[PATCH v3 1/2] blk-mq: add async quiesce interface

[Keith Busch]

On Mon, Jul 27, 2020 at 03:05:40PM -0600, Jens Axboe wrote:
> +void blk_mq_quiesce_queue_wait(struct request_queue *q)
>  {
>  	struct blk_mq_hw_ctx *hctx;
>  	unsigned int i;
>  	bool rcu = false;
>  
> -	blk_mq_quiesce_queue_nowait(q);
> -
>  	queue_for_each_hw_ctx(q, hctx, i) {
>  		if (hctx->flags & BLK_MQ_F_BLOCKING)
>  			synchronize_srcu(hctx->srcu);
>  		else
>  			rcu = true;
>  	}
> +
>  	if (rcu)
>  		synchronize_rcu();
>  }

Either all the hctx's are blocking or none of them are: we don't need to
iterate the hctx's to see which sync method to use. We can add at the
very beginning (and get rid of 'bool rcu'):

	if (!(q->tag_set->flags & BLK_MQ_F_BLOCKING)) {
		synchronize_rcu();
		return;
	}


But the issue Sagi is trying to address is quiescing a lot
request queues sharing a tagset where synchronize_rcu() is too time
consuming to do repeatedly. He wants to synchrnoize once for the entire
tagset rather than per-request_queue, so I think he needs an API taking
a 'struct blk_mq_tag_set' instead of a 'struct request_queue'.