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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>
This commit is contained in:
Tom Zanussi
Steven Rostedt (VMware)
parent
64836248dd
commit
34ed63573b
@ -525,3 +525,518 @@ The following commands are supported:
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event counts (hitcount).
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See Documentation/trace/histogram.rst for details and examples.
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6.3 In-kernel trace event API
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-----------------------------
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In most cases, the command-line interface to trace events is more than
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sufficient. Sometimes, however, applications might find the need for
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more complex relationships than can be expressed through a simple
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series of linked command-line expressions, or putting together sets of
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commands may be simply too cumbersome. An example might be an
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application that needs to 'listen' to the trace stream in order to
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maintain an in-kernel state machine detecting, for instance, when an
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illegal kernel state occurs in the scheduler.
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The trace event subsystem provides an in-kernel API allowing modules
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or other kernel code to generate user-defined 'synthetic' events at
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will, which can be used to either augment the existing trace stream
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and/or signal that a particular important state has occurred.
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A similar in-kernel API is also available for creating kprobe and
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kretprobe events.
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Both the synthetic event and k/ret/probe event APIs are built on top
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of a lower-level "dynevent_cmd" event command API, which is also
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available for more specialized applications, or as the basis of other
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higher-level trace event APIs.
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The API provided for these purposes is describe below and allows the
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following:
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- dynamically creating synthetic event definitions
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- dynamically creating kprobe and kretprobe event definitions
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- tracing synthetic events from in-kernel code
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- the low-level "dynevent_cmd" API
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6.3.1 Dyamically creating synthetic event definitions
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-----------------------------------------------------
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There are a couple ways to create a new synthetic event from a kernel
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module or other kernel code.
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The first creates the event in one step, using synth_event_create().
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In this method, the name of the event to create and an array defining
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the fields is supplied to synth_event_create(). If successful, a
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synthetic event with that name and fields will exist following that
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call. For example, to create a new "schedtest" synthetic event:
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ret = synth_event_create("schedtest", sched_fields,
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ARRAY_SIZE(sched_fields), THIS_MODULE);
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The sched_fields param in this example points to an array of struct
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synth_field_desc, each of which describes an event field by type and
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name:
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static struct synth_field_desc sched_fields[] = {
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{ .type = "pid_t", .name = "next_pid_field" },
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{ .type = "char[16]", .name = "next_comm_field" },
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{ .type = "u64", .name = "ts_ns" },
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{ .type = "u64", .name = "ts_ms" },
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{ .type = "unsigned int", .name = "cpu" },
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{ .type = "char[64]", .name = "my_string_field" },
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{ .type = "int", .name = "my_int_field" },
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};
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See synth_field_size() for available types. If field_name contains [n]
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the field is considered to be an array.
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If the event is created from within a module, a pointer to the module
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must be passed to synth_event_create(). This will ensure that the
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trace buffer won't contain unreadable events when the module is
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removed.
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At this point, the event object is ready to be used for generating new
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events.
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In the second method, the event is created in several steps. This
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allows events to be created dynamically and without the need to create
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and populate an array of fields beforehand.
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To use this method, an empty or partially empty synthetic event should
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first be created using synth_event_gen_cmd_start() or
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synth_event_gen_cmd_array_start(). For synth_event_gen_cmd_start(),
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the name of the event along with one or more pairs of args each pair
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representing a 'type field_name;' field specification should be
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supplied. For synth_event_gen_cmd_array_start(), the name of the
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event along with an array of struct synth_field_desc should be
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supplied. Before calling synth_event_gen_cmd_start() or
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synth_event_gen_cmd_array_start(), the user should create and
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initialize a dynevent_cmd object using synth_event_cmd_init().
