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

[牛志国 (Zhiguo Niu) <Zhiguo.Niu@xxxxxxxxxx>]

Hi Bart,

Just as mentioned in original patch, "dd->async_depth = max(1UL, 3 * q->nr_requests / 4);", this limitation methods look likes won't have a limit effect, because tag allocated is based on sbitmap, not based the whole nr_requests.
Right?
Thanks!

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.

-----邮件原件-----
发件人: Bart Van Assche <bvanassche@xxxxxxx> 
发送时间: 2024年3月14日 5:42
收件人: Jens Axboe <axboe@xxxxxxxxx>
抄送: linux-block@xxxxxxxxxxxxxxx; Christoph Hellwig <hch@xxxxxx>; Bart Van Assche <bvanassche@xxxxxxx>; stable@xxxxxxxxxxxxxxx; Damien Le Moal <dlemoal@xxxxxxxxxx>; Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>; 牛志国 (Zhiguo Niu) <Zhiguo.Niu@xxxxxxxxxx>
主题: [PATCH] Revert "block/mq-deadline: use correct way to throttling write requests"


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The code "max(1U, 3 * (1U << shift)  / 4)" comes from the Kyber I/O scheduler. The Kyber I/O scheduler maintains one internal queue per hwq and hence derives its async_depth from the number of hwq tags. Using this approach for the mq-deadline scheduler is wrong since the mq-deadline scheduler maintains one internal queue for all hwqs combined. Hence this revert.

Cc: stable@xxxxxxxxxxxxxxx
Cc: Damien Le Moal <dlemoal@xxxxxxxxxx>
Cc: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
Cc: Zhiguo Niu <Zhiguo.Niu@xxxxxxxxxx>
Fixes: d47f9717e5cf ("block/mq-deadline: use correct way to throttling write requests")
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/mq-deadline.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c index f958e79277b8..02a916ba62ee 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -646,9 +646,8 @@ 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(1U, 3 * (1U << shift)  / 4);
+       dd->async_depth = max(1UL, 3 * q->nr_requests / 4);

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