blk-mq: save memory by freeing requests on unused hardware queues

Depending on the topology of the machine and the number of queues
exposed by a device, we can end up in a situation where some of
the hardware queues are unused (as in, they don't map to any
software queues). For this case, free up the memory used by the
request map, as we will not use it. This can be a substantial
amount of memory, depending on the number of queues vs CPUs and
the queue depth of the device.

Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -597,8 +597,16 @@ static void blk_mq_rq_timer(unsigned long data)
 	unsigned long next = 0;
 	int i, next_set = 0;
 
-	queue_for_each_hw_ctx(q, hctx, i)
+	queue_for_each_hw_ctx(q, hctx, i) {
+		/*
+		 * If not software queues are currently mapped to this
+		 * hardware queue, there's nothing to check
+		 */
+		if (!hctx->nr_ctx || !hctx->tags)
+			continue;
+
 		blk_mq_hw_ctx_check_timeout(hctx, &next, &next_set);
+	}
 
 	if (next_set) {
 		next = blk_rq_timeout(round_jiffies_up(next));
@@ -1196,53 +1204,6 @@ void blk_mq_free_single_hw_queue(struct blk_mq_hw_ctx *hctx,
 }
 EXPORT_SYMBOL(blk_mq_free_single_hw_queue);
 
-static int blk_mq_hctx_notify(void *data, unsigned long action,
-			      unsigned int cpu)
-{
-	struct blk_mq_hw_ctx *hctx = data;
-	struct request_queue *q = hctx->queue;
-	struct blk_mq_ctx *ctx;
-	LIST_HEAD(tmp);
-
-	if (action != CPU_DEAD && action != CPU_DEAD_FROZEN)
-		return NOTIFY_OK;
-
-	/*
-	 * Move ctx entries to new CPU, if this one is going away.
-	 */
-	ctx = __blk_mq_get_ctx(q, cpu);
-
-	spin_lock(&ctx->lock);
-	if (!list_empty(&ctx->rq_list)) {
-		list_splice_init(&ctx->rq_list, &tmp);
-		blk_mq_hctx_clear_pending(hctx, ctx);
-	}
-	spin_unlock(&ctx->lock);
-
-	if (list_empty(&tmp))
-		return NOTIFY_OK;
-
-	ctx = blk_mq_get_ctx(q);
-	spin_lock(&ctx->lock);
-
-	while (!list_empty(&tmp)) {
-		struct request *rq;
-
-		rq = list_first_entry(&tmp, struct request, queuelist);
-		rq->mq_ctx = ctx;
-		list_move_tail(&rq->queuelist, &ctx->rq_list);
-	}
-
-	hctx = q->mq_ops->map_queue(q, ctx->cpu);
-	blk_mq_hctx_mark_pending(hctx, ctx);
-
-	spin_unlock(&ctx->lock);
-
-	blk_mq_run_hw_queue(hctx, true);
-	blk_mq_put_ctx(ctx);
-	return NOTIFY_OK;
-}
-
 static void blk_mq_free_rq_map(struct blk_mq_tag_set *set,
 		struct blk_mq_tags *tags, unsigned int hctx_idx)
 {
@@ -1384,6 +1345,77 @@ static int blk_mq_alloc_bitmap(struct blk_mq_ctxmap *bitmap, int node)
 	return 0;
 }
 
