Re: [PATCH] blk-mq: set default elevator as deadline in case of hctx shared tagset

[John Garry <john.garry@huawei.com>]

On 06/04/2021 04:19, Ming Lei wrote:

Hi Ming,

> Yanhui found that write performance is degraded a lot after applying
> hctx shared tagset on one test machine with megaraid_sas. And turns out
> it is caused by none scheduler which becomes default elevator caused by
> hctx shared tagset patchset.
> 
> Given more scsi HBAs will apply hctx shared tagset, and the similar
> performance exists for them too.
> 
> So keep previous behavior by still using default mq-deadline for queues
> which apply hctx shared tagset, just like before.

I think that there a some SCSI HBAs which have nr_hw_queues > 1 and 
don't use shared sbitmap - do you think that they want want this as well 
(without knowing it)?

IIRC, the upcoming broadcom SCSI HBA driver does this.

Thanks,
John

> 
> Fixes: 32bc15afed04 ("blk-mq: Facilitate a shared sbitmap per tagset")
> Reported-by: Yanhui Ma <yama@redhat.com>
> Cc: John Garry <john.garry@huawei.com>
> Cc: Hannes Reinecke <hare@suse.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   block/elevator.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/elevator.c b/block/elevator.c
> index 293c5c81397a..440699c28119 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -621,7 +621,8 @@ static inline bool elv_support_iosched(struct request_queue *q)
>    */
>   static struct elevator_type *elevator_get_default(struct request_queue *q)
>   {
> -	if (q->nr_hw_queues != 1)
> +	if (q->nr_hw_queues != 1 &&
> +			!blk_mq_is_sbitmap_shared(q->tag_set->flags))
>   		return NULL;
>   
>   	return elevator_get(q, "mq-deadline", false);
>