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

[Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>]

Hi Bart, Jens and Zhiguo,

On 14/03/24 03:12, Bart Van Assche wrote:
> 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.
> 

Thanks a lot for helping with this performance regression[1].


Regards,
Harshit

[1] 
https://lore.kernel.org/all/5ce2ae5d-61e2-4ede-ad55-551112602401@oracle.com/

> Cc: stable@vger.kernel.org
> Cc: Damien Le Moal <dlemoal@kernel.org>
> Cc: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
> Cc: Zhiguo Niu <Zhiguo.Niu@unisoc.com>
> Fixes: d47f9717e5cf ("block/mq-deadline: use correct way to throttling write requests")
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>   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);
>   }