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 226BBC761AF for ; Tue, 21 Mar 2023 08:48:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229662AbjCUIs1 (ORCPT ); Tue, 21 Mar 2023 04:48:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58034 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231172AbjCUIr5 (ORCPT ); Tue, 21 Mar 2023 04:47:57 -0400 Received: from mail-m118111.qiye.163.com (mail-m118111.qiye.163.com [115.236.118.111]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9FD5348E0D; Tue, 21 Mar 2023 01:47:13 -0700 (PDT) Received: from ubuntu.localdomain (unknown [117.133.56.22]) by mail-m118111.qiye.163.com (Hmail) with ESMTPA id 1CF7358063F; Tue, 21 Mar 2023 16:47:07 +0800 (CST) From: Donglin Peng To: mhiramat@kernel.org, rostedt@goodmis.org, linux@armlinux.org.uk, mark.rutland@arm.com, will@kernel.org, catalin.marinas@arm.com, palmer@dabbelt.com, paul.walmsley@sifive.com, tglx@linutronix.de, dave.hansen@linux.intel.com, x86@kernel.org, mingo@redhat.com, peterz@infradead.org, xiehuan09@gmail.com, dinghui@sangfor.com.cn, huangcun@sangfor.com.cn, dolinux.peng@gmail.com Cc: linux-trace-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Donglin Peng Subject: [PATCH v6 2/2] tracing: Add documentation for funcgraph-retval and graph_retval_hex Date: Tue, 21 Mar 2023 01:46:50 -0700 Message-Id: <20230321084650.769212-3-pengdonglin@sangfor.com.cn> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20230321084650.769212-1-pengdonglin@sangfor.com.cn> References: <20230321084650.769212-1-pengdonglin@sangfor.com.cn> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-HM-Spam-Status: e1kfGhgUHx5ZQUpXWQgPGg8OCBgUHx5ZQUlOS1dZFg8aDwILHllBWSg2Ly tZV1koWUFITzdXWS1ZQUlXWQ8JGhUIEh9ZQVkaGkkdVh0YGR1KHU1MQkNKHlUTARMWGhIXJBQOD1 lXWRgSC1lBWUpKTFVKSEhVTk1VSUlZV1kWGg8SFR0UWUFZT0tIVUpISkJIT1VKS0tVS1kG X-HM-Sender-Digest: e1kMHhlZQR0aFwgeV1kSHx4VD1lBWUc6NS46Azo6PT0UVhdNDyEZMBFP SDgaCi1VSlVKTUxCSENDT0lDT09OVTMWGhIXVQseFRwfFBUcFxIVOwgaFRwdFAlVGBQWVRgVRVlX WRILWUFZSkpMVUpISFVOTVVJSVlXWQgBWUFNQ05ONwY+ X-HM-Tid: 0a870359c6b12eb7kusn1cf7358063f X-HM-MType: 1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Add documentation for the two newly introduced options for the function_graph tracer. The new option funcgraph-retval is used to control whether or not to display the return value. The new option graph_retval_hex is used to control the display format of the return value. Signed-off-by: Donglin Peng --- v6: - Modify the limitations for funcgraph-retval - Optimize the English expression v5: - Describe the limitations of funcgraph-retval --- Documentation/trace/ftrace.rst | 73 ++++++++++++++++++++++++++++++++++ 1 file changed, 73 insertions(+) diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst index b927fb2b94dc..1807f7752037 100644 --- a/Documentation/trace/ftrace.rst +++ b/Documentation/trace/ftrace.rst @@ -1328,6 +1328,19 @@ Options for function_graph tracer: only a closing curly bracket "}" is displayed for the return of a function. + funcgraph-retval + When set, the return value of each traced function + will be printed after an equal sign "=". By default + this is off. + + graph_retval_hex + When set, the return value will always be printed + in hexadecimal format. If the option is not set and + the return value is an error code, it will be printed + in signed decimal format; otherwise it will also be + printed in hexadecimal format. By default, this option + is off. + sleep-time When running function graph tracer, to include the time a task schedules out in its function. @@ -2673,6 +2686,66 @@ It is default disabled. 0) 1.757 us | } /* kmem_cache_free() */ 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 +can be verfy helpful to quickly locate the function that first +returns an error code. + + - hide: echo nofuncgraph-retval > trace_options + - show: echo funcgraph-retval > trace_options + + Example with funcgraph-retval:: + + 1) | cgroup_migrate() { + 1) 0.651 us | cgroup_migrate_add_task(); /* = 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 */ + +The above example shows that the function cpu_cgroup_can_attach +returned the error code -22 firstly, then we can read the code +of this function to get the root cause. + +When the option graph_retval_hex is not set, the return value can +be displayed in a smart way. Specifically, if it is an error code, +it will be printed in signed decimal format, otherwise it will +printed in hexadecimal format. + + - smart: echo nograph_retval_hex > trace_options + - hexadecimal always: echo graph_retval_hex > trace_options + + Example with graph_retval_hex:: + + 1) | cgroup_migrate() { + 1) 0.651 us | cgroup_migrate_add_task(); /* = 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) 7.143 us | } /* cgroup_migrate = 0xffffffea */ + +There are some limitations when using the funcgraph-retval currently: + +- 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 two registers, only the value of + the first return register will be recorded and printed. For example, + in x86 architecture, both eax and edx are used to store a 64-bit + return value. Eax stores the low 32 bits, and edx stores the high + 32 bits. However, only the value stored in eax will be recorded and + printed. + You can put some comments on specific functions by using trace_printk() For example, if you want to put a comment inside the __might_sleep() function, you just have to include -- 2.25.1