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author | Tom Zanussi <zanussi@kernel.org> | 2020-01-29 12:59:32 -0600 |
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committer | Steven Rostedt (VMware) <rostedt@goodmis.org> | 2020-01-30 09:46:29 -0500 |
commit | 34ed63573b664f984a4b84df02a9ecdfaa3a6034 (patch) | |
tree | cefef689cb37c03ce1a6d9e84c18ab7941dcd707 /Documentation/trace | |
parent | 64836248dda20c8e7427b493f7e06d9bf8f58850 (diff) | |
download | linux-34ed63573b664f984a4b84df02a9ecdfaa3a6034.tar.bz2 |
tracing: Documentation for in-kernel synthetic event API
Add Documentation for creating and generating synthetic events from
modules.
Link: http://lkml.kernel.org/r/734bf8789ff8700000c9acde61a553427910ddb5.1580323897.git.zanussi@kernel.org
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Diffstat (limited to 'Documentation/trace')
-rw-r--r-- | Documentation/trace/events.rst | 515 |
1 files changed, 515 insertions, 0 deletions
diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst index f7e1fcc0953c..ed79b220bd07 100644 --- a/Documentation/trace/events.rst +++ b/Documentation/trace/events.rst @@ -525,3 +525,518 @@ 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. + +A similar in-kernel API is also available for creating kprobe and +kretprobe events. + +Both the synthetic event and k/ret/probe event APIs are built on top +of a lower-level "dynevent_cmd" event command API, which is also +available for more specialized applications, or as the basis of other +higher-level trace event APIs. + +The API provided for these purposes is describe below and allows the +following: + + - dynamically creating synthetic event definitions + - dynamically creating kprobe and kretprobe event definitions + - tracing synthetic events from in-kernel code + - the low-level "dynevent_cmd" API + +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 synth_event_create(). +In this method, the name of the event to create and an array defining +the fields is supplied to synth_event_create(). 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 = synth_event_create("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 synth_event_create(). 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 or partially empty synthetic event should +first be created using synth_event_gen_cmd_start() or +synth_event_gen_cmd_array_start(). For synth_event_gen_cmd_start(), +the name of the event along with one or more pairs of args each pair +representing a 'type field_name;' field specification should be +supplied. For synth_event_gen_cmd_array_start(), the name of the +event along with an array of struct synth_field_desc should be +supplied. Before calling synth_event_gen_cmd_start() or +synth_event_gen_cmd_array_start(), the user should create and +initialize a dynevent_cmd object using synth_event_cmd_init(). + +For example, to create a new "schedtest" synthetic event with two +fields: + + struct dynevent_cmd cmd; + char *buf; + + /* Create a buffer to hold the generated command */ + buf = kzalloc(MAX_DYNEVENT_CMD_LEN, GFP_KERNEL); + + /* Before generating the command, initialize the cmd object */ + synth_event_cmd_init(&cmd, buf, MAX_DYNEVENT_CMD_LEN); + + ret = synth_event_gen_cmd_start(&cmd, "schedtest", THIS_MODULE, + "pid_t", "next_pid_field", + "u64", "ts_ns"); + +Alternatively, using an array of struct synth_field_desc fields +containing the same information: + + ret = synth_event_gen_cmd_array_start(&cmd, "schedtest", THIS_MODULE, + fields, n_fields); + +Once the synthetic event object has been created, it can then be +populated with more fields. Fields are added one by one using +synth_event_add_field(), supplying the dynevent_cmd 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 = synth_event_add_field(&cmd, "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 = synth_event_add_fields(&cmd, sched_fields, 4); + +If you already have a string of the form 'type field_name', +synth_event_add_field_str() can be used to add it as-is; it will +also automatically append a ';' to the string. + +Once all the fields have been added, the event should be finalized and +registered by calling the synth_event_gen_cmd_end() function: + + ret = synth_event_gen_cmd_end(&cmd); + +At this point, the event object is ready to be used for tracing new +events. + +6.3.3 Tracing synthetic events from in-kernel code +-------------------------------------------------- + +To trace a synthetic event, there are several options. The first +option is to trace the event in one call, using synth_event_trace() +with a variable number of values, or synth_event_trace_array() 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 or list of arguments, via +synth_event_trace_start() and synth_event_trace_end() along with +synth_event_add_next_val() or synth_event_add_val() to add the values +piecewise. + +6.3.3.1 Tracing a synthetic event all at once +--------------------------------------------- + +To trace a synthetic event all at once, the synth_event_trace() or +synth_event_trace_array() functions can be used. + +The synth_event_trace() function is passed the trace_event_file +representing the synthetic event (which can be retrieved using +trace_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 variable number of u64 args, one for each +synthetic event field, and the number of values being passed. + +So, to trace an event corresponding