Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue

[Oliver Neukum]

On Tue, 2016-08-02 at 13:34 +0200, Michal Hocko wrote:
> On Tue 02-08-16 12:03:23, Oliver Neukum wrote:
> > On Tue, 2016-08-02 at 10:18 +0200, Michal Hocko wrote:
> > > On Tue 02-08-16 10:06:12, Oliver Neukum wrote:
> > > > On Mon, 2016-08-01 at 10:20 -0400, Tejun Heo wrote:
> > > > > Hello,
> > > > 
> > > > > > If any real IO depends on those devices then this is not sufficient and
> > > > > > they need some form of guarantee for progress (aka mempool).
> > > > > 
> > > > > Oliver, Alan, what do you think?  If USB itself can't operate without
> > > > > allocating memory during transactions, whatever USB storage drivers
> > > > 
> > > > It cannot. The IO must be described to the hardware with a data
> > > > structure in memory.
> > > > 
> > > > > are doing isn't all that meaningful.  Can we proceed with the
> > > > > workqueue patches?  Also, it could be that the only thing GFP_NOIO and
> > > > > GFP_ATOMIC are doing is increasing the chance of IO failures under
> > > > > memory pressure.  Maybe it'd be a good idea to reconsider the
> > > > > approach?
> > > > 
> > > > We had actual deadlocks with GFP_KERNEL. It seems to me that the SCSI
> > > > layer can deal with IO that cannot be completed due to a lack of memory
> > > > at least somewhat, but a deadlock within a driver would obviously be
> > > > deadly. So I don't think that mempools would remove the need for
> > > > GFP_NOIO as there are places in usbcore we cannot enter the page
> > > > laundering path from. They are an additional need.
> > > 
> > > OK, I guess there is some misunderstanding here. I believe that Tejun
> > > wasn't arguing to drop GFP_NOIO. It might be really needed for the dead
> > > lock avoidance. No question about that. The whole point is that
> > > WQ_RECLAIM might be completely pointless because a rescuer wouldn't help
> > > much if the work item would do GFP_NOIO and get stuck in the page
> > > allocator.
> > 
> > But that can be a problem only if the items on the work queue are
> > actually run and without WQ_MEM_RECLAIM that guarantee cannot be made.
> > We can deal with failures of memory allocation. But the requests
> > must actually terminate.
> 
> I think you have missed my point. So let me ask differently. What is the
> difference between your work item not running at all or looping
> endlessly with GFP_NOIO inside the page allocator? If that particular
> work item is necessary for the further progress then the system is
> screwed one way or another.

The key difference is that I could give the right parameters to the
kmalloc() call. If it doesn't run, I am surely screwed. Thus I conclude
that WQ_RECLAIM needs to be set.

	Regards
		Oliver