+static int blk_mq_hctx_cpu_offline(struct blk_mq_hw_ctx *hctx, int cpu)
+{
+	struct request_queue *q = hctx->queue;
+	struct blk_mq_ctx *ctx;
+	LIST_HEAD(tmp);
+
+	/*
+	 * Move ctx entries to new CPU, if this one is going away.
+	 */
+	ctx = __blk_mq_get_ctx(q, cpu);
+
+	spin_lock(&ctx->lock);
+	if (!list_empty(&ctx->rq_list)) {
+		list_splice_init(&ctx->rq_list, &tmp);
+		blk_mq_hctx_clear_pending(hctx, ctx);
+	}
+	spin_unlock(&ctx->lock);
+
+	if (list_empty(&tmp))
+		return NOTIFY_OK;
+
+	ctx = blk_mq_get_ctx(q);
+	spin_lock(&ctx->lock);
+
+	while (!list_empty(&tmp)) {
+		struct request *rq;
+
+		rq = list_first_entry(&tmp, struct request, queuelist);
+		rq->mq_ctx = ctx;
+		list_move_tail(&rq->queuelist, &ctx->rq_list);
+	}
+
+	hctx = q->mq_ops->map_queue(q, ctx->cpu);
+	blk_mq_hctx_mark_pending(hctx, ctx);
+
+	spin_unlock(&ctx->lock);
+
+	blk_mq_run_hw_queue(hctx, true);
+	blk_mq_put_ctx(ctx);
+	return NOTIFY_OK;
+}
+
+static int blk_mq_hctx_cpu_online(struct blk_mq_hw_ctx *hctx, int cpu)
+{
+	struct request_queue *q = hctx->queue;
+	struct blk_mq_tag_set *set = q->tag_set;
+
+	if (set->tags[hctx->queue_num])
+		return NOTIFY_OK;
+
+	set->tags[hctx->queue_num] = blk_mq_init_rq_map(set, hctx->queue_num);
+	if (!set->tags[hctx->queue_num])
+		return NOTIFY_STOP;
+
+	hctx->tags = set->tags[hctx->queue_num];
+	return NOTIFY_OK;
+}
+
+static int blk_mq_hctx_notify(void *data, unsigned long action,
+			      unsigned int cpu)
+{
+	struct blk_mq_hw_ctx *hctx = data;
+
+	if (action == CPU_DEAD || action == CPU_DEAD_FROZEN)
+		return blk_mq_hctx_cpu_offline(hctx, cpu);
+	else if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN)
+		return blk_mq_hctx_cpu_online(hctx, cpu);
+
+	return NOTIFY_OK;
+}
+
 static int blk_mq_init_hw_queues(struct request_queue *q,
 		struct blk_mq_tag_set *set)
 {
@@ -1513,6 +1545,24 @@ static void blk_mq_map_swqueue(struct request_queue *q)
 	}
 
 	queue_for_each_hw_ctx(q, hctx, i) {
+		/*
+		 * If not software queues are mapped to this hardware queue,
+		 * disable it and free the request entries
+		 */
+		if (!hctx->nr_ctx) {
+			struct blk_mq_tag_set *set = q->tag_set;
+
+			if (set->tags[i]) {
+				blk_mq_free_rq_map(set, set->tags[i], i);
+				set->tags[i] = NULL;
+				hctx->tags = NULL;
+			}
+			continue;
+		}
+
+		/*
+		 * Initialize batch roundrobin counts
+		 */
 		hctx->next_cpu = cpumask_first(hctx->cpumask);
 		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
 	}
@@ -1645,14 +1695,14 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
 	if (blk_mq_init_hw_queues(q, set))
 		goto err_flush_rq;
 
-	blk_mq_map_swqueue(q);
-
 	mutex_lock(&all_q_mutex);
 	list_add_tail(&q->all_q_node, &all_q_list);
 	mutex_unlock(&all_q_mutex);
 
 	blk_mq_add_queue_tag_set(set, q);
 
+	blk_mq_map_swqueue(q);
+
 	return q;
 
 err_flush_rq:
@@ -1790,8 +1840,11 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
 {
 	int i;
 
-	for (i = 0; i < set->nr_hw_queues; i++)
-		blk_mq_free_rq_map(set, set->tags[i], i);
+	for (i = 0; i < set->nr_hw_queues; i++) {
+		if (set->tags[i])
+			blk_mq_free_rq_map(set, set->tags[i], i);
+	}
+
 	kfree(set->tags);
 }
 EXPORT_SYMBOL(blk_mq_free_tag_set);