[RFC PATCH v2 4/4] blk-cgroup: move async bio punt state to blkcg

[Yu Kuai <yukuai@kernel.org>]

From: Yu Kuai <yukuai@fygo.io>

blkcg_punt_bio_submit() currently queues punted bios on blkg->async_bios,
so it has to call bio_blkg() to find or create a queue-local blkg.  Bios
now carry and pin the blkcg css, so punted bio lifetime no longer needs to
be anchored by a blkg.

Keeping the punt state in blkg can instantiate a blkg even when no blkcg
policy is enabled, just to bounce submission from a shared kthread.  Move
async_bio_lock, async_bios and async_bio_work to struct blkcg, and queue
punted bios on bio_blkcg() for non-root cgroups.  Root or unassociated bios
are submitted directly.

This preserves the priority-inversion avoidance while preventing
blkcg_punt_bio_submit() from creating blkgs that are not needed by any
policy.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
 block/blk-cgroup.c | 44 +++++++++++++++++++++-----------------------
 block/blk-cgroup.h | 14 ++++++--------
 2 files changed, 27 insertions(+), 31 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 1de2f2433d11..e76e9a52f95d 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -177,14 +177,10 @@ static void blkg_free(struct blkcg_gq *blkg)
 
 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
-
 	blkg_free(blkg);
 }
 
 /*
  * A group is RCU protected, but having an rcu lock does not mean that one
@@ -218,23 +214,22 @@ static void blkg_release(struct percpu_ref *ref)
 }
 
 #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
 static struct workqueue_struct *blkcg_punt_bio_wq;
 
-static void blkg_async_bio_workfn(struct work_struct *work)
+static void blkcg_async_bio_workfn(struct work_struct *work)
 {
-	struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
-					     async_bio_work);
+	struct blkcg *blkcg = container_of(work, struct blkcg, async_bio_work);
 	struct bio_list bios = BIO_EMPTY_LIST;
 	struct bio *bio;
 	struct blk_plug plug;
 	bool need_plug = false;
 
-	/* as long as there are pending bios, @blkg can't go away */
-	spin_lock(&blkg->async_bio_lock);
-	bio_list_merge_init(&bios, &blkg->async_bios);
-	spin_unlock(&blkg->async_bio_lock);
+	/* as long as there are pending bios, @blkcg can't go away */
+	spin_lock(&blkcg->async_bio_lock);
+	bio_list_merge_init(&bios, &blkcg->async_bios);
+	spin_unlock(&blkcg->async_bio_lock);
 
 	/* start plug only when bio_list contains at least 2 bios */
 	if (bios.head && bios.head->bi_next) {
 		need_plug = true;
 		blk_start_plug(&plug);
@@ -251,19 +246,19 @@ static void blkg_async_bio_workfn(struct work_struct *work)
  * 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_blkg(bio);
+	struct blkcg *blkcg = bio_blkcg(bio);
 
-	if (blkg && 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);
+	if (blkcg && cgroup_parent(blkcg->css.cgroup)) {
+		spin_lock(&blkcg->async_bio_lock);
+		bio_list_add(&blkcg->async_bios, bio);
+		spin_unlock(&blkcg->async_bio_lock);
+		queue_work(blkcg_punt_bio_wq, &blkcg->async_bio_work);
 	} else {
-		/* Never bounce if there is no non-root blkg to queue on. */
+		/* Never bounce if there is no non-root blkcg to queue on. */
 		submit_bio(bio);
 	}
 }
 EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);
 
@@ -342,15 +337,10 @@ 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;
 	blkg->blkcg_id = blkcg->css.id;
 	blkg->iostat.blkg = blkg;
-#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);
-#endif
 
 	u64_stats_init(&blkg->iostat.sync);
 	for_each_possible_cpu(cpu) {
 		u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
 		per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;
@@ -1370,10 +1360,13 @@ static void blkcg_css_free(struct cgroup_subsys_state *css)
 		if (blkcg->cpd[i])
 			blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);
 
 	mutex_unlock(&blkcg_pol_mutex);
 
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+	WARN_ON(!bio_list_empty(&blkcg->async_bios));
+#endif
 	free_percpu(blkcg->lhead);
 	kfree(blkcg);
 }
 
 static struct cgroup_subsys_state *
@@ -1418,10 +1411,15 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 	}
 
 	spin_lock_init(&blkcg->lock);
 	refcount_set(&blkcg->online_pin, 1);
 	INIT_HLIST_HEAD(&blkcg->blkg_list);
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+	spin_lock_init(&blkcg->async_bio_lock);
+	bio_list_init(&blkcg->async_bios);
+	INIT_WORK(&blkcg->async_bio_work, blkcg_async_bio_workfn);
+#endif
 #ifdef CONFIG_CGROUP_WRITEBACK
 	INIT_LIST_HEAD(&blkcg->cgwb_list);
 #endif
 	list_add_tail(&blkcg->all_blkcgs_node, &all_blkcgs);
 
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 3d113f6f6d25..efef0967d39e 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -74,18 +74,11 @@ struct blkcg_gq {
 
 	struct blkg_iostat_set __percpu	*iostat_cpu;
 	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;
-	};
+	struct work_struct		free_work;
 
 	atomic_t			use_delay;
 	atomic64_t			delay_nsec;
 	atomic64_t			delay_start;
 	u64				last_delay;
@@ -110,10 +103,15 @@ struct blkcg {
 	/*
 	 * List of updated percpu blkg_iostat_set's since the last flush.
 	 */
 	struct llist_head __percpu	*lhead;
 
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+	spinlock_t			async_bio_lock; /* protects async_bios */
+	struct bio_list			async_bios;
+	struct work_struct		async_bio_work;
+#endif
 #ifdef CONFIG_BLK_CGROUP_FC_APPID
 	char                            fc_app_id[FC_APPID_LEN];
 #endif
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct list_head		cgwb_list;
-- 
2.51.0