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

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Thu, 2017-06-01 at 10:21 +0800, Ming Lei wrote:
> 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:
> > >=20
> > > +	/* 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 =3D srcu_read_lock(&hctx->queue_rq_srcu) :
> > > +			rcu_read_lock());
> > > +
> > >  	if (q->elevator)
> > >  		goto insert;
> >=20
> > 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 wai=
tqueue
> > 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.
>=20
> OK, the reason is simple, and it is only related with direct issue.
>=20
> Under this situation, when the queue is quiesced, we have to
>=20
> 	- insert the current request into sw queue(scheduler queue)
> 	OR
> 	-wait until queue becomes unquiesced like what this patch is doing
>=20
> 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 quiescin=
g.
>=20
> For the 2nd way(what this patch is doing), one benefit is that applicatio=
n
> 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 co=
st
> of one waitqueue, the cost should be cheap, but if you persist on the 1st
> approach, I am fine to change to that.

Hello Ming,

Since the runtime overhead of the alternative approach (insert into queue) =
is
significantly smaller and since it will result in a simpler implementation =
I
prefer the alternative approach.

Thanks,

Bart.=