LTTng » LTTng-tools

Here is an abridged log that outlines the problem:

$ sudo -H lttng create asession
Session asession created.
Traces will be written in /root/lttng-traces/asession-20130214-134634
$ sudo -H lttng list asession
Tracing session asession: [inactive]
    Trace path: /root/lttng-traces/asession-20130214-134634

$ sudo -H lttng enable-event sched_switch -k --tracepoint
kernel event sched_switch created in channel channel0
$ sudo -H lttng list asession
Tracing session asession: [inactive]
      [...]
    Events:
      sched_switch (loglevel: TRACE_EMERG (0)) (type: tracepoint) [enabled]

$ sudo -H lttng disable-event sched_switch -k
kernel event sched_switch disabled in channel channel0 for session asession
$ sudo -H lttng list asession
Tracing session asession: [inactive]
      [...]
    Events:
      sched_switch (loglevel: TRACE_EMERG (0)) (type: tracepoint) [disabled]

$ sudo -H lttng enable-event sched_switch -k --function lttng_calibrate_kretprobe
kernel event sched_switch created in channel channel0
$ sudo -H lttng list asession
Tracing session asession: [inactive]
      [...]
    Events:
      sched_switch (loglevel: TRACE_EMERG (0)) (type: tracepoint) [enabled]

The problem is that when the user tries to redefine an event by issuing enable-event again, if the event is enabled he gets LTTNG_ERR_KERN_EVENT_EXIST "Kernel event already exists", but if it is disabled, he gets no error and apparent confirmation ("kernel event sched_switch created in channel channel0"). This can be misleading.

It seems the intent is that an event, once defined, is immutable. In that case, any redefinition attempt should yield LTTNG_ERR_KERN_EVENT_EXIST, whether the event is currently enabled or not. Having a dual-use command "enable-event" that acts upon a number of its options only when the event is first defined, while ignoring them when the event is merely re-enabled, is confusing for the user (it also makes the code unwieldy).

My suggestion is that event definition be split off into its own command, "define-event", while restricting "enable-event" to enabling already-defined events only. The corresponding --help could be something like this (abridged):

usage: lttng define-event [NAME (-k|-u)] [OPTIONS]

Options:
  -h, --help               Show this help and exit
      --list-options       List the options and exit
  -s, --session NAME       Apply to session NAME
  -k, --kernel             Apply to the kernel tracer
  -u, --userspace          Apply to the userspace tracer
  -c, --channel NAME       Apply to channel NAME

Event options:
    --tracepoint           Tracepoint event (default)
    --probe [addr | symbol | symbol+offset]
                           Dynamic probe. Kernel only.
                           Addr and offset can be octal (0NNN...),
                           decimal (NNN...) or hexadecimal (0xNNN...)
    --function [addr | symbol | symbol+offset]
                           Dynamic function entry/return probe. Kernel only.
    --syscall              System call event. Kernel only.
    --loglevel NAME        Tracepoint loglevel range from 0 to NAME.
                           Userspace only.
    --loglevel-only NAME   Tracepoint loglevel (only NAME)
                           Userspace only.
    --filter 'expression'  Filter expression on event fields,
                           event recording depends on evaluation.
                           Userspace only.

usage: lttng enable-event [[NAME[,NAME2,...]] (-k|-u)] [OPTIONS]

NAME can be omitted when using -a, --all.
The userspace tracer supports wildcards at the end of the NAME strings for
tracepoints. Don't forget to quote to deal with bash expansion. e.g.: "*",
"app_component:na*" 

Options:
  -h, --help               Show this help and exit
      --list-options       List the options and exit
  -s, --session NAME       Apply to session NAME
  -k, --kernel             Apply to the kernel tracer
  -u, --userspace          Apply to the user-space tracer
  -c, --channel NAME       Apply to channel NAME
  -a, --all [TYPE]         Enable all events
                           If TYPE is specified, enable all events of the TYPE;
                           TYPE can be tracepoint, syscall, probe, or function

Some features of the current enable-event could be kept in the new enable-event: for instance, an event could have its loglevel or filter settings adjusted (assuming this is possible now or in the future). Note how event definition now accepts a single event name to avoid bug #444.