Linux: Cleanup taskq threads spawn/exit

This changes taskq_thread_should_stop() to limit maximum exit rate
for idle threads to one per 5 seconds.  I believe the previous one
was broken, not allowing any thread exits for tasks arriving more
than one at a time and so completing while others are running.

Also while there:
 - Remove taskq_thread_spawn() calls on task allocation errors.
 - Remove extra taskq_thread_should_stop() call.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#15873

include/os/linux/spl/sys/taskq.h

@@ -104,7 +104,7 @@ typedef struct taskq {
 	/* list node for the cpu hotplug callback */
 	struct hlist_node	tq_hp_cb_node;
 	boolean_t		tq_hp_support;
-	unsigned long		lastshouldstop; /* when to purge dynamic */
+	unsigned long		lastspawnstop;	/* when to purge dynamic */
 } taskq_t;
 
 typedef struct taskq_ent {

man/man4/spl.4

@@ -186,18 +186,8 @@ reading it could cause a lock-up if the list grow too large
 without limiting the output.
 "(truncated)" will be shown if the list is larger than the limit.
 .
-.It Sy spl_taskq_thread_timeout_ms Ns = Ns Sy 10000 Pq uint
-(Linux-only)
-How long a taskq has to have had no work before we tear it down.
-Previously, we would tear down a dynamic taskq worker as soon
-as we noticed it had no work, but it was observed that this led
-to a lot of churn in tearing down things we then immediately
-spawned anew.
-In practice, it seems any nonzero value will remove the vast
-majority of this churn, while the nontrivially larger value
-was chosen to help filter out the little remaining churn on
-a mostly idle system.
-Setting this value to
-.Sy 0
-will revert to the previous behavior.
+.It Sy spl_taskq_thread_timeout_ms Ns = Ns Sy 5000 Pq uint
+Minimum idle threads exit interval for dynamic taskqs.
+Smaller values allow idle threads exit more often and potentially be
+respawned again on demand, causing more churn.
 .El

module/os/linux/spl/spl-taskq.c

@@ -36,12 +36,12 @@ static int spl_taskq_thread_bind = 0;
 module_param(spl_taskq_thread_bind, int, 0644);
 MODULE_PARM_DESC(spl_taskq_thread_bind, "Bind taskq thread to CPU by default");
 
-static uint_t spl_taskq_thread_timeout_ms = 10000;
+static uint_t spl_taskq_thread_timeout_ms = 5000;
 /* BEGIN CSTYLED */
 module_param(spl_taskq_thread_timeout_ms, uint, 0644);
 /* END CSTYLED */
 MODULE_PARM_DESC(spl_taskq_thread_timeout_ms,
-	"Time to require a dynamic thread be idle before it gets cleaned up");
+	"Minimum idle threads exit interval for dynamic taskqs");
 
 static int spl_taskq_thread_dynamic = 1;
 module_param(spl_taskq_thread_dynamic, int, 0444);
@@ -594,8 +594,7 @@ taskq_dispatch(taskq_t *tq, task_func_t func, void *arg, uint_t flags)
 	ASSERT(tq->tq_nactive <= tq->tq_nthreads);
 	if ((flags & TQ_NOQUEUE) && (tq->tq_nactive == tq->tq_nthreads)) {
 		/* Dynamic taskq may be able to spawn another thread */
-		if (!(tq->tq_flags & TASKQ_DYNAMIC) ||
-		    taskq_thread_spawn(tq) == 0)
+		if (taskq_thread_spawn(tq) == 0)
 			goto out;
 	}
 
@@ -629,11 +628,11 @@ taskq_dispatch(taskq_t *tq, task_func_t func, void *arg, uint_t flags)
 	spin_unlock(&t->tqent_lock);
 
 	wake_up(&tq->tq_work_waitq);
-out:
+
 	/* Spawn additional taskq threads if required. */
 	if (!(flags & TQ_NOQUEUE) && tq->tq_nactive == tq->tq_nthreads)
 		(void) taskq_thread_spawn(tq);
-
+out:
 	spin_unlock_irqrestore(&tq->tq_lock, irqflags);
 	return (rc);
 }
@@ -676,10 +675,11 @@ taskq_dispatch_delay(taskq_t *tq, task_func_t func, void *arg,
 	ASSERT(!(t->tqent_flags & TQENT_FLAG_PREALLOC));
 
 	spin_unlock(&t->tqent_lock);
-out:
+
 	/* Spawn additional taskq threads if required. */
 	if (tq->tq_nactive == tq->tq_nthreads)
 		(void) taskq_thread_spawn(tq);
+out:
 	spin_unlock_irqrestore(&tq->tq_lock, irqflags);
 	return (rc);
 }
@@ -704,9 +704,8 @@ taskq_dispatch_ent(taskq_t *tq, task_func_t func, void *arg, uint_t flags,
 
 	if ((flags & TQ_NOQUEUE) && (tq->tq_nactive == tq->tq_nthreads)) {
 		/* Dynamic taskq may be able to spawn another thread */
-		if (!(tq->tq_flags & TASKQ_DYNAMIC) ||
-		    taskq_thread_spawn(tq) == 0)
-			goto out2;
+		if (taskq_thread_spawn(tq) == 0)
+			goto out;
 		flags |= TQ_FRONT;
 	}
 
