[PATCH 10/11] vinyl: implement quota wait queue without fiber_cond

[Vladimir Davydov <vdavydov.dev@gmail.com>]

In the next patch we will need to split the queue of fibers waiting for
quota in two: one for high priority consumers and the other for low
priority ones. To balance wake-ups between the two queues, we'll need to
attach some additional info to fibers waiting on the queue (timestamp).
With fiber_cond it is impossible to do. So let's rewrite the quota wait
queue using plain rlist and fiber_yield.
---
 src/box/vy_quota.c | 47 ++++++++++++++++++++++++++++++++++++-----------
 src/box/vy_quota.h | 17 +++++++++++++----
 2 files changed, 49 insertions(+), 15 deletions(-)

diff --git a/src/box/vy_quota.c b/src/box/vy_quota.c
index 18cf4f35..3374aacc 100644
--- a/src/box/vy_quota.c
+++ b/src/box/vy_quota.c
@@ -34,11 +34,11 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <math.h>
+#include <small/rlist.h>
 #include <tarantool_ev.h>
 
 #include "diag.h"
 #include "fiber.h"
-#include "fiber_cond.h"
 #include "say.h"
 #include "histogram.h"
 #include "trivia/util.h"
@@ -141,6 +141,21 @@ vy_quota_check_watermark(struct vy_quota *q)
 		vy_quota_trigger_dump(q);
 }
 
+/**
+ * Wake up the next consumer in the line waiting for quota.
+ */
+static void
+vy_quota_signal(struct vy_quota *q)
+{
+	if (!rlist_empty(&q->wait_queue)) {
+		struct vy_quota_wait_node *n;
+		n = rlist_first_entry(&q->wait_queue,
+				      struct vy_quota_wait_node,
+				      in_wait_queue);
+		fiber_wakeup(n->fiber);
+	}
+}
+
 static void
 vy_quota_timer_cb(ev_loop *loop, ev_timer *timer, int events)
 {
@@ -179,7 +194,7 @@ vy_quota_timer_cb(ev_loop *loop, ev_timer *timer, int events)
 	 * Replenish quota and wake up throttled fibers, if any.
 	 */
 	vy_quota_rl_refill(&q->rl);
-	fiber_cond_signal(&q->cond);
+	vy_quota_signal(q);
 }
 
 int
@@ -213,7 +228,7 @@ vy_quota_create(struct vy_quota *q, size_t limit,
 	q->too_long_threshold = TIMEOUT_INFINITY;
 	q->dump_bw = VY_DEFAULT_DUMP_BANDWIDTH;
 	q->trigger_dump_cb = trigger_dump_cb;
-	fiber_cond_create(&q->cond);
+	rlist_create(&q->wait_queue);
 	ev_timer_init(&q->timer, vy_quota_timer_cb, 0,
 		      VY_QUOTA_UPDATE_INTERVAL);
 	q->timer.data = q;
@@ -237,8 +252,6 @@ vy_quota_destroy(struct vy_quota *q)
 {
 	ev_timer_stop(loop(), &q->timer);
 	histogram_delete(q->dump_bw_hist);
-	fiber_cond_broadcast(&q->cond);
-	fiber_cond_destroy(&q->cond);
 }
 
 void
@@ -246,7 +259,7 @@ vy_quota_set_limit(struct vy_quota *q, size_t limit)
 {
 	q->limit = q->watermark = limit;
 	vy_quota_check_watermark(q);
-	fiber_cond_signal(&q->cond);
+	vy_quota_signal(q);
 }
 
 void
@@ -260,7 +273,7 @@ vy_quota_dump(struct vy_quota *q, size_t size, double duration)
 	 */
 	assert(q->used >= size);
 	q->used -= size;
-	fiber_cond_signal(&q->cond);
+	vy_quota_signal(q);
 
 	/*
 	 * Update dump bandwidth.
@@ -340,8 +353,20 @@ vy_quota_use(struct vy_quota *q, size_t size, double timeout)
 		do {
 			if (q->used + size > q->limit)
 				vy_quota_trigger_dump(q);
-			if (fiber_cond_wait_deadline(&q->cond, deadline) != 0)
-				return -1; /* timed out */
+
+			double now = ev_monotonic_now(loop());
+			if (now >= deadline) {
+				diag_set(TimedOut);
+				return -1;
+			}
+
+			struct vy_quota_wait_node node = {
+				.fiber = fiber(),
+			};
+			rlist_add_tail_entry(&q->wait_queue,
+					     &node, in_wait_queue);
+			fiber_yield_timeout(deadline - now);
+			rlist_del_entry(&node, in_wait_queue);
 		} while (!vy_quota_may_use(q, size));
 
 		double wait_time = ev_monotonic_now(loop()) - start_time;
@@ -353,7 +378,7 @@ vy_quota_use(struct vy_quota *q, size_t size, double timeout)
 		 * Wake up the next fiber in the line waiting
 		 * for quota.
 		 */
-		fiber_cond_signal(&q->cond);
+		vy_quota_signal(q);
 	}
 	vy_quota_force_use(q, size);
 	return 0;
@@ -367,7 +392,7 @@ vy_quota_unuse(struct vy_quota *q, size_t size)
 	/* use_curr could have been reset on timeout. */
 	q->use_curr -= MIN(q->use_curr, size);
 	vy_quota_rl_unuse(&q->rl, size);
-	fiber_cond_signal(&q->cond);
+	vy_quota_signal(q);
 }
 
 void
diff --git a/src/box/vy_quota.h b/src/box/vy_quota.h
index 3070ff82..f53bb8a9 100644
--- a/src/box/vy_quota.h
+++ b/src/box/vy_quota.h
@@ -33,8 +33,8 @@
 
 #include <stdbool.h>
 #include <stddef.h>
+#include <small/rlist.h>
 #include <tarantool_ev.h>
-#include "fiber_cond.h"
 
 #if defined(__cplusplus)
 extern "C" {
@@ -42,6 +42,7 @@ extern "C" {
 
 struct vy_quota;
 struct histogram;
+struct fiber;
 
 typedef int
 (*vy_quota_trigger_dump_f)(struct vy_quota *quota);
@@ -60,6 +61,13 @@ struct vy_quota_rl {
 	ssize_t value;
 };
 
+struct vy_quota_wait_node {
+	/** Link in vy_quota::wait_queue. */
+	struct rlist in_wait_queue;
+	/** Fiber waiting for quota. */
+	struct fiber *fiber;
+};
+
 /**
  * Quota used for accounting and limiting memory consumption
  * in the vinyl engine. It is NOT multi-threading safe.
@@ -86,10 +94,11 @@ struct vy_quota {
 	 */
 	double too_long_threshold;
 	/**
-	 * Condition variable used for throttling consumers when
-	 * there is no quota left.
+	 * Queue of consumers waiting for quota, linked by
+	 * vy_quota_wait_node::state. Newcomers are added
+	 * to the tail.
 	 */
-	struct fiber_cond cond;
+	struct rlist wait_queue;
 	/**
 	 * Timer used for updating average use rate, calculating
 	 * quota watermark and refilling rate limit value.
-- 
2.11.0