Re: [PATCH 3/3] block: only allocate poll_stats if there's a user of them

[Jens Axboe <axboe@kernel.dk>]

On 11/23/21 9:41 AM, Christoph Hellwig wrote:
> On Tue, Nov 23, 2021 at 09:18:13AM -0700, Jens Axboe wrote:
>> 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.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>  block/blk-mq.c         | 20 ++++++++++++++++++--
>>  block/blk-stat.c       |  6 ------
>>  block/blk-sysfs.c      |  1 +
>>  include/linux/blkdev.h |  9 +++++++--
>>  4 files changed, 26 insertions(+), 10 deletions(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 20a6445f6a01..cb41c441aa8f 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -4577,9 +4577,25 @@ 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))
>> +	struct blk_rq_stat *poll_stat;
>> +
>> +	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
>>  		return true;
> 
> Can't we replace the checks for QUEUE_FLAG_POLL_STATS with checks for
> q->poll_stat now?

I think so:


diff --git a/block/blk-mq.c b/block/blk-mq.c
index f011fa3ebcc7..af4580bdf931 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4593,7 +4593,7 @@ static bool blk_poll_stats_enable(struct request_queue *q)
 {
 	struct blk_rq_stat *poll_stat;
 
-	if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
+	if (q->poll_stat)
 		return true;
 
 	poll_stat = kzalloc(BLK_MQ_POLL_STATS_BKTS * sizeof(*poll_stat),
@@ -4602,7 +4602,7 @@ static bool blk_poll_stats_enable(struct request_queue *q)
 		return false;
 
 	spin_lock_irq(&q->stats->lock);
-	if (blk_queue_flag_test_and_set(QUEUE_FLAG_POLL_STATS, q)) {
+	if (q->poll_stat) {
 		spin_unlock_irq(&q->stats->lock);
 		kfree(poll_stat);
 		return true;
@@ -4620,8 +4620,7 @@ 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-sysfs.c b/block/blk-sysfs.c
index e1b846ec58cb..c079be1c58a3 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -785,7 +785,7 @@ 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);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3d558cb397d5..20cf877d6627 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -414,7 +414,6 @@ 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 */

-- 
Jens Axboe