LTTng » LTTng-tools

Using:
lttng-modules 2.3.0-1 78c8710
lttng-tools 2.3.0-1 c9dc128
lttng-ust 2.3.0-1 9f00ce3
userspace-rcu 0.8.0-1 ca617ca
on:
Ubuntu 12.04.3 LTS precise,
3.2.0-53-virtual kernel

$ sudo -H lttng create mySession --snapshot
Spawning a session daemon
Session mySession created.
Default snapshot output set to: /root/lttng-traces/mySession-20130925-101114
Snapshot mode set. Every channel enabled for that session will be set in overwrite mode and mmap output.
$ sudo -H lttng enable-event -a -k --syscall
All kernel system calls are enabled in channel channel0
$ sudo -H lttng enable-channel channel0 -u --buffers-uid
$ sudo -H lttng enable-event -a -u -c channel0
$ sudo -H lttng start
Tracing started for session mySession
$ sudo -H lttng snapshot record
Snapshot recorded successfully for session mySession

What is bizarre here is that snapshot record will successfully capture the user-space trace if no kernel tracing is going on. But with kernel tracing active, as in this example, only the kernel domain gets committed to storage. Disabling kernel tracing (in this example by disabling the kernel's channel0 channel) does not help.