Re: Freezable workqueue blocks non-freezable workqueue during the system resume process

[Jan Kara <jack@suse.cz>]

On Mon 14-03-16 10:37:22, Alan Stern wrote:
> On Mon, 14 Mar 2016, Jan Kara wrote:
> 
> > On Fri 11-03-16 12:56:10, Tejun Heo wrote:
> > > Hello, Jan.
> > > 
> > > On Thu, Mar 03, 2016 at 10:33:10AM +0100, Jan Kara wrote:
> > > > > Ugh... that's nasty.  I wonder whether the right thing to do is making
> > > > > writeback workers non-freezable.  IOs are supposed to be blocked from
> > > > > lower layer anyway.  Jan, what do you think?
> > > > 
> > > > Well no, at least currently IO is not blocked in lower layers AFAIK - for
> > > > that you'd need to freeze block devices & filesystems and there are issues
> > > 
> > > At least libata does and I think SCSI does too, but yeah, there
> > > probably are drivers which depend on block layer blocking IOs, which
> > > btw is a pretty fragile way to go about as upper layers might not be
> > > the only source of activities.
> > > 
> > > > with that (Jiri Kosina was the last one which was trying to make this work
> > > > AFAIR). And I think you need to stop writeback (and generally any IO) to be
> > > > generated so that it doesn't interact in a strange way with device drivers
> > > > being frozen. So IMO until suspend freezes filesystems & devices properly
> > > > you have to freeze writeback workqueue.
> 
> What do you mean by "freezes ... devices"?  Only a piece of code can be 
> frozen -- not a device.

By that I meant block device and filesystem freezing. That way filesystem
is frozen so that it doesn't submit any more IO to the device.

> The kernel does suspend device drivers; that is, it invokes their
> suspend callbacks.  But it doesn't "freeze" them in any sense.  Once a 
> driver has been suspended, it assumes it won't receive any I/O requests 
> until it has been resumed.  Therefore the kernel first has to prevent 
> all the upper layers from generating such requests and/or sending them 
> to the low-level drivers.

OK, so Tejun and you should talk together because you both seem to want
something else... If I understand it right, Tejun wants suspended devices
to just queue requests that have been submitted after these devices were
suspended and complete them once they are resumed...

> > > I still think the right thing to do is plugging that block layer or
> > > low level drivers.  It's like we're trying to plug multiple sources
> > > when we can plug the point where they come together anyway.
> > 
> > I agree that freezing writeback workers is a workaround for real issues at
> > best and ideally we shouldn't have to do that. But at least for now I had
> > the impression that it is needed for suspend to work reasonably reliably.
> 
> The design is not to plug low-level drivers, but instead to prevent
> them from receiving any requests by plugging or freezing high-level
> code.
> 
> It's pretty clear that we don't want to have ongoing I/O during a 
> system suspend, right?  And that means the I/O has to be prevented (or 
> "plugged", if you prefer) somewhere -- either at an upper layer or at a 
> lower layer.  There was a choice to be made, and the decision was to do 
> it at an upper layer.

I agree the IO has to be plugged somewhere. And Tejun seems to want to plug
it at lower layer...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR