Re: [PATCH 1/4] block: blk-mq: support draining mq queue

[Christoph Hellwig <hch@infradead.org>]

On Tue, Dec 24, 2013 at 11:30:58AM +0800, Ming Lei wrote:
> Some drivers don't have LLDD, like null_blk, virtio-blk, ...

Well, they are the low-level driver in this case.

> Actually the below block APIs
>                blk_get_request()
>                blk_put_request()
>                blk_execute_rq_nowait()
>                blk_register_queue()
>                blk_unregister_queue()
>                blk_insert_flush()
>                ....
> 
> have been there to support MQ.

All these are called by higher level code and not the driver, so we need
a switch between MQ and non-MQ support somewhere.

> One problem about the 2nd way is that MQ drivers may become a bit
> ugly if the driver still need to support traditional request/bio mechanism.

On the other hand drivers have no legitimate case for long-term parallel
MQ/non-MQ code paths.  The only exception is null_blk as a benchmarking
vehicle, but we should not optimize for that.

> >> +             spin_unlock_irq(lock);
> >> +     }
> >
> > Why doesn't the mq case set QUEUE_FLAG_DEAD here?
> 
> Looks DEAD flag is set to prevent request_fn from being called,
> and there is no request_fn for MQ, so I don't set it and QUEUE_FLAG_DYING
> has been checked in blk_mq_queue_enter() already.
> 
> But we can do it easily.

Do we have a per-context flag to prevent calling into the driver
instead?  Sorry, it's been a while that I last looked at the code, but
anyone just looking over the difference will have the same question, so
the answer needs to go into a comment.

> > Also blk_execute_rq_nowait now needs to check the dying case for MQ as
> > well.
> 
> In this patch the dying flag is only checked in blk_mq_queue_enter()
> which is called in allocating request path, so once the request is
> allocated, we just simply wait for its completion in blk_mq_drain_queue().
> 
> Even for non-MQ case, looks blk_queue_dying(q) is checked to avoid
> being freed early before running queue, I am not sure if it is good
> because the I/O might or can have been completed.  And it isn't a problem
> for MQ because this request can't be reused when dying is set.

Good.  I think we'll want comments like that preferable in the code, but
at very least in the commit log.