to the synthetic event definition +above, code like the following could be used: + + ret = synth_event_trace(create_synth_test, 7, /* number of values */ + 444, /* next_pid_field */ + (u64)"clackers", /* next_comm_field */ + 1000000, /* ts_ns */ + 1000, /* ts_ms */ + smp_processor_id(),/* cpu */ + (u64)"Thneed", /* my_string_field */ + 999); /* my_int_field */ + +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. + +Alternatively, the synth_event_trace_array() function can be used to +accomplish the same thing. It is passed the trace_event_file +representing the synthetic event (which can be retrieved using +trace_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. + +To trace 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 trace a synthetic event, a pointer to the trace event file +is needed. The trace_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 = trace_get_event_file(NULL, "synthetic", + "schedtest"); + +Before tracing 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, synth_event_trace_array() can be used to actually trace the +event, which should be visible in the trace buffer afterwards: + + ret = synth_event_trace_array(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 trace_put_event_file(): + + trace_array_set_clr_event(schedtest_event_file->tr, + "synthetic", "schedtest", false); + trace_put_event_file(schedtest_event_file); + +If those have been successful, synth_event_delete() can be called to +remove the event: + + ret = synth_event_delete("schedtest"); + +6.3.3.1 Tracing a synthetic event piecewise +------------------------------------------- + +To trace a synthetic using the piecewise method described above, the +synth_event_trace_start() function is used to 'open' the synthetic +event trace: + + struct synth_trace_state trace_state; + + ret = synth_event_trace_start(schedtest_event_file, &trace_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_trace_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, +synth_event_add_next_val() should be used. Each call is passed the +same synth_trace_state object used in the synth_event_trace_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 = synth_event_add_next_val(777, &trace_state); + + /* next_comm_field */ + ret = synth_event_add_next_val((u64)"slinky", &trace_state); + + /* ts_ns */ + ret = synth_event_add_next_val(1000000, &trace_state); + + /* ts_ms */ + ret = synth_event_add_next_val(1000, &trace_state); + + /* cpu */ + ret = synth_event_add_next_val(smp_processor_id(), &trace_state); + + /* my_string_field */ + ret = synth_event_add_next_val((u64)"thneed_2.01", &trace_state); + + /* my_int_field */ + ret = synth_event_add_next_val(395, &trace_state); + +To assign the values in any order, synth_event_add_val() should be +used. Each call is passed the same synth_trace_state object used in +the synth_event_trace_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 = synth_event_add_val("next_pid_field", 777, &trace_state); + ret = synth_event_add_val("next_comm_field", (u64)"silly putty", + &trace_state); + ret = synth_event_add_val("ts_ns", 1000000, &trace_state); + ret = synth_event_add_val("ts_ms", 1000, &trace_state); + ret = synth_event_add_val("cpu", smp_processor_id(), &trace_state); + ret = synth_event_add_val("my_string_field", (u64)"thneed_9", + &trace_state); + ret = synth_event_add_val("my_int_field", 3999, &trace_state); + +Note that synth_event_add_next_val() and synth_event_add_val() are +incompatible if used within the same trace of an event - either one +can be used but not both at the same time. + +Finally, the event won't be actually traced until it's 'closed', +which is done using synth_event_trace_end(), which takes only the +struct synth_trace_state object used in the previous calls: + + ret = synth_event_trace_end(&trace_state); + +Note that synth_event_trace_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). + +6.3.4 Dyamically creating kprobe and kretprobe event definitions +---------------------------------------------------------------- + +To create a kprobe or kretprobe trace event from kernel code, the +kprobe_event_gen_cmd_start() or kretprobe_event_gen_cmd_start() +functions can be used. + +To create a kprobe event, an empty or partially empty kprobe event +should first be created using kprobe_event_gen_cmd_start(). The name +of the event and the probe location should be specfied along with one +or args each representing a probe field should be supplied to this +function. Before calling kprobe_event_gen_cmd_start(), the user +should create and initialize a dynevent_cmd object using +kprobe_event_cmd_init(). + +For example, to create a new "schedtest" kprobe event with two fields: + + struct dynevent_cmd cmd; + char *buf; + + /* Create a buffer to hold the generated command */ + buf = kzalloc(MAX_DYNEVENT_CMD_LEN, GFP_KERNEL); + + /* Before generating the command, initialize the cmd object */ + kprobe_event_cmd_init(&cmd, buf, MAX_DYNEVENT_CMD_LEN); + + /* + * Define the gen_kprobe_test event with the first 2 kprobe + * fields. + */ + ret = kprobe_event_gen_cmd_start(&cmd, "gen_kprobe_test", "do_sys_open", + "dfd=%ax", "filename=%dx"); + +Once the kprobe event object has been created, it can then be +populated with more fields. Fields can be added using +kprobe_event_add_fields(), supplying the dynevent_cmd object along +with a variable arg list of probe fields. For example, to