Re: [PATCH v6] block: loop: avoiding too many pending per work I/O

[Tejun Heo <tj@kernel.org>]

Hello,

On Sat, May 02, 2015 at 10:56:20PM +0800, Ming Lei wrote:
> > Maybe just cap max_active to NR_OF_LOOP_DEVS * 16 or sth?  But idk,
> 
> It might not work because there are nested loop devices like fedora live CD, and
> in theory the max_active should have been set as loop's queue depth *
> nr_loop, otherwise there may be possibility of hanging.
> 
> So this patch is introduced.

If loop devices can be stacked, regardless of what you do with
nr_active, it may deadlock.  There needs to be a rescuer per each
nesting level (or just one per device).  This means that the current
code is broken.

> > how many concurrent workers are we talking about and why are we
> > capping per-queue concurrency from worker pool side instead of command
> > tag side?
> 
> I think there should be performance advantage to make queue depth a bit more
> because it can help to make queue pipeline as full. Also queue depth often
> means how many requests the hardware can queue, and it is a bit different
> with per-queue concurrency.

I'm not really following.  Can you please elaborate?

Thanks.

-- 
tejun