linux-kernel - Re: [PATCH] Freeze bdevs when freezing processes.

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Message-Id: <200610261111.30486.rjw@sisk.pl>
Date:	Thu, 26 Oct 2006 11:11:29 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	David Chinner <dgc@....com>
Cc:	Nigel Cunningham <ncunningham@...uxmail.org>,
	Pavel Machek <pavel@....cz>, Andrew Morton <akpm@...l.org>,
	LKML <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: [PATCH] Freeze bdevs when freezing processes.

Hi,

On Thursday, 26 October 2006 10:57, David Chinner wrote:
> 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).

Well, I think this exactly is the problem, because worker_threads run with
PF_NOFREEZE set (as I've just said in another message).

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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