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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Thu, Jun 01, 2017 at 11:19:11PM +0000, Bart Van Assche wrote:
> 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:
> > > > 
> > > > +	/* 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.
> 
> 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.

OK, no problem, will change to the way of insert & run queue.

Thanks,
Ming



[Index of Archives]     [Linux RAID]     [Linux SCSI]     [Linux ATA RAID]     [IDE]     [Linux Wireless]     [Linux Kernel]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Device Mapper]

  Powered by Linux