On Tue, 2017-05-30 at 08:27 +0800, Ming Lei wrote: > 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.
Hello Ming, I think it's completely wrong to make blk_mq_unquiesce_queue() to clear the STOPPED state. Stopping a queue is typically used to tell the block layer to stop dispatching requests and not to resume dispatching requests until the STOPPED flag is cleared. If stopping and quiescing a queue happen simultaneously then dispatching should not occur before both blk_mq_unquiesce_queue() and blk_mq_start_hw_queue() have been called. Your patch would make dispatching start too early. Bart.
