diff options
| author | David Herrmann <dh.herrmann@gmail.com> | 2014-07-10 00:47:23 +0200 |
|---|---|---|
| committer | David Herrmann <dh.herrmann@gmail.com> | 2014-07-11 16:43:53 +0200 |
| commit | b63c8d4f0364457b0ead8793504012bb7113974f (patch) | |
| tree | fc5ec3e4181b72d7f9bee68d953679d2858ce024 /src/libsystemd | |
| parent | 1ca5fd003f5d3ffb32db581cb8d82d0721b5b12d (diff) | |
sd-event: always call epoll_ctl() on mask-updates if edge-triggered
A call to sd_event_source_set_io_events() skipps calling into the kernel
if the new event-mask matches the old one. This is safe for
level-triggered sources as the kernel moves them onto the ready-list
automatically if events change. However, edge-triggered sources might not
be on the ready-list even though events are present.
A call to sd_event_source_set_io_events() with EPOLLET set might thus be
used to just move the io-source onto the ready-list so the next poll
will return it again. This is very useful to avoid starvation in
priority-based event queues.
Imagine a read() loop on an edge-triggered fd. If we cannot read data fast
enough to drain the receive queue, we might decide to skip reading for now
and schedule it for later. On edge-triggered io-sources we have to make
sure it's put on the ready-list so the next dispatch-round will return it
again if it's still the highest priority task. We could make sd-event
handle edge-triggered sources directly and allow marking them ready again.
However, it's much simpler to let the kernel do that for now via
EPOLL_CTL_MOD.
Diffstat (limited to 'src/libsystemd')
| -rw-r--r-- | src/libsystemd/sd-event/sd-event.c | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/src/libsystemd/sd-event/sd-event.c b/src/libsystemd/sd-event/sd-event.c index 53f1904d3d..a21f7db8eb 100644 --- a/src/libsystemd/sd-event/sd-event.c +++ b/src/libsystemd/sd-event/sd-event.c @@ -1282,7 +1282,8 @@ _public_ int sd_event_source_set_io_events(sd_event_source *s, uint32_t events) assert_return(s->event->state != SD_EVENT_FINISHED, -ESTALE); assert_return(!event_pid_changed(s->event), -ECHILD); - if (s->io.events == events) + /* edge-triggered updates are never skipped, so we can reset edges */ + if (s->io.events == events && !(events & EPOLLET)) return 0; if (s->enabled != SD_EVENT_OFF) { |