Re: [PATCH v3 5/9] blk-mq: fix blk_mq_quiesce_queue

[Ming Lei <ming.lei@redhat.com>]

On Wed, May 31, 2017 at 03:37:30PM +0000, Bart Van Assche wrote:
> On Wed, 2017-05-31 at 20:37 +0800, Ming Lei wrote:
> > 
> > +	/* wait until queue is unquiesced */
> > +	wait_event_cmd(q->quiesce_wq, !blk_queue_quiesced(q),
> > +			may_sleep ?
> > +			srcu_read_unlock(&hctx->queue_rq_srcu, *srcu_idx) :
> > +			rcu_read_unlock(),
> > +			may_sleep ?
> > +			*srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu) :
> > +			rcu_read_lock());
> > +
> >  	if (q->elevator)
> >  		goto insert;
> 
> What I see is that in this patch a new waitqueue has been introduced
> (quiesce_wq) and also that an explanation of why you think this new waitqueue
> is needed is missing completely. Why is it that you think that the
> synchronize_scru() and synchronize_rcu() calls in blk_mq_quiesce_queue() are
> not sufficient? If this new waitqueue is not needed, please remove that
> waitqueue again.

OK, the reason is simple, and it is only related with direct issue.

Under this situation, when the queue is quiesced, we have to

	- insert the current request into sw queue(scheduler queue)
	OR
	-wait until queue becomes unquiesced like what this patch is doing

The disadvantage of the 1st way is that we have to consider to run queue
again in blk_mq_unquiesce_queue() for the queued requests during quiescing.

For the 2nd way(what this patch is doing), one benefit is that application
can avoid to submit I/O to a quiesced queue. Another benefit is that we
needn't to consider to run queue in blk_mq_unquiesce_queue(). But with cost
of one waitqueue, the cost should be cheap, but if you persist on the 1st
approach, I am fine to change to that.


Thanks,
Ming