47 lines
2.1 KiB
Diff
47 lines
2.1 KiB
Diff
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From b63c8d4f0364457b0ead8793504012bb7113974f Mon Sep 17 00:00:00 2001
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From: David Herrmann <dh.herrmann@gmail.com>
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Date: Thu, 10 Jul 2014 00:47:23 +0200
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Subject: [PATCH] sd-event: always call epoll_ctl() on mask-updates if
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edge-triggered
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A call to sd_event_source_set_io_events() skipps calling into the kernel
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if the new event-mask matches the old one. This is safe for
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level-triggered sources as the kernel moves them onto the ready-list
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automatically if events change. However, edge-triggered sources might not
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be on the ready-list even though events are present.
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A call to sd_event_source_set_io_events() with EPOLLET set might thus be
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used to just move the io-source onto the ready-list so the next poll
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will return it again. This is very useful to avoid starvation in
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priority-based event queues.
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Imagine a read() loop on an edge-triggered fd. If we cannot read data fast
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enough to drain the receive queue, we might decide to skip reading for now
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and schedule it for later. On edge-triggered io-sources we have to make
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sure it's put on the ready-list so the next dispatch-round will return it
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again if it's still the highest priority task. We could make sd-event
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handle edge-triggered sources directly and allow marking them ready again.
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However, it's much simpler to let the kernel do that for now via
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EPOLL_CTL_MOD.
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---
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src/libsystemd/sd-event/sd-event.c | 3 ++-
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1 file changed, 2 insertions(+), 1 deletion(-)
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diff --git src/libsystemd/sd-event/sd-event.c src/libsystemd/sd-event/sd-event.c
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index 53f1904..a21f7db 100644
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--- src/libsystemd/sd-event/sd-event.c
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+++ src/libsystemd/sd-event/sd-event.c
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@@ -1282,7 +1282,8 @@ _public_ int sd_event_source_set_io_events(sd_event_source *s, uint32_t events)
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assert_return(s->event->state != SD_EVENT_FINISHED, -ESTALE);
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assert_return(!event_pid_changed(s->event), -ECHILD);
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- if (s->io.events == events)
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+ /* edge-triggered updates are never skipped, so we can reset edges */
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+ if (s->io.events == events && !(events & EPOLLET))
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return 0;
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if (s->enabled != SD_EVENT_OFF) {
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--
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1.7.9.2
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