add a +couple additional fields, the following call could be made: + + ret = kprobe_event_add_fields(&cmd, "flags=%cx", "mode=+4($stack)"); + +Once all the fields have been added, the event should be finalized and +registered by calling the kprobe_event_gen_cmd_end() or +kretprobe_event_gen_cmd_end() functions, depending on whether a kprobe +or kretprobe command was started: + + ret = kprobe_event_gen_cmd_end(&cmd); + +or + + ret = kretprobe_event_gen_cmd_end(&cmd); + +At this point, the event object is ready to be used for tracing new +events. + +Similarly, a kretprobe event can be created using +kretprobe_event_gen_cmd_start() with a probe name and location and +additional params such as $retval: + + ret = kretprobe_event_gen_cmd_start(&cmd, "gen_kretprobe_test", + "do_sys_open", "$retval"); + +Similar to the synthetic event case, code like the following can be +used to enable the newly created kprobe event: + + gen_kprobe_test = trace_get_event_file(NULL, "kprobes", "gen_kprobe_test"); + + ret = trace_array_set_clr_event(gen_kprobe_test->tr, + "kprobes", "gen_kprobe_test", true); + +Finally, also similar to synthetic events, the following code can be +used to give the kprobe event file back and delete the event: + + trace_put_event_file(gen_kprobe_test); + + ret = kprobe_event_delete("gen_kprobe_test"); + +6.3.4 The "dynevent_cmd" low-level API +-------------------------------------- + +Both the in-kernel synthetic event and kprobe interfaces are built on +top of a lower-level "dynevent_cmd" interface. This interface is +meant to provide the basis for higher-level interfaces such as the +synthetic and kprobe interfaces, which can be used as examples. + +The basic idea is simple and amounts to providing a general-purpose +layer that can be used to generate trace event commands. The +generated command strings can then be passed to the command-parsing +and event creation code that already exists in the trace event +subystem for creating the corresponding trace events. + +In a nutshell, the way it works is that the higher-level interface +code creates a struct dynevent_cmd object, then uses a couple +functions, dynevent_arg_add() and dynevent_arg_pair_add() to build up +a command string, which finally causes the command to be executed +using the dynevent_create() function. The details of the interface +are described below. + +The first step in building a new command string is to create and +initialize an instance of a dynevent_cmd. Here, for instance, we +create a dynevent_cmd on the stack and initialize it: + + struct dynevent_cmd cmd; + char *buf; + int ret; + + buf = kzalloc(MAX_DYNEVENT_CMD_LEN, GFP_KERNEL); + + dynevent_cmd_init(cmd, buf, maxlen, DYNEVENT_TYPE_FOO, + foo_event_run_command); + +The dynevent_cmd initialization needs to be given a user-specified +buffer and the length of the buffer (MAX_DYNEVENT_CMD_LEN can be used +for this purpose - at 2k it's generally too big to be comfortably put +on the stack, so is dynamically allocated), a dynevent type id, which +is meant to be used to check that further API calls are for the +correct command type, and a pointer to an event-specific run_command() +callback that will be called to actually execute the event-specific +command function. + +Once that's done, the command string can by built up by successive +calls to argument-adding functions. + +To add a single argument, define and initialize a struct dynevent_arg +or struct dynevent_arg_pair object. Here's an example of the simplest +possible arg addition, which is simply to append the given string as +a whitespace-separated argument to the command: + + struct dynevent_arg arg; + + dynevent_arg_init(&arg, NULL, 0); + + arg.str = name; + + ret = dynevent_arg_add(cmd, &arg); + +The arg object is first initialized using dynevent_arg_init() and in +this case the parameters are NULL or 0, which means there's no +optional sanity-checking function or separator appended to the end of +the arg. + +Here's another more complicated example using an 'arg pair', which is +used to create an argument that consists of a couple components added +together as a unit, for example, a 'type field_name;' arg or a simple +expression arg e.g. 'flags=%cx': + + struct dynevent_arg_pair arg_pair; + + dynevent_arg_pair_init(&arg_pair, dynevent_foo_check_arg_fn, 0, ';'); + + arg_pair.lhs = type; + arg_pair.rhs = name; + + ret = dynevent_arg_pair_add(cmd, &arg_pair); + +Again, the arg_pair is first initialized, in this case with a callback +function used to check the sanity of the args (for example, that +neither part of the pair is NULL), along with a character to be used +to add an operator between the pair (here none) and a separator to be +appended onto the end of the arg pair (here ';'). + +There's also a dynevent_str_add() function that can be used to simply +add a string as-is, with no spaces, delimeters, or arg check. + +Any number of dynevent_*_add() calls can be made to build up the string +(until its length surpasses cmd->maxlen). When all the arguments have +been added and the command string is complete, the only thing left to +do is run the command, which happens by simply calling +dynevent_create(): + + ret = dynevent_create(&cmd); + +At that point, if the return value is 0, the dynamic event has been +created and is ready to use. + +See the dynevent_cmd function definitions themselves for the details +of the API. |