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

Ming Lei
2020-07-29T15:50:20Z

On Wed, Jul 29, 2020 at 08:42:31AM -0700, Sagi Grimberg 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, can you test for non-blocking on the same setup?

OK, I can do that, but this patch supposes to not affect non-blocking,
care to share your motivation for testing non-blocking?

Thanks,
Ming