Re: [PATCH] Freeze bdevs when freezing processes.

[David Chinner <dgc@sgi.com>]

Hi Nigel,

On Thu, Oct 26, 2006 at 06:18:29PM +1000, Nigel Cunningham wrote:
> On Thu, 2006-10-26 at 17:30 +1000, David Chinner wrote:
> > We have daemons running in the background that can definitely do stuff
> > after a sync. hmm - one does try_to_freeze() after a wakeup, the
> > other does:
> > 
> >                 if (unlikely(freezing(current))) {
> >                         set_bit(XBT_FORCE_SLEEP, &target->bt_flags);
> >                         refrigerator();
> >                 } else {
> >                         clear_bit(XBT_FORCE_SLEEP, &target->bt_flags);
> >                 }
> > 
> > before it goes to sleep. So that one (xfsbufd - metadata buffer flushing)
> > can definitely wake up after the sync and do work, and the other could if
> > the kernel thread freeze occurs after the sync.
> > 
> > Another good question at this point - exactly how should we be putting
> > these thread to to sleep? Are both these valid methods for freezing them?
> > And should we be freezing when we wake up instead of before we go to
> > sleep? i.e. what are teh rules we are supposed to be following?
> 
> As you have them at the moment, the threads seem to be freezing fine.
> The issue I've seen in the past related not to threads but to timer
> based activity. Admittedly it was 2.6.14 when I last looked at it, but
> there used to be a possibility for XFS to submit I/O from a timer when
> the threads are frozen but the bdev isn't frozen. Has that changed?

I didn't think we've ever done that - periodic or delayed operations
are passed off to the kernel threads to execute. A stack trace
(if you still have it) would be really help here.

Hmmm - we have a couple of per-cpu work queues as well that are
used on I/O completion and that can, in some circumstances,
trigger new transactions. If we are only flush metadata, then
I don't think that any more I/o will be issued, but I could be
wrong (maze of twisty passages).

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group