From: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Date: Fri, 7 Dec 2018 22:06:39 +0000 (-0500)
Subject: Fix: only wait if work queue is empty in real-time mode
X-Git-Tag: v0.10.2~1
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=803e59362b6d94edff3a36f47a9da3e8dca4bde2;p=userspace-rcu.git

Fix: only wait if work queue is empty in real-time mode

Unconditionally waiting for 10 ms after the completion of every batch
of jobs of the work queue in real-time mode appears to be a behaviour
inherited from the call-rcu thread.

While this is a fair trade-off in the context of call-rcu, it is less
evident that it is desirable in the context of a general-purpose
work queue. Waiting when work is available artificially degrades the
latency characteristics of the work queue.

If a workqueue user even need the explicit delay for batching (e.g. if
a call-rcu implementation would ever use the workqueue worker thread),
it can add it within the worker_before_wait_fct callback received as
argument from workqueue creation.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/src/workqueue.c b/src/workqueue.c
index 39d0e07..14957a0 100644
--- a/src/workqueue.c
+++ b/src/workqueue.c
@@ -246,7 +246,10 @@ static void *workqueue_thread(void *arg)
 				cmm_smp_mb();
 			}
 		} else {
-			(void) poll(NULL, 0, 10);
+			if (cds_wfcq_empty(&workqueue->cbs_head,
+					&workqueue->cbs_tail)) {
+				(void) poll(NULL, 0, 10);
+			}
 		}
 		if (workqueue->worker_after_wake_up_fct)
 			workqueue->worker_after_wake_up_fct(workqueue, workqueue->priv);