callback= will be cancelled. +Note that the @entry_ip is saved at function entry and passed to exit +handler. +If the entry callback function returns !0, the corresponding exit callback +will be cancelled. =20 @fp This is the address of `fprobe` data structure related to this han= dler. @@ -112,12 +115,10 @@ If the entry callback function returns !0, the corres= ponding exit callback will This is the return address that the traced function will return to, somewhere in the caller. This can be used at both entry and exit. =20 -@regs - This is the `pt_regs` data structure at the entry and exit. Note t= hat - the instruction pointer of @regs may be different from the @entry_= ip - in the entry_handler. If you need traced instruction pointer, you = need - to use @entry_ip. On the other hand, in the exit_handler, the inst= ruction - pointer of @regs is set to the current return address. +@fregs + This is the `ftrace_regs` data structure at the entry and exit. Th= is + includes the function parameters, or the return values. So user can + access thos values via appropriate `ftrace_regs_*` APIs. =20 @entry_data This is a local storage to share the data between entry and exit h= andlers. @@ -125,6 +126,17 @@ If the entry callback function returns !0, the corresp= onding exit callback will and `entry_data_size` field when registering the fprobe, the stora= ge is allocated and passed to both `entry_handler` and `exit_handler`. =20 +Entry data size and exit handlers on the same function +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D + +Since the entry data is passed via per-task stack and it is has limited si= ze, +the entry data size per probe is limited to `15 * sizeof(long)`. You also = need +to take care that the different fprobes are probing on the same function, = this +limit becomes smaller. The entry data size is aligned to `sizeof(long)` and +each fprobe which has exit handler uses a `sizeof(long)` space on the stac= k, +you should keep the number of fprobes on the same function as small as +possible. + Share the callbacks with kprobes =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D =20 @@ -165,8 +177,8 @@ This counter counts up when; - fprobe fails to take ftrace_recursion lock. This usually means that a f= unction which is traced by other ftrace users is called from the entry_handler. =20 - - fprobe fails to setup the function exit because of the shortage of reth= ook - (the shadow stack for hooking the function return.) + - fprobe fails to setup the function exit because of failing to allocate = the + data buffer from the per-task shadow stack. =20 The `fprobe::nmissed` field counts up in both cases. Therefore, the former skips both of entry and exit callback and the latter skips the exit