[PATCH RFC] io-wq: handle hashed writes in chains

[Jens Axboe <axboe@xxxxxxxxx>]

RFC patch for now - I've tested it, works for me. Basically it allows
io-wq to grab a whole chain of identically hashed writes, avoiding
hammering on the wqe->lock for detaching hashed work.

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 9541df2729de..d9a50670d47b 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -380,32 +380,44 @@ static inline unsigned int io_get_work_hash(struct io_wq_work *work)
 	return work->flags >> IO_WQ_HASH_SHIFT;
 }
 
-static struct io_wq_work *io_get_next_work(struct io_wqe *wqe)
+static struct io_wq_work *io_get_next_work(struct io_wqe *wqe,
+					   struct io_wq_work_list *list)
 	__must_hold(wqe->lock)
 {
-	struct io_wq_work_node *node, *prev;
-	struct io_wq_work *work;
-	unsigned int hash;
+	struct io_wq_work *work, *ret;
+	unsigned int new_hash, hash = -1U;
 
-	wq_list_for_each(node, prev, &wqe->work_list) {
-		work = container_of(node, struct io_wq_work, list);
+	ret = NULL;
+	while (!wq_list_empty(&wqe->work_list)) {
+		work = container_of(wqe->work_list.first, struct io_wq_work,
+					list);
 
 		/* not hashed, can run anytime */
 		if (!io_wq_is_hashed(work)) {
-			wq_node_del(&wqe->work_list, node, prev);
-			return work;
+			if (hash != -1U)
+				break;
+			wq_node_del(&wqe->work_list, &work->list, NULL);
+			ret = work;
+			break;
 		}
 
 		/* hashed, can run if not already running */
-		hash = io_get_work_hash(work);
-		if (!(wqe->hash_map & BIT(hash))) {
-			wqe->hash_map |= BIT(hash);
-			wq_node_del(&wqe->work_list, node, prev);
-			return work;
-		}
+		new_hash = io_get_work_hash(work);
+		if (hash == -1U)
+			hash = new_hash;
+		else if (hash != new_hash)
+			break;
+
+		wqe->hash_map |= BIT(hash);
+		wq_node_del(&wqe->work_list, &work->list, NULL);
+		/* return first node, add subsequent same hash to the list */
+		if (ret)
+			wq_list_add_tail(&work->list, list);
+		else
+			ret = work;
 	}
 
-	return NULL;
+	return ret;
 }
 
 static void io_wq_switch_mm(struct io_worker *worker, struct io_wq_work *work)
@@ -481,6 +493,7 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work);
 static void io_worker_handle_work(struct io_worker *worker)
 	__releases(wqe->lock)
 {
+	struct io_wq_work_list list = { .first = NULL, .last = NULL };
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
 
@@ -495,7 +508,7 @@ static void io_worker_handle_work(struct io_worker *worker)
 		 * can't make progress, any work completion or insertion will
 		 * clear the stalled flag.
 		 */
-		work = io_get_next_work(wqe);
+		work = io_get_next_work(wqe, &list);
 		if (work)
 			__io_worker_busy(wqe, worker, work);
 		else if (!wq_list_empty(&wqe->work_list))
@@ -504,6 +517,7 @@ static void io_worker_handle_work(struct io_worker *worker)
 		spin_unlock_irq(&wqe->lock);
 		if (!work)
 			break;
+got_work:
 		io_assign_current_work(worker, work);
 
 		/* handle a whole dependent link */
@@ -530,6 +544,14 @@ static void io_worker_handle_work(struct io_worker *worker)
 				work = NULL;
 			}
 			if (hash != -1U) {
+				if (!wq_list_empty(&list)) {
+					work = container_of(list.first,
+							    struct io_wq_work,
+							    list);
+					wq_node_del(&list, &work->list, NULL);
+					goto got_work;
+				}
+
 				spin_lock_irq(&wqe->lock);
 				wqe->hash_map &= ~BIT_ULL(hash);
 				wqe->flags &= ~IO_WQE_FLAG_STALLED;

-- 
Jens Axboe