Re: [PATCH v2 6/8] blk-mq: don't stop queue for quiescing

Ming Lei
2017-05-30T00:27:27Z

On Sun, May 28, 2017 at 04:03:20PM +0000, Bart Van Assche wrote:
> On Sun, 2017-05-28 at 18:50 +0800, Ming Lei wrote:
> > On Sat, May 27, 2017 at 09:49:27PM +0000, Bart Van Assche wrote:
> > > On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote:
> > > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > > index 032045841856..84cce67caee3 100644
> > > > --- a/block/blk-mq.c
> > > > +++ b/block/blk-mq.c
> > > > @@ -168,8 +168,6 @@ void blk_mq_quiesce_queue(struct request_queue *q)
> > > > unsigned int i;
> > > > bool rcu = false;
> > > >
> > > > - __blk_mq_stop_hw_queues(q, true);
> > > > -
> > > > spin_lock_irq(q->queue_lock);
> > > > queue_flag_set(QUEUE_FLAG_QUIESCED, q);
> > > > spin_unlock_irq(q->queue_lock);
> > > > @@ -198,7 +196,12 @@ void blk_mq_unquiesce_queue(struct request_queue *q)
> > > > queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
> > > > spin_unlock_irq(q->queue_lock);
> > > >
> > > > - blk_mq_start_stopped_hw_queues(q, true);
> > > > + /*
> > > > + * During quiescing, requests can be inserted
> > > > + * to scheduler's queue or sw queue, so we run
> > > > + * queues for dispatching these requests.
> > > > + */
> > > > + blk_mq_start_hw_queues(q);
> > > > }
> > > > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
> > >
> > > This looks really weird to me. If blk_mq_quiesce_queue() doesn't stop any
> > > hardware queues, why should blk_mq_unquiesce_queue() start any hardware
> > > queues?
> >
> > Please see the above comment, especially in direct issue path, we have to
> > insert the request into sw/scheduler queue when queue is quiesced,
> > and these queued requests have to be dispatched out during unquiescing.
> >
> > I considered to sleep the current context under this situation, but
> > turns out it is more complicated to handle, given we have very limited
> > queue depth, just let applications consumes all, then wait.
>
> Hello Ming,
>
> The above seems to me to be an argument to *run* the queue from inside
> blk_mq_unquiesce_queue() instead of *starting* stopped queues from inside
> that function.

Yes, it is run queues, as you can see in my comment.

The reality is that we can't run queue without clearing the STOPPED state.

Thanks,
Ming