linux-kernel - Re: [Xen-devel] Re: [linux-pm] [PATCH 0/2] Fix hangup after creating checkpoint on Xen.

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Message-ID: <Pine.LNX.4.44L0.1102101043010.2029-100000@iolanthe.rowland.org>
Date:	Thu, 10 Feb 2011 11:00:51 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ian Campbell <ijc@...lion.org.uk>
cc:	Brendan Cully <brendan@...ubc.ca>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"linux-pm@...ts.linux-foundation.org" 
	<linux-pm@...ts.linux-foundation.org>,
	"SUZUKI, Kazuhiro" <kaz@...fujitsu.com>
Subject: Re: [Xen-devel] Re: [linux-pm] [PATCH 0/2] Fix hangup after creating
 checkpoint on Xen.

On Thu, 10 Feb 2011, Ian Campbell wrote:

> On Wed, 2011-02-09 at 23:42 +0000, Alan Stern wrote:
> > In fact there already is a "fast suspend & resume" path in the PM core.  
> > It's the freeze/thaw procedure used when starting to hibernate.  The
> > documentation specifically says that drivers' freeze methods are
> > supposed to quiesce their devices but not change power levels.  In
> > addition, the thaw method is invoked as part of recovery from a failed
> > hibernation attempt, so it already has the "cancel" semantics that xen 
> > seems to want.
> 
> Sounds like that would work and I would much prefer to simply make
> correct use of the core functionality.

It seems like a reasonable approach.  Whether it will actually _work_ 
is a harder question...  :-)

> So PMSG_FREEZE is balanced by either PMSG_RECOVER or PMSG_THAW depending
> on whether the suspend was cancelled or not?

Basically yes.  It is also "balanced" by PMSG_RESTORE, which is used
after a memory image has been restored (although this isn't relevant to
your snapshotting).  See the comments in include/linux/pm.h.

>  So the sequence of events
> is something like:
> 	dpm_suspend_start(PMSG_FREEZE);
>          
> 		dpm_suspend_noirq(PMSG_FREEZE);
>                          
> 			sysdev_suspend(PMSG_QUIESCE);

This should say sysdev_suspend(PMSG_FREEZE).

> 			cancelled = suspend_hypercall()

At this point swsusp_arch_suspend() is called.  If that translates to 
suspend_hypercall() in your setting, then yes.

> 			sysdev_resume();
>                  
> 		dpm_resume_noirq(cancelled ? PMSG_RECOVER : PMSG_THAW);
>          
> 	dpm_resume_end(cancelled ? PMSG_RECOVER : PMSG_THAW);
> ?

Yes.

> (For comparison we currently have:
> > > >         dpm_suspend_start(PMSG_SUSPEND);
> > > >         
> > > >                 dpm_suspend_noirq(PMSG_SUSPEND);
> > > >                         
> > > >                         sysdev_suspend(PMSG_SUSPEND);
> > > >                         /* suspend hypercall */
> > > >                         sysdev_resume();
> > > >                 
> > > >                 dpm_resume_noirq(PMSG_RESUME);
> > > >         
> > > >         dpm_resume_end(PMSG_RESUME);
> )

Right.  The sequence of calls is the same, but the PMSG_ argument is 
different so drivers are expected to act differently in response.

Alan Stern

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