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For example, to create a new "schedtest" synthetic event with two
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fields:
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struct dynevent_cmd cmd;
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char *buf;
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/* Create a buffer to hold the generated command */
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buf = kzalloc(MAX_DYNEVENT_CMD_LEN, GFP_KERNEL);
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/* Before generating the command, initialize the cmd object */
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synth_event_cmd_init(&cmd, buf, MAX_DYNEVENT_CMD_LEN);
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ret = synth_event_gen_cmd_start(&cmd, "schedtest", THIS_MODULE,
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"pid_t", "next_pid_field",
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"u64", "ts_ns");
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Alternatively, using an array of struct synth_field_desc fields
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containing the same information:
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ret = synth_event_gen_cmd_array_start(&cmd, "schedtest", THIS_MODULE,
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fields, n_fields);
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Once the synthetic event object has been created, it can then be
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populated with more fields. Fields are added one by one using
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synth_event_add_field(), supplying the dynevent_cmd object, a field
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type, and a field name. For example, to add a new int field named
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"intfield", the following call should be made:
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ret = synth_event_add_field(&cmd, "int", "intfield");
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See synth_field_size() for available types. If field_name contains [n]
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the field is considered to be an array.
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A group of fields can also be added all at once using an array of
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synth_field_desc with add_synth_fields(). For example, this would add
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just the first four sched_fields:
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ret = synth_event_add_fields(&cmd, sched_fields, 4);
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If you already have a string of the form 'type field_name',
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synth_event_add_field_str() can be used to add it as-is; it will
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also automatically append a ';' to the string.
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Once all the fields have been added, the event should be finalized and
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registered by calling the synth_event_gen_cmd_end() function:
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ret = synth_event_gen_cmd_end(&cmd);
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At this point, the event object is ready to be used for tracing new
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events.
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6.3.3 Tracing synthetic events from in-kernel code
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--------------------------------------------------
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To trace a synthetic event, there are several options. The first
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option is to trace the event in one call, using synth_event_trace()
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with a variable number of values, or synth_event_trace_array() with an
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array of values to be set. A second option can be used to avoid the
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need for a pre-formed array of values or list of arguments, via
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synth_event_trace_start() and synth_event_trace_end() along with
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synth_event_add_next_val() or synth_event_add_val() to add the values
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piecewise.
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6.3.3.1 Tracing a synthetic event all at once
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---------------------------------------------
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To trace a synthetic event all at once, the synth_event_trace() or
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synth_event_trace_array() functions can be used.
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The synth_event_trace() function is passed the trace_event_file
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representing the synthetic event (which can be retrieved using
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trace_get_event_file() using the synthetic event name, "synthetic" as
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the system name, and the trace instance name (NULL if using the global
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trace array)), along with an variable number of u64 args, one for each
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synthetic event field, and the number of values being passed.
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So, to trace an event corresponding to the synthetic event definition
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above, code like the following could be used:
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ret = synth_event_trace(create_synth_test, 7, /* number of values */
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444, /* next_pid_field */
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(u64)"clackers", /* next_comm_field */
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1000000, /* ts_ns */
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1000, /* ts_ms */
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smp_processor_id(),/* cpu */
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(u64)"Thneed", /* my_string_field */
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999); /* my_int_field */
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All vals should be cast to u64, and string vals are just pointers to
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strings, cast to u64. Strings will be copied into space reserved in
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the event for the string, using these pointers.
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Alternatively, the synth_event_trace_array() function can be used to
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accomplish the same thing. It is passed the trace_event_file
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representing the synthetic event (which can be retrieved using
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trace_get_event_file() using the synthetic event name, "synthetic" as
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the system name, and the trace instance name (NULL if using the global
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trace array)), along with an array of u64, one for each synthetic
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event field.
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To trace an event corresponding to the synthetic event definition
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above, code like the following could be used:
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u64 vals[7];
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vals[0] = 777; /* next_pid_field */
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vals[1] = (u64)"tiddlywinks"; /* next_comm_field */
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vals[2] = 1000000; /* ts_ns */
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vals[3] = 1000; /* ts_ms */
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vals[4] = smp_processor_id(); /* cpu */
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vals[5] = (u64)"thneed"; /* my_string_field */
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vals[6] = 398; /* my_int_field */
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The 'vals' array is just an array of u64, the number of which must
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match the number of field in the synthetic event, and which must be in
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the same order as the synthetic event fields.
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All vals should be cast to u64, and string vals are just pointers to
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strings, cast to u64. Strings will be copied into space reserved in
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the event for the string, using these pointers.
