Fix: notification: kernel: consumption of event notification stalls

Observed issue
==============

Using:

 lttng add-trigger --condition event-rule-matches --type kernel:tracepoint --name "sched_waking" --capture comm --action notify

The sessiond receives multiple event notifications from the kernel event
source then stop receiving despite the kernel event source buffer
being full.

Cause
=====

It turns out that the kernel event source, when reaching near the end of
its buffer capacity, raises the POLLPRI [1] flag and not the POLLIN
flag.

Solution
========

lttng-modules stretches a bit the usage of POLLPRI as defined by the man
page (man 2 poll):

 There is some exceptional condition on the  file  descriptor. Possibilities
 include:

 *  There is out-of-band data on a TCP socket (see tcp(7)).

 *  A  pseudoterminal  master  in  packet  mode has seen a state change on the
    slave (see ioctl_tty(2)).

 *  A cgroup.events file has been modified (see cgroups(7)).

Still, even if lttng-modules changes how it does things, lttng-tools
needs to support other lttng-modules versions.

Thus, add LPOLLPRI (EPOLLPRI/POLLPRI) to the event mask when dealing
with notification event sources.

Note
=====

In the future, during the poll loop we could also prioritize
event sources in POLLPRI 'state'.

Known drawbacks
=========

None.

References
==========

[1] https://github.com/lttng/lttng-modules/blob/c312bda00d2dc10ce5f6c1189acbefee5c6c8c6c/src/lttng-abi.c#L1169

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ieb428ef1037c8eb197b489a38a1ae5216ac63d4b

diff --git a/src/bin/lttng-sessiond/notification-thread-events.cpp b/src/bin/lttng-sessiond/notification-thread-events.cpp
index 48ba7a00bccf30da93cfd02b560433da32179239..279bc7c5a26aa797c08d800b38b16ae98aa04627 100644 (file)
--- a/src/bin/lttng-sessiond/notification-thread-events.cpp
+++ b/src/bin/lttng-sessiond/notification-thread-events.cpp
@@ -2049,7 +2049,7 @@ int handle_notification_thread_command_add_tracer_event_source(
                        lttng_domain_type_str(domain_type));
 
        /* Adding the read side pipe to the event poll. */
-       ret = lttng_poll_add(&state->events, tracer_event_source_fd, LPOLLIN | LPOLLERR);
+       ret = lttng_poll_add(&state->events, tracer_event_source_fd, LPOLLPRI | LPOLLIN | LPOLLERR);
        if (ret < 0) {
                ERR("Failed to add tracer event source to poll set: tracer_event_source_fd = %d, domain = '%s'",
                                tracer_event_source_fd,