From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 961E4C2D0BF for ; Wed, 18 Dec 2019 15:28:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5888F2467F for ; Wed, 18 Dec 2019 15:28:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576682889; bh=I5rJItCtVyG3ifmXpdh0BqpQLoPh7lJkZPAAE05a4Go=; h=From:To:Cc:Subject:Date:In-Reply-To:References:In-Reply-To: References:List-ID:From; b=UR9LZ6fnG0uP9esJbHKsiFjhj/TBoPBmSWbgqmvnEOnUw9oVu3+YOucDlyzGpc9i0 Eo7GfuGFxBvZ6E/KXEdJ012Y7RgEquQ9+vY8LonUi5aJBB3RDU2p/L4K40lJs4qWLE Wa27PN1FT8qVN40MUVHvyx/k4cPCNuM0qTguqSfc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727384AbfLRP2F (ORCPT ); Wed, 18 Dec 2019 10:28:05 -0500 Received: from mail.kernel.org ([198.145.29.99]:60116 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727297AbfLRP2B (ORCPT ); Wed, 18 Dec 2019 10:28:01 -0500 Received: from tzanussi-mobl.hsd1.il.comcast.net (c-98-220-238-81.hsd1.il.comcast.net [98.220.238.81]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2960824684; Wed, 18 Dec 2019 15:27:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576682879; bh=I5rJItCtVyG3ifmXpdh0BqpQLoPh7lJkZPAAE05a4Go=; h=From:To:Cc:Subject:Date:In-Reply-To:References:In-Reply-To: References:From; b=XCADSztVAeCgWdV5flnUpYQdSQcSf48Dnry3WPQOb+YKYQicVxcfAGK+Ix2ylJmKB 66huqAAFi/NzYh/eaTmtDSzUEfUqhF/TG4rpLOGOj1RxLT+p+pg8nwTq7PWeAUaQHu 3TwTCmuNFkJlt5ZqP3l+W80e7JoIxk3KCtSmjqHM= From: Tom Zanussi To: rostedt@goodmis.org Cc: artem.bityutskiy@linux.intel.com, mhiramat@kernel.org, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Subject: [PATCH 7/7] tracing: Documentation for in-kernel synthetic event API Date: Wed, 18 Dec 2019 09:27:43 -0600 Message-Id: <62dde4c759cbeda827ddcd110ecf0488d5a8890d.1576679206.git.zanussi@kernel.org> X-Mailer: git-send-email 2.14.1 In-Reply-To: References: In-Reply-To: References: Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org Add Documentation for creating and generating synthetic events from modules. Signed-off-by: Tom Zanussi --- Documentation/trace/events.rst | 268 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 268 insertions(+) diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst index f7e1fcc0953c..084d983df3a2 100644 --- a/Documentation/trace/events.rst +++ b/Documentation/trace/events.rst @@ -525,3 +525,271 @@ The following commands are supported: event counts (hitcount). See Documentation/trace/histogram.rst for details and examples. + +6.3 In-kernel trace event API +----------------------------- + +In most cases, the command-line interface to trace events is more than +sufficient. Sometimes, however, applications might find the need for +more complex relationships than can be expressed through a simple +series of linked command-line expressions, or putting together sets of +commands may be simply too cumbersome. An example might be an +application that needs to 'listen' to the trace stream in order to +maintain an in-kernel state machine detecting, for instance, when an +illegal kernel state occurs in the scheduler. + +The trace event subsystem provides an in-kernel API allowing modules +or other kernel code to generate user-defined 'synthetic' events at +will, which can be used to either augment the existing trace stream +and/or signal that a particular important state has occurred. + +The API provided for these purposes is describe below and allows the +following: + + - dynamically creating synthetic event definitions + - generating synthetic events from in-kernel code + +6.3.1 Dyamically creating synthetic event definitions +----------------------------------------------------- + +There are a couple ways to create a new synthetic event from a kernel +module or other kernel code. + +The first creates the event in one step, using create_synth_event(). +In this method, the name of the event to create and an array defining +the fields is supplied to create_synth_event(). If successful, a +synthetic event with that name and fields will exist following that +call. For example, to create a new "schedtest" synthetic event: + + ret = create_synth_event("schedtest", sched_fields, + ARRAY_SIZE(sched_fields), THIS_MODULE); + +The sched_fields param in this example points to an array of struct +synth_field_desc, each of which describes an event field by type and +name: + + static struct synth_field_desc sched_fields[] = { + { .type = "pid_t", .name = "next_pid_field" }, + { .type = "char[16]", .name = "next_comm_field" }, + { .type = "u64", .name = "ts_ns" }, + { .type = "u64", .name = "ts_ms" }, + { .type = "unsigned int", .name = "cpu" }, + { .type = "char[64]", .name = "my_string_field" }, + { .type = "int", .name = "my_int_field" }, + }; + +See synth_field_size() for available types. If field_name contains [n] +the field is considered to be an array. + +If the event is created from within a module, a pointer to the module +must be passed to create_synth_event(). This will ensure that the +trace buffer won't contain unreadable events when the module is +removed. + +At this point, the event object is ready to be used for generating new +events. + +In the second method, the event is created in several steps. This +allows events to be created dynamically and without the need to create +and populate an array of fields beforehand. + +To use this method, an empty synthetic event should first be created +using create_empty_synth_event(). The name of the event should be +supplied to this function. For example, to create a new "schedtest" +synthetic event: + + struct synth_event *se = create_empty_synth_event("schedtest", THIS_MODULE); + +Once the synthetic event object has been created, it can then be +populated with fields. Fields are added one by one using +add_synth_field(), supplying the new synthetic event object, a field +type, and a field name. For example, to add a new int field named +"intfield", the following call should be made: + + ret = add_synth_field(se, "int", "intfield"); + +See synth_field_size() for available types. If field_name contains [n] +the field is considered to be an array. + +A group of fields can also be added all at once using an array of +synth_field_desc with add_synth_fields(). For example, this would add +just the first four sched_fields: + + ret = add_synth_fields(se, sched_fields, 4); + +Once all the fields have been added, the event should be finalized and +registered by calling the finalize_synth_event() function: + + ret = finalize_synth_event(se); + +At this point, the event object is ready to be used for generating new +events. + +6.3.3 Generating synthetic events from in-kernel code +----------------------------------------------------- + +To generate a synthetic event, there are a couple of options. The +first option is to generate the event in one call, using +generate_synth_event() with an array of values to be set. A second +option can be used to avoid the need for a pre-formed array of values, +using generate_synth_event_start() and generate_synth_event_end() +along with add_next_synth_val() or add_synth_val() to add the values +piecewise. + +6.3.3.1 Generating a synthetic event all at once +------------------------------------------------ + +To generate a synthetic event all at once, the generate_synth_event() +function is used. It's passed the trace_event_file representing the +synthetic event (which can be retrieved using get_event_file() using +the synthetic event name, "synthetic" as the system name, and the +trace instance name (NULL if using the global trace array)), along +with an array of u64, one for each synthetic event field. + +So, to generate an event corresponding to the synthetic event +definition above, code like the following could be used: + + u64 vals[7]; + + vals[0] = 777; /* next_pid_field */ + vals[1] = (u64)"tiddlywinks"; /* next_comm_field */ + vals[2] = 1000000; /* ts_ns */ + vals[3] = 1000; /* ts_ms */ + vals[4] = smp_processor_id(); /* cpu */ + vals[5] = (u64)"thneed"; /* my_string_field */ + vals[6] = 398; /* my_int_field */ + +The 'vals' array is just an array of u64, the number of which must +match the number of field in the synthetic event, and which must be in +the same order as the synthetic event fields. + +All vals should be cast to u64, and string vals are just pointers to +strings, cast to u64. Strings will be copied into space reserved in +the event for the string, using these pointers. + +In order to generate a synthetic event, a pointer to the trace event +file is needed. The get_event_file() function can be used to get it - +it will find the file in the given trace instance (in this case NULL +since the top trace array is being used) while at the same time +preventing the instance containing it from going away: + + schedtest_event_file = get_event_file(NULL, "synthetic", + "schedtest"); + +Before generating the event, it should be enabled in some way, +otherwise the synthetic event won't actually show up in the trace +buffer. + +To enable a