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In order to trace a synthetic event, a pointer to the trace event file
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is needed. The trace_get_event_file() function can be used to get
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it - it will find the file in the given trace instance (in this case
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NULL since the top trace array is being used) while at the same time
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preventing the instance containing it from going away:
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schedtest_event_file = trace_get_event_file(NULL, "synthetic",
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"schedtest");
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Before tracing the event, it should be enabled in some way, otherwise
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the synthetic event won't actually show up in the trace buffer.
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To enable a synthetic event from the kernel, trace_array_set_clr_event()
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can be used (which is not specific to synthetic events, so does need
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the "synthetic" system name to be specified explicitly).
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To enable the event, pass 'true' to it:
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trace_array_set_clr_event(schedtest_event_file->tr,
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"synthetic", "schedtest", true);
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To disable it pass false:
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trace_array_set_clr_event(schedtest_event_file->tr,
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"synthetic", "schedtest", false);
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Finally, synth_event_trace_array() can be used to actually trace the
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event, which should be visible in the trace buffer afterwards:
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ret = synth_event_trace_array(schedtest_event_file, vals,
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ARRAY_SIZE(vals));
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To remove the synthetic event, the event should be disabled, and the
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trace instance should be 'put' back using trace_put_event_file():
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trace_array_set_clr_event(schedtest_event_file->tr,
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"synthetic", "schedtest", false);
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trace_put_event_file(schedtest_event_file);
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If those have been successful, synth_event_delete() can be called to
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remove the event:
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ret = synth_event_delete("schedtest");
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6.3.3.1 Tracing a synthetic event piecewise
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-------------------------------------------
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To trace a synthetic using the piecewise method described above, the
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synth_event_trace_start() function is used to 'open' the synthetic
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event trace:
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struct synth_trace_state trace_state;
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ret = synth_event_trace_start(schedtest_event_file, &trace_state);
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It's passed the trace_event_file representing the synthetic event
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using the same methods as described above, along with a pointer to a
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struct synth_trace_state object, which will be zeroed before use and
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used to maintain state between this and following calls.
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Once the event has been opened, which means space for it has been
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reserved in the trace buffer, the individual fields can be set. There
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are two ways to do that, either one after another for each field in
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the event, which requires no lookups, or by name, which does. The
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tradeoff is flexibility in doing the assignments vs the cost of a
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lookup per field.
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To assign the values one after the other without lookups,
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synth_event_add_next_val() should be used. Each call is passed the
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same synth_trace_state object used in the synth_event_trace_start(),
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along with the value to set the next field in the event. After each
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field is set, the 'cursor' points to the next field, which will be set
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by the subsequent call, continuing until all the fields have been set
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in order. The same sequence of calls as in the above examples using
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this method would be (without error-handling code):
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/* next_pid_field */
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ret = synth_event_add_next_val(777, &trace_state);
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/* next_comm_field */
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ret = synth_event_add_next_val((u64)"slinky", &trace_state);
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/* ts_ns */
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ret = synth_event_add_next_val(1000000, &trace_state);
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/* ts_ms */
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ret = synth_event_add_next_val(1000, &trace_state);
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/* cpu */
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ret = synth_event_add_next_val(smp_processor_id(), &trace_state);
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/* my_string_field */
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ret = synth_event_add_next_val((u64)"thneed_2.01", &trace_state);
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/* my_int_field */
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ret = synth_event_add_next_val(395, &trace_state);
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To assign the values in any order, synth_event_add_val() should be
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used. Each call is passed the same synth_trace_state object used in
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the synth_event_trace_start(), along with the field name of the field
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to set and the value to set it to. The same sequence of calls as in
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the above examples using this method would be (without error-handling
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code):
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ret = synth_event_add_val("next_pid_field", 777, &trace_state);
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ret = synth_event_add_val("next_comm_field", (u64)"silly putty",
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&trace_state);
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ret = synth_event_add_val("ts_ns", 1000000, &trace_state);
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ret = synth_event_add_val("ts_ms", 1000, &trace_state);
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ret = synth_event_add_val("cpu", smp_processor_id(), &trace_state);
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ret = synth_event_add_val("my_string_field", (u64)"thneed_9",
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&trace_state);
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ret = synth_event_add_val("my_int_field", 3999, &trace_state);
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Note that synth_event_add_next_val() and synth_event_add_val() are
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incompatible if used within the same trace of an event - either one
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can be used but not both at the same time.
