LTTng » LTTng-tools

Thinking about this bug here, not sure we can make it work without having UST accept multiple filters for a single event. Consider this case here:

$ lttng create test1
$ lttng enable-event -j event1
$ lttng enable-event -j event2

So for JUL, we enable a UST tracepoint wildcard which is "lttng_jul:user*" and "lttng_jul:sys*" for root sessiond. Fixing this issue would mean having a way to "add" filters to an already enabled event in UST. Pseudo code here:

enable UST event "lttng_jul:user*" with filter "name == event1" 
[random wait period]
add filter to "lttng_jul:user*" for "name == event2" 

Hope I'm clear here but the problem in a nutshell is to apply multiple filters to the same UST event. Is UST ready for that or am I failing to understand what we are trying to do here?

I just looked at the UST code and enabling the same event twice should work.

By the way, there is another aspect to this issue: enabling different loglevels for different sessions.

Let's suppose we have events:

event1 -> prio high
event2 -> prio low

In SessionA, we enable high prio JUL events only. In SessionB, we enable low (and high) prio loglevel events.

I'm pretty sure the result currently is that SessionA will be polluted by low prio events, because there is no filtering on a per-session basis based on the JUL loglevel within lttng-ust. We should use the lttng-ust filters to filter by JUL loglevel too, in addition to filtering by JUL logger name.