From patchwork Sat Apr 8 12:42:16 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: pengdonglin X-Patchwork-Id: 13205693 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 DB6F4C77B70 for ; Sat, 8 Apr 2023 12:42:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230215AbjDHMmt (ORCPT ); Sat, 8 Apr 2023 08:42:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51564 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229984AbjDHMmr (ORCPT ); Sat, 8 Apr 2023 08:42:47 -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 3CFA9AD3D; Sat, 8 Apr 2023 05:42:44 -0700 (PDT) Received: from ubuntu.localdomain (unknown [121.32.254.147]) by mail-m118111.qiye.163.com (Hmail) with ESMTPA id 049785802B3; Sat, 8 Apr 2023 20:42:29 +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, rmk+kernel@armlinux.org.uk, palmer@dabbelt.com, paul.walmsley@sifive.com, aou@eecs.berkeley.edu, tglx@linutronix.de, dave.hansen@linux.intel.com, x86@kernel.org, bp@alien8.de, hpa@zytor.com, chenhuacai@kernel.org, zhangqing@loongson.cn, kernel@xen0n.name, 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, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Donglin Peng Subject: [PATCH v11 2/8] tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex Date: Sat, 8 Apr 2023 05:42:16 -0700 Message-Id: <2b5635f05146161b54c9ea6307e25efe5ccebdad.1680954589.git.pengdonglin@sangfor.com.cn> X-Mailer: git-send-email 2.25.1 In-Reply-To: References: MIME-Version: 1.0 X-HM-Spam-Status: e1kfGhgUHx5ZQUpXWQgPGg8OCBgUHx5ZQUlOS1dZFg8aDwILHllBWSg2Ly tZV1koWUFITzdXWS1ZQUlXWQ8JGhUIEh9ZQVkaHR5IVkhISh9KQ0saQhpCS1UTARMWGhIXJBQOD1 lXWRgSC1lBWUpJSlVISVVJTk9VSk9MWVdZFhoPEhUdFFlBWU9LSFVKSEpCSE9VSktLVUtZBg++ X-HM-Sender-Digest: e1kMHhlZQR0aFwgeV1kSHx4VD1lBWUc6NE06Djo*Cj0PGjcOMxwBVg4p Q0oKC05VSlVKTUNLQk5MTE5KTUxJVTMWGhIXVQseFRwfFBUcFxIVOwgaFRwdFAlVGBQWVRgVRVlX WRILWUFZSklKVUhJVUlOT1VKT0xZV1kIAVlBQkpLSTcG X-HM-Tid: 0a8760e3bf4c2eb7kusn049785802b3 X-HM-MType: 1 Precedence: bulk List-ID: X-Mailing-List: linux-trace-kernel@vger.kernel.org Add documentation for the two newly introduced options for the function_graph tracer. The funcgraph-retval option is used to control whether or not to display the return value, while the funcgraph-retval-hex option is used to control the display format of the return value. Acked-by: Masami Hiramatsu (Google) Signed-off-by: Donglin Peng --- v11: - Add a limitation discovered by Mark. v9: - Update limitation description v7: - Rename trace option 'graph_retval_hex' to 'funcgraph-retval-hex' - Update documentation description v6: - Modify the limitations for funcgraph-retval - Optimize the English expression v5: - Describe the limitations of funcgraph-retval --- Documentation/trace/ftrace.rst | 126 +++++++++++++++++++++++++++++++++ 1 file changed, 126 insertions(+) diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst index b927fb2b94dc..bd3e2b96f3f3 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. + + funcgraph-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 +2686,119 @@ 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 funcgraph-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 nofuncgraph-retval-hex > trace_options + - hexadecimal: echo funcgraph-retval-hex > trace_options + + Example with funcgraph-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 */ + +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: + + **Case One**:: + + The function narrow_to_u8 is defined as follows:: + + u8 narrow_to_u8(u64 val) + { + // implicitly truncated + return val; + } + + It may be compiled to:: + + narrow_to_u8: + < ... ftrace instrumentation ... > + RET + + If you pass 0x123456789abcdef to this function and want to narrow it, + it may be recorded as 0x123456789abcdef instead of 0xef. + + **Case Two**:: + + The function error_if_not_4g_aligned is defined as follows:: + + int error_if_not_4g_aligned(u64 val) + { + if (val & GENMASK(31, 0)) + return -EINVAL; + + return 0; + } + + It could be compiled to:: + + error_if_not_4g_aligned: + CBNZ w0, .Lnot_aligned + RET // bits [31:0] are zero, bits + // [63:32] are UNKNOWN + .Lnot_aligned: + MOV x0, #-EINVAL + RET + + When passing 0x2_0000_0000 to it, the return value may be recorded as + 0x2_0000_0000 instead of 0. + 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