From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f174.google.com (mail-pf1-f174.google.com [209.85.210.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DFC0D2EFDB4 for ; Sun, 7 Dec 2025 14:27:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765117677; cv=none; b=kcTmuPHO+yExT1A5T13hQ+6mGTGVNqiA+0fQz+JWKVQWaSZ9EbK6KECD/5OsZEC2bMwbXaY5/UrpVlGh41yWEoV73q/xNyllZc2+TwMH502rbnABdc4rnM59QWZQn4jmszBJViiHcfRnLXbZ+eQaeYmh0iG8CSe6jmrN0q48Oq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765117677; c=relaxed/simple; bh=Kwz/i/ToDupsWxXvYUeC2N41dqM/je07XCbIeJOAi3k=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=BU0oXf3f90GDZlehMba+wUfS+SsSBvwD0O2AcHD5f3oI9JW42/MUU3gM4gKmQwpOhaS+WVMgYwFMGhoLBsv0l8162S1ighgqgi9sUYxeab00E/Syw/YKsfl0FKGHr8MHiY8QSDBLwf2uRqCgLROEPx2MqiBxPVPIIHJ6Yc9HjrM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=SXaGq34C; arc=none smtp.client-ip=209.85.210.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="SXaGq34C" Received: by mail-pf1-f174.google.com with SMTP id d2e1a72fcca58-7aab7623f42so4250443b3a.2 for ; Sun, 07 Dec 2025 06:27:54 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1765117674; x=1765722474; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=Mim4L7D6RiJD0Xfk3ctgRLF/8EuUIkjvDV4bArvMP3Q=; b=SXaGq34CXlpeoNBDzbUGPiHuL3CU04HqnpfziHqQ/QjOcq7/gvHhefaZl3ZDVI7Epj tXDs2cqvmjv3wKSLum04JL0DJIIaVX9GGBtFrACstU0Xi3q/WsoYGGw8dCwfjhacePJt IqvJGleKv+1e4l5BU+wpyx1xgE/KU41BvlSqWmTetvgmw8vtcty4qSWjejJ0cRtNA7np W02iVs3p1ZNPKzMXOVw94rqrCyy/5DWWtHgO8Apwvw1CW/ztsPxWS+IItl0L7sM0NSJf AIMWKlRMlirZrOD/S3+HSL2VztYH3S8KIoEYLq7aHgID9KEM+JYzY50R6fxNE2Tv+Lto sjAg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1765117674; x=1765722474; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=Mim4L7D6RiJD0Xfk3ctgRLF/8EuUIkjvDV4bArvMP3Q=; b=YH8QF4v2r1WAOwltk/YJUyYHdjahaEOcJqI8MNcq8VYjUxQcyH7qrufVQfd/YcPtMl WwlqMS/ZbVaFVMcQk5yNrOnND0CBOmD8uNg9GzJIYbejSRKNbSQB8xScsvYERFzZAbdP eY1FnQyP9wpyjCE3vFzhM6ttPOPG5/2Qmo9mq4Ywbi0iXHvtK82lJf9TYs3E+sJHqYf5 QFffCDo9koefqChTswg5/irA+B62DFkarlQ3rk1cbl3hBMDohYA8gHi430Qkf/5VxVej pN77qVxinePpiprVpyfsMiZYSCHkpbmvqb+xYOkxppPRraCW/RTpkSP21ATfAHBvfE3R XSTQ== X-Gm-Message-State: AOJu0YwmbkAEMurSXK8E3CnhVaWTcz7/08BTRXQKUyZKhGQkvYxLJ0/2 nF+e4g7B1vQ6AnDrwHrwaTvajCDcMnFAwZt1cOFQc5ffz582Ue5KGOZg X-Gm-Gg: ASbGncvAka/ZCWydVpo+fSlINaN1NTDIaUv7l8jRgEMolaa0fbtvHIAmOceottW1GG3 oImmo2SeqTDXqPBR/hShrvxqfZxdvKxjP517nzNGrmh9eLF82zD8vjCj94HxzzwonLGdIkkq/I9 1qI0pl9YJGQ4vyfWGbmj6PcCIBigTuhQbdGaASk1buuPwoZk7iO2rV6Gh2YDgnjk8Q4BHHCYTTW +zskgTcyl+uePP1tc2OOLfxqqRKirI158Wi1wU8TwY9fSvqrFSib465LCluVIoksGrpVpK+7Uil chm/tMyFU2RwAC8dqbZAYOGVGUpe+/z2aAK7lwydvrQQrEwLjgeQYf1ScohhPiSPEwGJYSr0h/N rnsFSyhp0Mqj3LzpHy7nYmxezxtQab05b/+pd5NdnZknh+dEA71asOQk4lR+YDd4mKeUzRj84gZ u2ltQq1Q5YCZIYhjjNBuXbz+MEfJI= X-Google-Smtp-Source: AGHT+IEVUhB8KEX4xXFxWmTBAiOJNAYgHsDvlU9jrkKUE/cQ/kDX+/4BFflD9NpwiP62P5/AXuU/kQ== X-Received: by 2002:a05:6a20:3d87:b0:334:8d1f:fa8d with SMTP id adf61e73a8af0-36617e74cfemr4778913637.18.1765117673713; Sun, 07 Dec 2025 06:27:53 -0800 (PST) Received: from pengdl-pc.mioffice.cn ([43.224.245.249]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-bf686c32deesm9514792a12.13.2025.12.07.06.27.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 07 Dec 2025 06:27:52 -0800 (PST) From: Donglin Peng To: rostedt@goodmis.org Cc: linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, pengdonglin , Masami Hiramatsu , Xiaoqin Zhang Subject: [PATCH v1 2/2] tracing: Update funcgraph-retval documentation Date: Sun, 7 Dec 2025 22:27:42 +0800 Message-Id: <20251207142742.229924-3-dolinux.peng@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251207142742.229924-1-dolinux.peng@gmail.com> References: <20251207142742.229924-1-dolinux.peng@gmail.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: pengdonglin The existing documentation for funcgraph-retval is outdated and partially incorrect, as it describes limitations that have now been resolved. Recent changes (e.g., using BTF to obtain function return types) have addressed key issues: 1. Return values are now printed only for non-void functions. 2. Values are trimmed to the correct width of the return type, avoiding garbage data from high bits. Cc: Steven Rostedt (Google) Cc: Masami Hiramatsu Cc: Xiaoqin Zhang Signed-off-by: pengdonglin --- Documentation/trace/ftrace.rst | 74 ++++++++++++++++++---------------- 1 file changed, 40 insertions(+), 34 deletions(-) diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst index d1f313a5f4ad..de97b65433cc 100644 --- a/Documentation/trace/ftrace.rst +++ b/Documentation/trace/ftrace.rst @@ -1454,6 +1454,10 @@ Options for function_graph tracer: printed in hexadecimal format. By default, this option is off. + funcgraph-retaddr + When set, the return address will always be printed. + By default, this option is off. + sleep-time When running function graph tracer, to include the time a task schedules out in its function. @@ -2800,7 +2804,7 @@ It is default disabled. 