synthetic event from the kernel, trace_array_set_clr_event() +can be used (which is not specific to synthetic events, so does need +the "synthetic" system name to be specified explicitly). + +To enable the event, pass 'true' to it: + + trace_array_set_clr_event(schedtest_event_file->tr, + "synthetic", "schedtest", true); + +To disable it pass false: + + trace_array_set_clr_event(schedtest_event_file->tr, + "synthetic", "schedtest", false); + +Finally, generate_synth_event() can be used to actually generate the +event, which should be visible in the trace buffer afterwards: + + ret = generate_synth_event(schedtest_event_file, vals, + ARRAY_SIZE(vals)); + +To remove the synthetic event, the event should be disabled, and the +trace instance should be 'put' back using put_event_file(): + + trace_array_set_clr_event(schedtest_event_file->tr, + "synthetic", "schedtest", false); + put_event_file(schedtest_event_file); + +If those have been successful, delete_synth_event() can be called to +remove the event: + + ret = delete_synth_event("schedtest"); + +6.3.3.1 Generating a synthetic event piecewise +---------------------------------------------- + +To generate a synthetic using the piecewise method described above, +the generate_synth_event_start() function is used to 'open' the +synthetic event generation: + + struct synth_gen_state gen_state; + + ret = generate_synth_event_start(schedtest_event_file, &gen_state); + +It's passed the trace_event_file representing the synthetic event +using the same methods as described above, along with a pointer to a +struct synth_gen_state object, which will be zeroed before use and +used to maintain state between this and following calls. + +Once the event has been opened, which means space for it has been +reserved in the trace buffer, the individual fields can be set. There +are two ways to do that, either one after another for each field in +the event, which requires no lookups, or by name, which does. The +tradeoff is flexibility in doing the assignments vs the cost of a +lookup per field. + +To assign the values one after the other without lookups, +add_next_synth_val() should be used. Each call is passed the same +synth_gen_state object used in the generate_synth_event_start(), along +with the value to set the next field in the event. After each field +is set, the 'cursor' points to the next field, which will be set by +the subsequent call, continuing until all the fields have been set in +order. The same sequence of calls as in the above examples using this +method would be (without error-handling code): + + /* next_pid_field */ + ret = add_next_synth_val(777, &gen_state); + + /* next_comm_field */ + ret = add_next_synth_val((u64)"slinky", &gen_state); + + /* ts_ns */ + ret = add_next_synth_val(1000000, &gen_state); + + /* ts_ms */ + ret = add_next_synth_val(1000, &gen_state); + + /* cpu */ + ret = add_next_synth_val(smp_processor_id(), &gen_state); + + /* my_string_field */ + ret = add_next_synth_val((u64)"thneed_2.01", &gen_state); + + /* my_int_field */ + ret = add_next_synth_val(395, &gen_state); + +To assign the values in any order, add_synth_val() should be used. +Each call is passed the same synth_gen_state object used in the +generate_synth_event_start(), along with the field name of the field +to set and the value to set it to. The same sequence of calls as in +the above examples using this method would be (without error-handling +code): + + ret = add_synth_val("next_pid_field", 777, &gen_state); + ret = add_synth_val("next_comm_field", (u64)"silly putty", &gen_state); + ret = add_synth_val("ts_ns", 1000000, &gen_state); + ret = add_synth_val("ts_ms", 1000, &gen_state); + ret = add_synth_val("cpu", smp_processor_id(), &gen_state); + ret = add_synth_val("my_string_field", (u64)"thneed_9", &gen_state); + ret = add_synth_val("my_int_field", 3999, &gen_state); + +Note that add_next_synth_val() and add_synth_val() are incompatible if +used within the same generation of an event - either one can be used +but not both at the same time. + +Finally, the event won't be actually generated until it's 'closed', +which is done using generate_synth_event_end(), which takes only the +struct synth_gen_state object used in the previous calls: + + ret = generate_synth_event_end(&gen_state); + +Note that generate_synth_event_end() must be called at the end +regardless of whether any of the add calls failed (say due to a bad +field name being passed in). -- 2.14.1