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Finally, the event won't be actually traced until it's 'closed',
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which is done using synth_event_trace_end(), which takes only the
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struct synth_trace_state object used in the previous calls:
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ret = synth_event_trace_end(&trace_state);
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Note that synth_event_trace_end() must be called at the end regardless
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of whether any of the add calls failed (say due to a bad field name
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being passed in).
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6.3.4 Dyamically creating kprobe and kretprobe event definitions
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----------------------------------------------------------------
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To create a kprobe or kretprobe trace event from kernel code, the
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kprobe_event_gen_cmd_start() or kretprobe_event_gen_cmd_start()
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functions can be used.
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To create a kprobe event, an empty or partially empty kprobe event
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should first be created using kprobe_event_gen_cmd_start(). The name
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of the event and the probe location should be specfied along with one
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or args each representing a probe field should be supplied to this
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function. Before calling kprobe_event_gen_cmd_start(), the user
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should create and initialize a dynevent_cmd object using
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kprobe_event_cmd_init().
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For example, to create a new "schedtest" kprobe event with two fields:
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struct dynevent_cmd cmd;
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char *buf;
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/* Create a buffer to hold the generated command */
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buf = kzalloc(MAX_DYNEVENT_CMD_LEN, GFP_KERNEL);
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/* Before generating the command, initialize the cmd object */
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kprobe_event_cmd_init(&cmd, buf, MAX_DYNEVENT_CMD_LEN);
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/*
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* Define the gen_kprobe_test event with the first 2 kprobe
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* fields.
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*/
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ret = kprobe_event_gen_cmd_start(&cmd, "gen_kprobe_test", "do_sys_open",
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"dfd=%ax", "filename=%dx");
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Once the kprobe event object has been created, it can then be
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populated with more fields. Fields can be added using
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kprobe_event_add_fields(), supplying the dynevent_cmd object along
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with a variable arg list of probe fields. For example, to add a
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couple additional fields, the following call could be made:
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ret = kprobe_event_add_fields(&cmd, "flags=%cx", "mode=+4($stack)");
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Once all the fields have been added, the event should be finalized and
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registered by calling the kprobe_event_gen_cmd_end() or
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kretprobe_event_gen_cmd_end() functions, depending on whether a kprobe
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or kretprobe command was started:
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ret = kprobe_event_gen_cmd_end(&cmd);
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or
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ret = kretprobe_event_gen_cmd_end(&cmd);
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At this point, the event object is ready to be used for tracing new
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events.
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Similarly, a kretprobe event can be created using
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kretprobe_event_gen_cmd_start() with a probe name and location and
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additional params such as $retval:
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ret = kretprobe_event_gen_cmd_start(&cmd, "gen_kretprobe_test",
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"do_sys_open", "$retval");
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Similar to the synthetic event case, code like the following can be
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used to enable the newly created kprobe event:
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gen_kprobe_test = trace_get_event_file(NULL, "kprobes", "gen_kprobe_test");
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ret = trace_array_set_clr_event(gen_kprobe_test->tr,
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"kprobes", "gen_kprobe_test", true);
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Finally, also similar to synthetic events, the following code can be
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used to give the kprobe event file back and delete the event:
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trace_put_event_file(gen_kprobe_test);
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ret = kprobe_event_delete("gen_kprobe_test");
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6.3.4 The "dynevent_cmd" low-level API
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--------------------------------------
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Both the in-kernel synthetic event and kprobe interfaces are built on
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top of a lower-level "dynevent_cmd" interface. This interface is
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meant to provide the basis for higher-level interfaces such as the
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synthetic and kprobe interfaces, which can be used as examples.
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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.
|
||||
|
||||
Loading…
Reference in New Issue
Block a user