0) 2.861 us | } /* putname() */ The return value of each traced function can be displayed after -an equal sign "=". When encountering system call failures, it +an equal sign "ret =". When encountering system call failures, it can be very helpful to quickly locate the function that first returns an error code. @@ -2810,16 +2814,16 @@ returns an error code. Example with funcgraph-retval:: 1) | cgroup_migrate() { - 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ + 1) 0.651 us | cgroup_migrate_add_task(); /* ret=0xffff93fcfd346c00 */ 1) | cgroup_migrate_execute() { 1) | cpu_cgroup_can_attach() { 1) | cgroup_taskset_first() { - 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ - 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ - 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ - 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */ - 1) 4.369 us | } /* cgroup_migrate_execute = -22 */ - 1) 7.143 us | } /* cgroup_migrate = -22 */ + 1) 0.732 us | cgroup_taskset_next(); /* ret=0xffff93fc8fb20000 */ + 1) 1.232 us | } /* cgroup_taskset_first ret=0xffff93fc8fb20000 */ + 1) 0.380 us | sched_rt_can_attach(); /* ret=0x0 */ + 1) 2.335 us | } /* cpu_cgroup_can_attach ret=-22 */ + 1) 4.369 us | } /* cgroup_migrate_execute ret=-22 */ + 1) 7.143 us | } /* cgroup_migrate ret=-22 */ The above example shows that the function cpu_cgroup_can_attach returned the error code -22 firstly, then we can read the code @@ -2836,37 +2840,39 @@ printed in hexadecimal format. Example with funcgraph-retval-hex:: 1) | cgroup_migrate() { - 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ + 1) 0.651 us | cgroup_migrate_add_task(); /* ret=0xffff93fcfd346c00 */ 1) | cgroup_migrate_execute() { 1) | cpu_cgroup_can_attach() { 1) | cgroup_taskset_first() { - 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */ - 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */ - 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */ - 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */ - 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */ + 1) 0.732 us | cgroup_taskset_next(); /* ret=0xffff93fc8fb20000 */ + 1) 1.232 us | } /* cgroup_taskset_first ret=0xffff93fc8fb20000 */ + 1) 0.380 us | sched_rt_can_attach(); /* ret=0x0 */ + 1) 2.335 us | } /* cpu_cgroup_can_attach ret=0xffffffea */ + 1) 4.369 us | } /* cgroup_migrate_execute ret=0xffffffea */ 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */ -At present, there are some limitations when using the funcgraph-retval -option, and these limitations will be eliminated in the future: - -- Even if the function return type is void, a return value will still - be printed, and you can just ignore it. - -- Even if return values are stored in multiple registers, only the - value contained in the first register will be recorded and printed. - To illustrate, in the x86 architecture, eax and edx are used to store - a 64-bit return value, with the lower 32 bits saved in eax and the - upper 32 bits saved in edx. However, only the value stored in eax - will be recorded and printed. - -- In certain procedure call standards, such as arm64's AAPCS64, when a - type is smaller than a GPR, it is the responsibility of the consumer - to perform the narrowing, and the upper bits may contain UNKNOWN values. - Therefore, it is advisable to check the code for such cases. For instance, - when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values, - especially when larger types are truncated, whether explicitly or implicitly. - Here are some specific cases to illustrate this point: +Note that there are some limitations when using the funcgraph-retval +option: + +- If CONFIG_DEBUG_INFO_BTF is disabled (n), a return value is printed even for + functions with a void return type. When CONFIG_DEBUG_INFO_BTF is enabled (y), + the return value is printed only for non-void functions. + +- If a return value occupies multiple registers, only the value in the first + register is recorded and printed. For example, on the x86 architecture, a + 64-bit return value is stored across eax (lower 32 bits) and edx (upper 32 bits), + but only the contents of eax are captured. + +- Under certain procedure-call standards (e.g., arm64's AAPCS64), when the return + type is smaller than a general-purpose register (GPR), the caller is responsible + for narrowing the value; the upper bits of the register may contain undefined data. + For instance, when a u8 is returned in 64-bit GPR, bits [63:8] can hold arbitrary + values, especially when larger types are truncated (explicitly or implicitly). It + is therefore advisable to inspect the code in such cases. If CONFIG_DEBUG_INFO_BTF + is enabled (y), the return value is automatically trimmed to the width of the return + type. + + The following examples illustrate the behavior: **Case One**: -- 2.34.1