bcachefs: Split out btree_write_submit_wq

Split the workqueues for btree read completions and btree write
submissions; we don't want concurrency control on btree read
completions, but we do want concurrency control on write submissions,
else blocking in submit_bio() will cause a ton of kworkers to be
allocated.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -790,7 +790,8 @@ struct bch_fs {
 
 	/* BTREE CACHE */
 	struct bio_set		btree_bio;
-	struct workqueue_struct	*io_complete_wq;
+	struct workqueue_struct	*btree_read_complete_wq;
+	struct workqueue_struct	*btree_write_submit_wq;
 
 	struct btree_root	btree_roots_known[BTREE_ID_NR];
 	DARRAY(struct btree_root) btree_roots_extra;

fs/bcachefs/btree_io.c

@@ -1389,7 +1389,7 @@ static void btree_node_read_endio(struct bio *bio)
 		bch2_latency_acct(ca, rb->start_time, READ);
 	}
 
-	queue_work(c->io_complete_wq, &rb->work);
+	queue_work(c->btree_read_complete_wq, &rb->work);
 }
 
 struct btree_node_read_all {
@@ -1656,7 +1656,7 @@ static int btree_node_read_all_replicas(struct bch_fs *c, struct btree *b, bool
 		btree_node_read_all_replicas_done(&ra->cl.work);
 	} else {
 		continue_at(&ra->cl, btree_node_read_all_replicas_done,
-			    c->io_complete_wq);
+			    c->btree_read_complete_wq);
 	}
 
 	return 0;
@@ -1737,7 +1737,7 @@ void bch2_btree_node_read(struct btree_trans *trans, struct btree *b,
 		if (sync)
 			btree_node_read_work(&rb->work);
 		else
-			queue_work(c->io_complete_wq, &rb->work);
+			queue_work(c->btree_read_complete_wq, &rb->work);
 	}
 }
 
@@ -2229,7 +2229,7 @@ void __bch2_btree_node_write(struct bch_fs *c, struct btree *b, unsigned flags)
 	atomic64_add(bytes_to_write, &c->btree_write_stats[type].bytes);
 
 	INIT_WORK(&wbio->work, btree_write_submit);
-	queue_work(c->io_complete_wq, &wbio->work);
+	queue_work(c->btree_write_submit_wq, &wbio->work);
 	return;
 err:
 	set_btree_node_noevict(b);

fs/bcachefs/super.c

@@ -582,8 +582,10 @@ static void __bch2_fs_free(struct bch_fs *c)
 
 	if (c->write_ref_wq)
 		destroy_workqueue(c->write_ref_wq);
-	if (c->io_complete_wq)
-		destroy_workqueue(c->io_complete_wq);
+	if (c->btree_write_submit_wq)
+		destroy_workqueue(c->btree_write_submit_wq);
+	if (c->btree_read_complete_wq)
+		destroy_workqueue(c->btree_read_complete_wq);
 	if (c->copygc_wq)
 		destroy_workqueue(c->copygc_wq);
 	if (c->btree_io_complete_wq)
@@ -878,8 +880,10 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 				WQ_HIGHPRI|WQ_FREEZABLE|WQ_MEM_RECLAIM, 1)) ||
 	    !(c->copygc_wq = alloc_workqueue("bcachefs_copygc",
 				WQ_HIGHPRI|WQ_FREEZABLE|WQ_MEM_RECLAIM|WQ_CPU_INTENSIVE, 1)) ||
-	    !(c->io_complete_wq = alloc_workqueue("bcachefs_io",
+	    !(c->btree_read_complete_wq = alloc_workqueue("bcachefs_btree_read_complete",
 				WQ_HIGHPRI|WQ_FREEZABLE|WQ_MEM_RECLAIM, 512)) ||
+	    !(c->btree_write_submit_wq = alloc_workqueue("bcachefs_btree_write_sumit",
+				WQ_HIGHPRI|WQ_FREEZABLE|WQ_MEM_RECLAIM, 1)) ||
 	    !(c->write_ref_wq = alloc_workqueue("bcachefs_write_ref",
 				WQ_FREEZABLE, 0)) ||
 #ifndef BCH_WRITE_REF_DEBUG