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

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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.

Bart.




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