blk-throttle: remove asynchrnous percpu stats allocation mechanism

Because percpu allocator couldn't do non-blocking allocations,
blk-throttle was forced to implement an ad-hoc asynchronous allocation
mechanism for its percpu stats for cases where blkg's (blkcg_gq's) are
allocated from an IO path without sleepable context.

Now that percpu allocator can handle gfp_mask and blkg_policy_data
alloc / free are handled by policy methods, the ad-hoc asynchronous
allocation mechanism can be replaced with direct allocation from
tg_stats_alloc_fn().  Rit it out.

This ensures that an active throtl_grp always has valid non-NULL
->stats_cpu.  Remove checks on it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-throttle.c

@@ -144,9 +144,6 @@ struct throtl_grp {
 
 	/* Per cpu stats pointer */
 	struct tg_stats_cpu __percpu *stats_cpu;
-
-	/* List of tgs waiting for per cpu stats memory to be allocated */
-	struct list_head stats_alloc_node;
 };
 
 struct throtl_data
@@ -168,13 +165,6 @@ struct throtl_data
 	struct work_struct dispatch_work;
 };
 
-/* list and work item to allocate percpu group stats */
-static DEFINE_SPINLOCK(tg_stats_alloc_lock);
-static LIST_HEAD(tg_stats_alloc_list);
-
-static void tg_stats_alloc_fn(struct work_struct *);
-static DECLARE_DELAYED_WORK(tg_stats_alloc_work, tg_stats_alloc_fn);
-
 static void throtl_pending_timer_fn(unsigned long arg);
 
 static inline struct throtl_grp *pd_to_tg(struct blkg_policy_data *pd)
@@ -256,53 +246,6 @@ static struct throtl_data *sq_to_td(struct throtl_service_queue *sq)
 	}								\
 } while (0)
 
-static void tg_stats_init(struct tg_stats_cpu *tg_stats)
-{
-	blkg_rwstat_init(&tg_stats->service_bytes);
-	blkg_rwstat_init(&tg_stats->serviced);
-}
-
-/*
- * Worker for allocating per cpu stat for tgs. This is scheduled on the
- * system_wq once there are some groups on the alloc_list waiting for
- * allocation.
- */
-static void tg_stats_alloc_fn(struct work_struct *work)
-{
-	static struct tg_stats_cpu *stats_cpu;	/* this fn is non-reentrant */
-	struct delayed_work *dwork = to_delayed_work(work);
-	bool empty = false;
-
-alloc_stats:
-	if (!stats_cpu) {
-		int cpu;
-
-		stats_cpu = alloc_percpu(struct tg_stats_cpu);
-		if (!stats_cpu) {
-			/* allocation failed, try again after some time */
-			schedule_delayed_work(dwork, msecs_to_jiffies(10));
-			return;
-		}
-		for_each_possible_cpu(cpu)
-			tg_stats_init(per_cpu_ptr(stats_cpu, cpu));
-	}
-
-	spin_lock_irq(&tg_stats_alloc_lock);
-
-	if (!list_empty(&tg_stats_alloc_list)) {
-		struct throtl_grp *tg = list_first_entry(&tg_stats_alloc_list,
-							 struct throtl_grp,
-							 stats_alloc_node);
-		swap(tg->stats_cpu, stats_cpu);
-		list_del_init(&tg->stats_alloc_node);
-	}
-
-	empty = list_empty(&tg_stats_alloc_list);
-	spin_unlock_irq(&tg_stats_alloc_lock);
-	if (!empty)
-		goto alloc_stats;
-}
-
 static void throtl_qnode_init(struct throtl_qnode *qn, struct throtl_grp *tg)
 {
 	INIT_LIST_HEAD(&qn->node);
@@ -405,7 +348,27 @@ static void throtl_service_queue_exit(struct throtl_service_queue *sq)
 
