Hrm, tried this on an s390x VM:

$ uname -a
Linux ckingvm2 4.4.0-101-generic #124-Ubuntu SMP Fri Nov 10 18:29:39 UTC 2017 s390x s390x s390x GNU/Linux

dpkg -l | grep lttng
ii  liblttng-ctl0:s390x              2.7.1-2ubuntu1                             s390x        LTTng control and utility library
ii  liblttng-ust-ctl2:s390x          2.7.1-1                                    s390x        LTTng 2.0 Userspace Tracer (trace control library)
ii  liblttng-ust-dev:s390x           2.7.1-1                                    s390x        LTTng 2.0 Userspace Tracer (development files)
ii  liblttng-ust-python-agent0:s390x 2.7.1-1                                    s390x        LTTng 2.0 Userspace Tracer (Python agent native library)
ii  liblttng-ust0:s390x              2.7.1-1                                    s390x        LTTng 2.0 Userspace Tracer (tracing libraries)
ii  lttng-modules-dkms               2.8.0-1ubuntu1~16.04.3                     all          Linux Trace Toolkit (LTTng) kernel modules (DKMS)
ii  lttng-tools                      2.7.1-2ubuntu1                             s390x        LTTng control and utility programs

ubuntu@ckingvm2:~$ sudo autotest/client/autotest-local autotest/client/tests/ubuntu_lttng_smoke_test/control
11:29:35 INFO | Writing results to /home/ubuntu/autotest/client/results/default
11:29:35 INFO | START	----	----	timestamp=1510918175	localtime=Nov 17 11:29:35	
11:29:35 INFO | 	START	ubuntu_lttng_smoke_test.lttng-smoke-test	ubuntu_lttng_smoke_test.lttng-smoke-test	timestamp=1510918175	localtime=Nov 17 11:29:35	
11:29:40 ERROR| [stderr] [warning] Tracer discarded 1340 events between [11:29:38.065061128] and [11:29:38.150240071] in trace UUID 6614a11b486b459bce703191741816, at path: "/tmp/lttng-kernel-trace-1494-session/kernel", within stream id 0, at relative path: "channel0_0". You should consider recording a new trace with larger buffers or with fewer events enabled.
11:29:40 INFO | == lttng smoke test of session create/destroy ==
11:29:40 INFO | Session test-kernel-session created.
11:29:40 INFO | Traces will be written in /tmp/lttng-kernel-trace-1494-session
11:29:40 INFO | PASSED (lttng create)
11:29:40 INFO | Session test-kernel-session destroyed
11:29:40 INFO | PASSED (lttng destroy)
11:29:40 INFO |  
11:29:40 INFO | == lttng smoke test list kernel events ==
11:29:40 INFO | PASSED (lttng list --kernel)
11:29:40 INFO |  
11:29:40 INFO | lttng smoke test trace open/close system calls SKIPPED for s390x
11:29:40 INFO |  
11:29:40 INFO | == lttng smoke test trace context switches ==
11:29:40 INFO | Session test-kernel-session created.
11:29:40 INFO | Traces will be written in /tmp/lttng-kernel-trace-1494-session
11:29:40 INFO | PASSED (lttng create)
11:29:40 INFO | Kernel event sched_switch created in channel channel0
11:29:40 INFO | PASSED (lttng enable-event)
11:29:40 INFO | Tracing started for session test-kernel-session
11:29:40 INFO | PASSED (lttng start)
11:29:40 INFO | Waiting for data availability.
11:29:40 INFO | Tracing stopped for session test-kernel-session
11:29:40 INFO | PASSED (lttng stop)
11:29:40 INFO | Session test-kernel-session destroyed
11:29:40 INFO | PASSED (lttng destroy)
11:29:40 INFO | Found 19932 dd and 44645 context switches
11:29:40 INFO | PASSED (simple system call tracing with babeltrace)
11:29:40 INFO |  
11:29:40 INFO | Summary: 8 passed, 0 failed
11:29:40 INFO | 		GOOD	ubuntu_lttng_smoke_test.lttng-smoke-test	ubuntu_lttng_smoke_test.lttng-smoke-test	timestamp=1510918180	localtime=Nov 17 11:29:40	completed successfully
11:29:40 INFO | 	END GOOD	ubuntu_lttng_smoke_test.lttng-smoke-test	ubuntu_lttng_smoke_test.lttng-smoke-test	timestamp=1510918180	localtime=Nov 17 11:29:40	
11:29:40 INFO | END GOOD	----	----	timestamp=1510918180	localtime=Nov 17 11:29:40	
11:29:40 INFO | Report successfully generated at /home/ubuntu/autotest/client/results/default/job_report.html

I just can't seem reproduce this issue.