block: make blkcg_punt_bio_submit optional

Guard all the code to punt bios to a per-cgroup submission helper by a
new CONFIG_BLK_CGROUP_PUNT_BIO symbol that is selected by btrfs.
This way non-btrfs kernel builds don't need to have this code.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

block/Kconfig

@@ -41,6 +41,9 @@ config BLK_RQ_ALLOC_TIME
 config BLK_CGROUP_RWSTAT
 	bool
 
+config BLK_CGROUP_PUNT_BIO
+	bool
+
 config BLK_DEV_BSG_COMMON
 	tristate
 

block/blk-cgroup.c

@@ -56,7 +56,6 @@ static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];
 static LIST_HEAD(all_blkcgs);		/* protected by blkcg_pol_mutex */
 
 bool blkcg_debug_stats = false;
-static struct workqueue_struct *blkcg_punt_bio_wq;
 
 #define BLKG_DESTROY_BATCH_SIZE  64
 
@@ -166,7 +165,9 @@ static void __blkg_release(struct rcu_head *rcu)
 {
 	struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);
 
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
 	WARN_ON(!bio_list_empty(&blkg->async_bios));
+#endif
 
 	/* release the blkcg and parent blkg refs this blkg has been holding */
 	css_put(&blkg->blkcg->css);
@@ -188,6 +189,9 @@ static void blkg_release(struct percpu_ref *ref)
 	call_rcu(&blkg->rcu_head, __blkg_release);
 }
 
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+static struct workqueue_struct *blkcg_punt_bio_wq;
+
 static void blkg_async_bio_workfn(struct work_struct *work)
 {
 	struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
@@ -214,6 +218,40 @@ static void blkg_async_bio_workfn(struct work_struct *work)
 		blk_finish_plug(&plug);
 }
 
+/*
+ * When a shared kthread issues a bio for a cgroup, doing so synchronously can
+ * lead to priority inversions as the kthread can be trapped waiting for that
+ * cgroup.  Use this helper instead of submit_bio to punt the actual issuing to
+ * a dedicated per-blkcg work item to avoid such priority inversions.
+ */
+void blkcg_punt_bio_submit(struct bio *bio)
+{
+	struct blkcg_gq *blkg = bio->bi_blkg;
+
+	if (blkg->parent) {
+		spin_lock(&blkg->async_bio_lock);
+		bio_list_add(&blkg->async_bios, bio);
+		spin_unlock(&blkg->async_bio_lock);
+		queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
+	} else {
+		/* never bounce for the root cgroup */
+		submit_bio(bio);
+	}
+}
+EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);
+
+static int __init blkcg_punt_bio_init(void)
+{
+	blkcg_punt_bio_wq = alloc_workqueue("blkcg_punt_bio",
+					    WQ_MEM_RECLAIM | WQ_FREEZABLE |
+					    WQ_UNBOUND | WQ_SYSFS, 0);
+	if (!blkcg_punt_bio_wq)
+		return -ENOMEM;
+	return 0;
+}
+subsys_initcall(blkcg_punt_bio_init);
+#endif /* CONFIG_BLK_CGROUP_PUNT_BIO */
+
 /**
  * bio_blkcg_css - return the blkcg CSS associated with a bio
  * @bio: target bio
@@ -269,10 +307,12 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
 
 	blkg->q = disk->queue;
 	INIT_LIST_HEAD(&blkg->q_node);
+	blkg->blkcg = blkcg;
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
 	spin_lock_init(&blkg->async_bio_lock);
 	bio_list_init(&blkg->async_bios);
 	INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
-	blkg->blkcg = blkcg;
+#endif
 
 	u64_stats_init(&blkg->iostat.sync);
 	for_each_possible_cpu(cpu) {
@@ -1688,28 +1728,6 @@ out_unlock:
 }
 EXPORT_SYMBOL_GPL(blkcg_policy_unregister);
 
-/*
- * When a shared kthread issues a bio for a cgroup, doing so synchronously can
- * lead to priority inversions as the kthread can be trapped waiting for that
- * cgroup.  Use this helper instead of submit_bio to punt the actual issuing to
- * a dedicated per-blkcg work item to avoid such priority inversions.
- */
-void blkcg_punt_bio_submit(struct bio *bio)
-{
-	struct blkcg_gq *blkg = bio->bi_blkg;
-
-	if (blkg->parent) {
-		spin_lock(&blkg->async_bio_lock);
-		bio_list_add(&blkg->async_bios, bio);
-		spin_unlock(&blkg->async_bio_lock);
-		queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
-	} else {
-		/* never bounce for the root cgroup */
-		submit_bio(bio);
-	}
-}
-EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);
-
 /*
  * Scale the accumulated delay based on how long it has been since we updated
  * the delay.  We only call this when we are adding delay, in case it's been a
@@ -2088,16 +2106,5 @@ bool blk_cgroup_congested(void)
 	return ret;
 }
 
-static int __init blkcg_init(void)
-{
-	blkcg_punt_bio_wq = alloc_workqueue("blkcg_punt_bio",
-					    WQ_MEM_RECLAIM | WQ_FREEZABLE |
-					    WQ_UNBOUND | WQ_SYSFS, 0);
-	if (!blkcg_punt_bio_wq)
-		return -ENOMEM;
-	return 0;
-}
-subsys_initcall(blkcg_init);
-
 module_param(blkcg_debug_stats, bool, 0644);
 MODULE_PARM_DESC(blkcg_debug_stats, "True if you want debug stats, false if not");

block/blk-cgroup.h

@@ -72,9 +72,10 @@ struct blkcg_gq {
 	struct blkg_iostat_set		iostat;
 
 	struct blkg_policy_data		*pd[BLKCG_MAX_POLS];
-
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
 	spinlock_t			async_bio_lock;
 	struct bio_list			async_bios;
+#endif
 	union {
 		struct work_struct	async_bio_work;
 		struct work_struct	free_work;

fs/btrfs/Kconfig

@@ -2,6 +2,7 @@
 
 config BTRFS_FS
 	tristate "Btrfs filesystem support"
+	select BLK_CGROUP_PUNT_BIO
 	select CRYPTO
 	select CRYPTO_CRC32C
 	select LIBCRC32C