btrfs: async_thread: Fix workqueue 'max_active' value when initializing

At initializing time, for threshold-able workqueue, it's max_active
of kernel workqueue should be 1 and grow if it hits threshold.

But due to the bad naming, there is both 'max_active' for kernel
workqueue and btrfs workqueue.
So wrong value is given at workqueue initialization.

This patch fixes it, and to avoid further misunderstanding, change the
member name of btrfs_workqueue to 'current_active' and 'limit_active'.

Also corresponding comment is added for readability.

Reported-by: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

Author: Qu Wenruo

fs/btrfs/async-thread.c

@@ -42,8 +42,14 @@ struct __btrfs_workqueue {
 
 	/* Thresholding related variants */
 	atomic_t pending;
-	int max_active;
-	int current_max;
+
+	/* Up limit of concurrency workers */
+	int limit_active;
+
+	/* Current number of concurrency workers */
+	int current_active;
+
+	/* Threshold to change current_active */
 	int thresh;
 	unsigned int count;
 	spinlock_t thres_lock;
@@ -88,34 +94,39 @@ BTRFS_WORK_HELPER(scrubnc_helper);
 BTRFS_WORK_HELPER(scrubparity_helper);
 
 static struct __btrfs_workqueue *
-__btrfs_alloc_workqueue(const char *name, unsigned int flags, int max_active,
+__btrfs_alloc_workqueue(const char *name, unsigned int flags, int limit_active,
 			 int thresh)
 {
 	struct __btrfs_workqueue *ret = kzalloc(sizeof(*ret), GFP_NOFS);
 
 	if (!ret)
 		return NULL;
 
-	ret->max_active = max_active;
+	ret->limit_active = limit_active;
 	atomic_set(&ret->pending, 0);
 	if (thresh == 0)
 		thresh = DFT_THRESHOLD;
 	/* For low threshold, disabling threshold is a better choice */
 	if (thresh < DFT_THRESHOLD) {
-		ret->current_max = max_active;
+		ret->current_active = limit_active;
 		ret->thresh = NO_THRESHOLD;
 	} else {
-		ret->current_max = 1;
+		/*
+		 * For threshold-able wq, let its concurrency grow on demand.
+		 * Use minimal max_active at alloc time to reduce resource
+		 * usage.
+		 */
+		ret->current_active = 1;
 		ret->thresh = thresh;
 	}
 
 	if (flags & WQ_HIGHPRI)
 		ret->normal_wq = alloc_workqueue("%s-%s-high", flags,
-						 ret->max_active,
-						 "btrfs", name);
+						 ret->current_active, "btrfs",
+						 name);
 	else
 		ret->normal_wq = alloc_workqueue("%s-%s", flags,
-						 ret->max_active, "btrfs",
+						 ret->current_active, "btrfs",
 						 name);
 	if (!ret->normal_wq) {
 		kfree(ret);
@@ -134,7 +145,7 @@ __btrfs_destroy_workqueue(struct __btrfs_workqueue *wq);
 
 struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name,
 					      unsigned int flags,
-					      int max_active,
+					      int limit_active,
 					      int thresh)
 {
 	struct btrfs_workqueue *ret = kzalloc(sizeof(*ret), GFP_NOFS);
@@ -143,14 +154,14 @@ struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name,
 		return NULL;
 
 	ret->normal = __btrfs_alloc_workqueue(name, flags & ~WQ_HIGHPRI,
-					      max_active, thresh);
+					      limit_active, thresh);
 	if (!ret->normal) {
 		kfree(ret);
 		return NULL;
 	}
 
 	if (flags & WQ_HIGHPRI) {
-		ret->high = __btrfs_alloc_workqueue(name, flags, max_active,
+		ret->high = __btrfs_alloc_workqueue(name, flags, limit_active,
 						    thresh);
 		if (!ret->high) {
 			__btrfs_destroy_workqueue(ret->normal);
@@ -180,7 +191,7 @@ static inline void thresh_queue_hook(struct __btrfs_workqueue *wq)
  */
 static inline void thresh_exec_hook(struct __btrfs_workqueue *wq)
 {
-	int new_max_active;
+	int new_current_active;
 	long pending;
 	int need_change = 0;
 
@@ -197,27 +208,27 @@ static inline void thresh_exec_hook(struct __btrfs_workqueue *wq)
 	wq->count %= (wq->thresh / 4);
 	if (!wq->count)
 		goto  out;
-	new_max_active = wq->current_max;
+	new_current_active = wq->current_active;
 
 	/*
 	 * pending may be changed later, but it's OK since we really
 	 * don't need it so accurate to calculate new_max_active.
 	 */
 	pending = atomic_read(&wq->pending);
 	if (pending > wq->thresh)
-		new_max_active++;
+		new_current_active++;
 	if (pending < wq->thresh / 2)
-		new_max_active--;
-	new_max_active = clamp_val(new_max_active, 1, wq->max_active);
-	if (new_max_active != wq->current_max)  {
+		new_current_active--;
+	new_current_active = clamp_val(new_current_active, 1, wq->limit_active);
+	if (new_current_active != wq->current_active)  {
 		need_change = 1;
-		wq->current_max = new_max_active;
+		wq->current_active = new_current_active;
 	}
 out:
 	spin_unlock(&wq->thres_lock);
 
 	if (need_change) {
-		workqueue_set_max_active(wq->normal_wq, wq->current_max);
+		workqueue_set_max_active(wq->normal_wq, wq->current_active);
 	}
 }
 
@@ -351,13 +362,13 @@ void btrfs_destroy_workqueue(struct btrfs_workqueue *wq)
 	kfree(wq);
 }
 
-void btrfs_workqueue_set_max(struct btrfs_workqueue *wq, int max)
+void btrfs_workqueue_set_max(struct btrfs_workqueue *wq, int limit_active)
 {
 	if (!wq)
 		return;
-	wq->normal->max_active = max;
+	wq->normal->limit_active = limit_active;
 	if (wq->high)
-		wq->high->max_active = max;
+		wq->high->limit_active = limit_active;
 }
 
 void btrfs_set_work_high_priority(struct btrfs_work *work)

fs/btrfs/async-thread.h

@@ -69,7 +69,7 @@ BTRFS_WORK_HELPER_PROTO(scrubparity_helper);
 
 struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name,
 					      unsigned int flags,
-					      int max_active,
+					      int limit_active,
 					      int thresh);
 void btrfs_init_work(struct btrfs_work *work, btrfs_work_func_t helper,
 		     btrfs_func_t func,