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

[Michal Hocko]

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.
-- 
Michal Hocko
SUSE Labs