Re: [linux-pm] Re: Hibernation considerations

Rafael J. Wysocki
2007-07-23T22:02:31Z

On Monday, 23 July 2007 23:55, Nigel Cunningham wrote:
> Hi.
>
> On Tuesday 24 July 2007 01:23:15 Alan Stern wrote:
> > On Mon, 23 Jul 2007, Nigel Cunningham wrote:
> >
> > > Take a step back for a second.
> > >
> > > The problem we're facing now is that we're getting some userspace threads,
> > > used in processing I/O, that are functioning as exceptions to the "freeze
> > > userspace, then freezeable kernel threads" rule. They are only exceptions
> > > because of that role in processing I/O - because they're de facto kernel
> > > threads. So, if we orient our thinking more in terms of I/O processing and
> > > less in terms of the userspace/kernelspace distinction, we'll have a
> > > solution:
> > >
> > > 1) Freeze processes that aren't fs related (ie stop them generating I/O).
> >
> > The problem here is that with things like FUSE, _every_ process is
> > potentially fs related. Nothing prevents a FUSE thread from doing IPC
> > with any other thread.
>
> Yes, but the fuse thread is going to know what other thread it's doing IPC
> with, so it can get that thread flagged too.

Yes, but that thread may do IPC with yet another one and so on.

> > > 2) Flush pending I/O.
> > > 3) Freeze filesystems in reverse order of dependency, the primary purpose
> > > being to stop them generating further I/O on their metadata.
> > >
> > > Locks that are being held are only being held because work is being done.
> If
> > > we progressively focus on threads in terms of their create/process work
> > > dependencies, we'll see that the problem isn't at all intractable.
> >
> > As has been mentioned before, keeping track of all that dependency
> > information would be very fragile and time-consuming.
>
> I disagree. It's at least going to be less fragile and time-consuming then
> maintaining new/extra code for kexec.

Well, I think the issue is real, so we need to find a solution (the simpler,
the better) and that need not be related to kexec. ;-)

Greetings,
Rafael

--
"Premature optimization is the root of all evil." - Donald Knuth