LTTng » LTTng-tools

Enable UST and/or kernel events in their "default channel":

lttng create a
lttng enable-event -u red

Enable any agent event:

lttng enable-event -j blue

This has the effect of creating an UST channel lttng_jul_channel, making the session have a "non-default channel" existing.

So you cannot, for instance, disable the red UST event without specifying its default channel, channel0:

$ lttng disable-event -u red 
Error: Event red: Non-default channel exists within session: channel name needs to be specified with '-c name' (channel <default>, session a)

$ lttng disable-event -u red -c channel0
UST event red disabled in channel channel0 for session a

I understand that two UST channels exist now and the command doesn't know which one to choose. But this seems counterintuitive anyway from a UX point of view: the user never created another UST channel purposely.

Should agent UST channels be ignored when the only "true" UST/kernel channels are the default channels?

One step further: should the commands that need a channel apply to any channel of the given domain if there's only one (agent UST channels excluded)?

Typical situation:

$ lttng create a
$ lttng enable-channel -k kchan
$ lttng enable-event -k sched_switch
Error: Event sched_switch: Non-default channel exists within session: channel name needs to be specified with '-c name' (channel <default>, session a)

Why? It's not ambiguous where to put this event.