[PATCH 1/2] block: only allocate poll_stats if there's a user of them

[Jens Axboe <axboe@kernel.dk>]

This is essentially never used, yet it's about 1/3rd of the total
queue size. Allocate it when needed, and don't embed it in the queue.

Kill the queue flag for this while at it, since we can just check the
assigned pointer now.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-mq-debugfs.c |  1 -
 block/blk-mq.c         | 10 ++++------
 block/blk-stat.c       | 18 ++++++++++++++++++
 block/blk-stat.h       |  1 +
 block/blk-sysfs.c      |  3 ++-
 include/linux/blkdev.h |  3 +--
 6 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 4f2cf8399f3d..f4022b198580 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -122,7 +122,6 @@ static const char *const blk_queue_flag_name[] = {
 	QUEUE_FLAG_NAME(FUA),
 	QUEUE_FLAG_NAME(DAX),
 	QUEUE_FLAG_NAME(STATS),
-	QUEUE_FLAG_NAME(POLL_STATS),
 	QUEUE_FLAG_NAME(REGISTERED),
 	QUEUE_FLAG_NAME(QUIESCED),
 	QUEUE_FLAG_NAME(PCI_P2PDMA),
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4c00b22590cc..9d29245511ec 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4580,11 +4580,10 @@ EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);
 /* Enable polling stats and return whether they were already enabled. */
 static bool blk_poll_stats_enable(struct request_queue *q)
 {
-	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags) ||
-	    blk_queue_flag_test_and_set(QUEUE_FLAG_POLL_STATS, q))
+	if (q->poll_stat)
 		return true;
-	blk_stat_add_callback(q, q->poll_cb);
-	return false;
+
+	return blk_stats_alloc_enable(q);
 }
 
 static void blk_mq_poll_stats_start(struct request_queue *q)
@@ -4593,8 +4592,7 @@ static void blk_mq_poll_stats_start(struct request_queue *q)
 	 * We don't arm the callback if polling stats are not enabled or the
 	 * callback is already active.
 	 */
-	if (!test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags) ||
-	    blk_stat_is_active(q->poll_cb))
+	if (!q->poll_stat || blk_stat_is_active(q->poll_cb))
 		return;
 
 	blk_stat_activate_msecs(q->poll_cb, 100);
diff --git a/block/blk-stat.c b/block/blk-stat.c
index ae3dd1fb8e61..1b927b6e49bc 100644
--- a/block/blk-stat.c
+++ b/block/blk-stat.c
@@ -219,3 +219,21 @@ void blk_free_queue_stats(struct blk_queue_stats *stats)
 
 	kfree(stats);
 }
+
+bool blk_stats_alloc_enable(struct request_queue *q)
+{
+	struct blk_rq_stat *poll_stat;
+
+	poll_stat = kcalloc(BLK_MQ_POLL_STATS_BKTS, sizeof(*poll_stat),
+				GFP_ATOMIC);
+	if (!poll_stat)
+		return false;
+
+	if (cmpxchg(&q->poll_stat, poll_stat, NULL) != poll_stat) {
+		kfree(poll_stat);
+		return true;
+	}
+
+	blk_stat_add_callback(q, q->poll_cb);
+	return false;
+}
diff --git a/block/blk-stat.h b/block/blk-stat.h
index 17b47a86eefb..58f029af49e5 100644
--- a/block/blk-stat.h
+++ b/block/blk-stat.h
@@ -64,6 +64,7 @@ struct blk_stat_callback {
 
 struct blk_queue_stats *blk_alloc_queue_stats(void);
 void blk_free_queue_stats(struct blk_queue_stats *);
+bool blk_stats_alloc_enable(struct request_queue *q);
 
 void blk_stat_add(struct request *rq, u64 now);
 
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index cd75b0f73dc6..c079be1c58a3 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -785,11 +785,12 @@ static void blk_release_queue(struct kobject *kobj)
 
 	might_sleep();
 
-	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
+	if (q->poll_stat)
 		blk_stat_remove_callback(q, q->poll_cb);
 	blk_stat_free_callback(q->poll_cb);
 
 	blk_free_queue_stats(q->stats);
+	kfree(q->poll_stat);
 
 	blk_exit_queue(q);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index bd4370baccca..74118e67f649 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -267,7 +267,7 @@ struct request_queue {
 	int			poll_nsec;
 
 	struct blk_stat_callback	*poll_cb;
-	struct blk_rq_stat	poll_stat[BLK_MQ_POLL_STATS_BKTS];
+	struct blk_rq_stat	*poll_stat;
 
 	struct timer_list	timeout;
 	struct work_struct	timeout_work;
@@ -397,7 +397,6 @@ struct request_queue {
 #define QUEUE_FLAG_FUA		18	/* device supports FUA writes */
 #define QUEUE_FLAG_DAX		19	/* device supports DAX */
 #define QUEUE_FLAG_STATS	20	/* track IO start and completion times */
-#define QUEUE_FLAG_POLL_STATS	21	/* collecting stats for hybrid polling */
 #define QUEUE_FLAG_REGISTERED	22	/* queue has been registered to a disk */
 #define QUEUE_FLAG_QUIESCED	24	/* queue has been quiesced */
 #define QUEUE_FLAG_PCI_P2PDMA	25	/* device supports PCI p2p requests */
-- 
2.34.0