 static struct blkg_policy_data *throtl_pd_alloc(gfp_t gfp, int node)
 {
-	return kzalloc_node(sizeof(struct throtl_grp), gfp, node);
+	struct throtl_grp *tg;
+	int cpu;
+
+	tg = kzalloc_node(sizeof(*tg), gfp, node);
+	if (!tg)
+		return NULL;
+
+	tg->stats_cpu = alloc_percpu_gfp(struct tg_stats_cpu, gfp);
+	if (!tg->stats_cpu) {
+		kfree(tg);
+		return NULL;
+	}
+
+	for_each_possible_cpu(cpu) {
+		struct tg_stats_cpu *stats_cpu = per_cpu_ptr(tg->stats_cpu, cpu);
+
+		blkg_rwstat_init(&stats_cpu->service_bytes);
+		blkg_rwstat_init(&stats_cpu->serviced);
+	}
+
+	return &tg->pd;
 }
 
 static void throtl_pd_init(struct blkcg_gq *blkg)
@@ -413,7 +376,6 @@ static void throtl_pd_init(struct blkcg_gq *blkg)
 	struct throtl_grp *tg = blkg_to_tg(blkg);
 	struct throtl_data *td = blkg->q->td;
 	struct throtl_service_queue *parent_sq;
-	unsigned long flags;
 	int rw;
 
 	/*
@@ -448,16 +410,6 @@ static void throtl_pd_init(struct blkcg_gq *blkg)
 	tg->bps[WRITE] = -1;
 	tg->iops[READ] = -1;
 	tg->iops[WRITE] = -1;
-
-	/*
-	 * Ugh... We need to perform per-cpu allocation for tg->stats_cpu
-	 * but percpu allocator can't be called from IO path.  Queue tg on
-	 * tg_stats_alloc_list and allocate from work item.
-	 */
-	spin_lock_irqsave(&tg_stats_alloc_lock, flags);
-	list_add(&tg->stats_alloc_node, &tg_stats_alloc_list);
-	schedule_delayed_work(&tg_stats_alloc_work, 0);
-	spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);
 }
 
 /*
@@ -487,20 +439,16 @@ static void throtl_pd_online(struct blkcg_gq *blkg)
 static void throtl_pd_exit(struct blkcg_gq *blkg)
 {
 	struct throtl_grp *tg = blkg_to_tg(blkg);
-	unsigned long flags;
-
-	spin_lock_irqsave(&tg_stats_alloc_lock, flags);
-	list_del_init(&tg->stats_alloc_node);
-	spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);
-
-	free_percpu(tg->stats_cpu);
 
 	throtl_service_queue_exit(&tg->service_queue);
 }
 
 static void throtl_pd_free(struct blkg_policy_data *pd)
 {
-	kfree(pd);
+	struct throtl_grp *tg = pd_to_tg(pd);
+
+	free_percpu(tg->stats_cpu);
+	kfree(tg);
 }
 
 static void throtl_pd_reset_stats(struct blkcg_gq *blkg)
@@ -508,9 +456,6 @@ static void throtl_pd_reset_stats(struct blkcg_gq *blkg)
 	struct throtl_grp *tg = blkg_to_tg(blkg);
 	int cpu;
 
-	if (tg->stats_cpu == NULL)
-		return;
-
 	for_each_possible_cpu(cpu) {
 		struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);
 
@@ -973,10 +918,6 @@ static void throtl_update_dispatch_stats(struct blkcg_gq *blkg, u64 bytes,
 	struct tg_stats_cpu *stats_cpu;
 	unsigned long flags;
 
-	/* If per cpu stats are not allocated yet, don't do any accounting. */
-	if (tg->stats_cpu == NULL)
-		return;
-
 	/*
 	 * Disabling interrupts to provide mutual exclusion between two
 	 * writes on same cpu. It probably is not needed for 64bit. Not
@@ -1302,9 +1243,6 @@ static u64 tg_prfill_cpu_rwstat(struct seq_file *sf,
 	struct blkg_rwstat rwstat = { }, tmp;
 	int i, cpu;
 
-	if (tg->stats_cpu == NULL)
-		return 0;
-
 	for_each_possible_cpu(cpu) {
 		struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);