@@ -742,11 +741,11 @@ taskq_dispatch_ent(taskq_t *tq, task_func_t func, void *arg, uint_t flags,
 	spin_unlock(&t->tqent_lock);
 
 	wake_up(&tq->tq_work_waitq);
-out:
+
 	/* Spawn additional taskq threads if required. */
 	if (tq->tq_nactive == tq->tq_nthreads)
 		(void) taskq_thread_spawn(tq);
-out2:
+out:
 	spin_unlock_irqrestore(&tq->tq_lock, irqflags);
 }
 EXPORT_SYMBOL(taskq_dispatch_ent);
@@ -825,6 +824,7 @@ taskq_thread_spawn(taskq_t *tq)
 	if (!(tq->tq_flags & TASKQ_DYNAMIC))
 		return (0);
 
+	tq->lastspawnstop = jiffies;
 	if ((tq->tq_nthreads + tq->tq_nspawn < tq->tq_maxthreads) &&
 	    (tq->tq_flags & TASKQ_ACTIVE)) {
 		spawning = (++tq->tq_nspawn);
@@ -836,9 +836,9 @@ taskq_thread_spawn(taskq_t *tq)
 }
 
 /*
- * Threads in a dynamic taskq should only exit once it has been completely
- * drained and no other threads are actively servicing tasks.  This prevents
- * threads from being created and destroyed more than is required.
+ * Threads in a dynamic taskq may exit once there is no more work to do.
+ * To prevent threads from being created and destroyed too often limit
+ * the exit rate to one per spl_taskq_thread_timeout_ms.
  *
  * The first thread is the thread list is treated as the primary thread.
  * There is nothing special about the primary thread but in order to avoid
@@ -847,44 +847,22 @@ taskq_thread_spawn(taskq_t *tq)
 static int
 taskq_thread_should_stop(taskq_t *tq, taskq_thread_t *tqt)
 {
-	if (!(tq->tq_flags & TASKQ_DYNAMIC))
+	ASSERT(!taskq_next_ent(tq));
+	if (!(tq->tq_flags & TASKQ_DYNAMIC) || !spl_taskq_thread_dynamic)
 		return (0);
-
+	if (!(tq->tq_flags & TASKQ_ACTIVE))
+		return (1);
 	if (list_first_entry(&(tq->tq_thread_list), taskq_thread_t,
 	    tqt_thread_list) == tqt)
 		return (0);
-
-	int no_work =
-	    ((tq->tq_nspawn == 0) &&	/* No threads are being spawned */
-	    (tq->tq_nactive == 0) &&	/* No threads are handling tasks */
-	    (tq->tq_nthreads > 1) &&	/* More than 1 thread is running */
-	    (!taskq_next_ent(tq)) &&	/* There are no pending tasks */
-	    (spl_taskq_thread_dynamic)); /* Dynamic taskqs are allowed */
-
-	/*
-	 * If we would have said stop before, let's instead wait a bit, maybe
-	 * we'll see more work come our way soon...
-	 */
-	if (no_work) {
-		/* if it's 0, we want the old behavior. */
-		/* if the taskq is being torn down, we also want to go away. */
-		if (spl_taskq_thread_timeout_ms == 0 ||
-		    !(tq->tq_flags & TASKQ_ACTIVE))
-			return (1);
-		unsigned long lasttime = tq->lastshouldstop;
-		if (lasttime > 0) {
-			if (time_after(jiffies, lasttime +
-			    msecs_to_jiffies(spl_taskq_thread_timeout_ms)))
-				return (1);
-			else
-				return (0);
-		} else {
-			tq->lastshouldstop = jiffies;
-		}
-	} else {
-		tq->lastshouldstop = 0;
-	}
-	return (0);
+	ASSERT3U(tq->tq_nthreads, >, 1);
+	if (tq->tq_nspawn != 0)
+		return (0);
+	if (time_before(jiffies, tq->lastspawnstop +
+	    msecs_to_jiffies(spl_taskq_thread_timeout_ms)))
+		return (0);
+	tq->lastspawnstop = jiffies;
+	return (1);
 }
 
 static int
@@ -935,10 +913,8 @@ taskq_thread(void *args)
 		if (list_empty(&tq->tq_pend_list) &&
 		    list_empty(&tq->tq_prio_list)) {
 
-			if (taskq_thread_should_stop(tq, tqt)) {
-				wake_up_all(&tq->tq_wait_waitq);
+			if (taskq_thread_should_stop(tq, tqt))
 				break;
-			}
 
 			add_wait_queue_exclusive(&tq->tq_work_waitq, &wait);
 			spin_unlock_irqrestore(&tq->tq_lock, flags);
@@ -1013,9 +989,6 @@ taskq_thread(void *args)
 			tqt->tqt_id = TASKQID_INVALID;
 			tqt->tqt_flags = 0;
 			wake_up_all(&tq->tq_wait_waitq);
-		} else {
-			if (taskq_thread_should_stop(tq, tqt))
-				break;
 		}
 
 		set_current_state(TASK_INTERRUPTIBLE);
@@ -1122,7 +1095,7 @@ taskq_create(const char *name, int threads_arg, pri_t pri,
 	tq->tq_flags = (flags | TASKQ_ACTIVE);
 	tq->tq_next_id = TASKQID_INITIAL;
 	tq->tq_lowest_id = TASKQID_INITIAL;
-	tq->lastshouldstop = 0;
+	tq->lastspawnstop = jiffies;
 	INIT_LIST_HEAD(&tq->tq_free_list);
 	INIT_LIST_HEAD(&tq->tq_pend_list);
 	INIT_LIST_HEAD(&tq->tq_prio_list);