Re: [RFC PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

Ming Lei
2020-07-29T10:29:13Z

On Tue, Jul 28, 2020 at 09:49:38PM +0800, Ming Lei wrote:
> In case of BLK_MQ_F_BLOCKING, blk-mq uses SRCU to mark read critical
> section during dispatching request, then request queue quiesce is based on
> SRCU. What we want to get is low cost added in fast path.
>
> However, from srcu_read_lock/srcu_read_unlock implementation, not see
> it is quicker than percpu refcount, so use percpu_ref to implement
> queue quiesce. This usage is cleaner and simpler & enough for implementing
> queue quiesce. The main requirement is to make sure all read sections to observe
> QUEUE_FLAG_QUIESCED once blk_mq_quiesce_queue() returns.
>
> Also it becomes much easier to add interface of async queue quiesce.

BTW, no obvious IOPS difference is observed with this patch applied when running
io on null_blk(blocking, submit_queues=32) in one dual-socket, 32cores system.

Thanks,
Ming