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 <j...@suse.com>
SUSE Labs, CR
