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

On Wed, 9 Feb 2011, Brendan Cully wrote:

> > > That is odd, and it is quite different from the intended design of the 
> > > PM core.  Drivers are supposed to put their devices into a known 
> > > suspended state; then afterwards they put the devices back into an 
> > > operational state.  What happens while the devices are in the suspended 
> > > state isn't supposed to matter -- the system transition can fail, but 
> > > devices get treated exactly the same way as if it succeeded.
> > > 
> > > Why do your drivers need to recover differently based on the success of 
> > > the hypercall?
> > 
> > checkpointing isn't really my area but AIUI you don't want to do a full
> > device teardown and reconnect like you would with a proper suspend
> > because of the time that takes which prevents you from doing continuous
> > rolling checkpoints at granularity which people want to implement
> > various disaster recovery schemes.
> > 
> > Hopefully one of the Xen checkpointing folks will chime in and explain
> > why this is not possible to achieve at the individual driver level (or,
> > even better, with a patch which does it that way ;-)).
> As Ian says, Xen has suspend_cancel because while the normal full
> suspend/resume path works, it is much slower, and the work done during
> resume is redundant. I don't remember the numbers offhand, but when we
> added suspend_cancel I think we could do full suspend/resume in under
> 100us, which was maybe a couple of orders of magnitude faster than
> full resume (largely due to slow xenstore handshaking on resume,
> IIRC). It made a big difference for our Remus HA project, which was
> checkpointing tens of times per second.
> I'd like to keep the fast resume option, and expect that it can be
> contained entirely in Xen-specific code. I'll try to get someone to
> look into it here.
> I think fast resume is somewhat orthogonal to the problem of hanging
> on resume, which just sounds like a xen-specific bug in the slow
> path.

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.

Alan Stern

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