Commit 64f1c21e authored by Jens Axboe's avatar Jens Axboe
Browse files

blk-mq: make the polling code adaptive 



The previous commit introduced the hybrid sleep/poll mode. Take
that one step further, and use the completion latencies to
automatically sleep for half the mean completion time. This is
a good approximation.

This changes the 'io_poll_delay' sysfs file a bit to expose the
various options. Depending on the value, the polling code will
behave differently:

-1	Never enter hybrid sleep mode
 0	Use half of the completion mean for the sleep delay
>0	Use this specific value as the sleep delay

Signed-off-by: default avatarJens Axboe <axboe@fb.com>
Tested-By: default avatarStephen Bates <sbates@raithlin.com>
Reviewed-By: default avatarStephen Bates <sbates@raithlin.com>
parent 06426adf

block/blk-mq.c

+64 −3
Original line number Diff line number Diff line
@@ -2132,6 +2132,11 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set, 
	 */

	q->nr_requests = set->queue_depth;



	/*

	 * Default to classic polling

	 */

	q->poll_nsec = -1;



	if (set->ops->complete)

		blk_queue_softirq_done(q, set->ops->complete);
@@ -2469,14 +2474,70 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues) 
}

EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);



static unsigned long blk_mq_poll_nsecs(struct request_queue *q,

				       struct blk_mq_hw_ctx *hctx,

				       struct request *rq)

{

	struct blk_rq_stat stat[2];

	unsigned long ret = 0;



	/*

	 * If stats collection isn't on, don't sleep but turn it on for

	 * future users

	 */

	if (!blk_stat_enable(q))

		return 0;



	/*

	 * We don't have to do this once per IO, should optimize this

	 * to just use the current window of stats until it changes

	 */

	memset(&stat, 0, sizeof(stat));

	blk_hctx_stat_get(hctx, stat);



	/*

	 * As an optimistic guess, use half of the mean service time

	 * for this type of request. We can (and should) make this smarter.

	 * For instance, if the completion latencies are tight, we can

	 * get closer than just half the mean. This is especially

	 * important on devices where the completion latencies are longer

	 * than ~10 usec.

	 */

	if (req_op(rq) == REQ_OP_READ && stat[BLK_STAT_READ].nr_samples)

		ret = (stat[BLK_STAT_READ].mean + 1) / 2;

	else if (req_op(rq) == REQ_OP_WRITE && stat[BLK_STAT_WRITE].nr_samples)

		ret = (stat[BLK_STAT_WRITE].mean + 1) / 2;



	return ret;

}



static bool blk_mq_poll_hybrid_sleep(struct request_queue *q,

				     struct blk_mq_hw_ctx *hctx,

				     struct request *rq)

{

	struct hrtimer_sleeper hs;

	enum hrtimer_mode mode;

	unsigned int nsecs;

	ktime_t kt;



	if (!q->poll_nsec || test_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags))

	if (test_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags))

		return false;



	/*

	 * poll_nsec can be:

	 *

	 * -1:	don't ever hybrid sleep

	 *  0:	use half of prev avg

	 * >0:	use this specific value

	 */

	if (q->poll_nsec == -1)

		return false;

	else if (q->poll_nsec > 0)

		nsecs = q->poll_nsec;

	else

		nsecs = blk_mq_poll_nsecs(q, hctx, rq);



	if (!nsecs)

		return false;



	set_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
@@ -2485,7 +2546,7 @@ static bool blk_mq_poll_hybrid_sleep(struct request_queue *q, 
	 * This will be replaced with the stats tracking code, using

	 * 'avg_completion_time / 2' as the pre-sleep target.

	 */

	kt = ktime_set(0, q->poll_nsec);

	kt = ktime_set(0, nsecs);



	mode = HRTIMER_MODE_REL;

	hrtimer_init_on_stack(&hs.timer, CLOCK_MONOTONIC, mode);
@@ -2520,7 +2581,7 @@ static bool __blk_mq_poll(struct blk_mq_hw_ctx *hctx, struct request *rq) 
	 * the IO isn't complete, we'll get called again and will go

	 * straight to the busy poll loop.

	 */

	if (blk_mq_poll_hybrid_sleep(q, rq))

	if (blk_mq_poll_hybrid_sleep(q, hctx, rq))

		return true;



	hctx->poll_considered++;

block/blk-sysfs.c

+18 −8
Original line number Diff line number Diff line
@@ -352,24 +352,34 @@ queue_rq_affinity_store(struct request_queue *q, const char *page, size_t count) 


static ssize_t queue_poll_delay_show(struct request_queue *q, char *page)

{

	return queue_var_show(q->poll_nsec / 1000, page);

	int val;



	if (q->poll_nsec == -1)

		val = -1;

	else

		val = q->poll_nsec / 1000;



	return sprintf(page, "%d\n", val);

}



static ssize_t queue_poll_delay_store(struct request_queue *q, const char *page,

				size_t count)

{

	unsigned long poll_usec;

	ssize_t ret;

	int err, val;



	if (!q->mq_ops || !q->mq_ops->poll)

		return -EINVAL;



	ret = queue_var_store(&poll_usec, page, count);

	if (ret < 0)

		return ret;

	err = kstrtoint(page, 10, &val);

	if (err < 0)

		return err;



	q->poll_nsec = poll_usec * 1000;

	return ret;

	if (val == -1)

		q->poll_nsec = -1;

	else

		q->poll_nsec = val * 1000;



	return count;

}



static ssize_t queue_poll_show(struct request_queue *q, char *page)

include/linux/blkdev.h

+1 −1
Original line number Diff line number Diff line
@@ -509,7 +509,7 @@ struct request_queue { 
	unsigned int		request_fn_active;



	unsigned int		rq_timeout;

	unsigned int		poll_nsec;

	int			poll_nsec;

	struct timer_list	timeout;

	struct work_struct	timeout_work;

	struct list_head	timeout_list;

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