2023-08-03 11:49:29

by Zhiguo Niu

Subject: [PATCH] block/mq-deadline: use correct way to throttling write requests

The original formula was inaccurate:
dd->async_depth = max(1UL, 3 * q->nr_requests / 4);

For write requests, when we assign a tags from sched_tags,
data->shallow_depth will be passed to sbitmap_find_bit,
see the following code:

nr = sbitmap_find_bit_in_word(&sb->map[index],
min_t (unsigned int,
__map_depth(sb, index),
depth),
alloc_hint, wrap);

The smaller of data->shallow_depth and __map_depth(sb, index)
will be used as the maximum range when allocating bits.

For a mmc device (one hw queue, deadline I/O scheduler):
q->nr_requests = sched_tags = 128, so according to the previous
calculation method, dd->async_depth = data->shallow_depth = 96,
and the platform is 64bits with 8 cpus, sched_tags.bitmap_tags.sb.shift=5,
sb.maps[]=32/32/32/32, 32 is smaller than 96, whether it is a read or
a write I/O, tags can be allocated to the maximum range each time,
which has not throttling effect.

In addition, refer to the methods of bfg/kyber I/O scheduler,
limit ratiois are calculated base on sched_tags.bitmap_tags.sb.shift.

This patch can throttle write requests really.

Fixes: 07757588e507 ("block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests")

Signed-off-by: Zhiguo Niu <[email protected]>

---
block/mq-deadline.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 5839a027e0f0..7e043d4a78f8 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -620,8 +620,9 @@ static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
struct request_queue *q = hctx->queue;
struct deadline_data *dd = q->elevator->elevator_data;
struct blk_mq_tags *tags = hctx->sched_tags;
+ unsigned int shift = tags->bitmap_tags.sb.shift;

- dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
+ dd->async_depth = max(1U, 3 * (1U << shift) / 4);

sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, dd->async_depth);
}
--
2.37.3

2023-08-03 17:43:36

by Bart Van Assche

Subject: Re: [PATCH] block/mq-deadline: use correct way to throttling write requests

On 8/3/23 04:12, Zhiguo Niu wrote:
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 5839a027e0f0..7e043d4a78f8 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -620,8 +620,9 @@ static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
> struct request_queue *q = hctx->queue;
> struct deadline_data *dd = q->elevator->elevator_data;
> struct blk_mq_tags *tags = hctx->sched_tags;
> + unsigned int shift = tags->bitmap_tags.sb.shift;
>
> - dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
> + dd->async_depth = max(1U, 3 * (1U << shift) / 4);
>
> sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, dd->async_depth);
> }

Reviewed-by: Bart Van Assche <[email protected]>

2023-08-08 22:55:32

by Jens Axboe

Subject: Re: [PATCH] block/mq-deadline: use correct way to throttling write requests

On Thu, 03 Aug 2023 19:12:42 +0800, Zhiguo Niu wrote:
> The original formula was inaccurate:
> dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
>
> For write requests, when we assign a tags from sched_tags,
> data->shallow_depth will be passed to sbitmap_find_bit,
> see the following code:
>
> [...]

Applied, thanks!

[1/1] block/mq-deadline: use correct way to throttling write requests
commit: d47f9717e5cfd0dd8c0ba2ecfa47c38d140f1bb6

Best regards,
--
Jens Axboe