Re: freeze vs freezer

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On Nov 27, 2007, at 17:49:18, Jeremy Fitzhardinge wrote:
Rafael J. Wysocki wrote:
Well, this is more-or-less how we all imagine that should be done eventually.
The main problem is how to implement it without causing too much
breakage. Also, there are some dirty details that need to be
taken into consideration.
For Xen suspend/resume, I'd like to use the freezer to get all
threads into a known consistent state (where, specifically, they
don't have any outstanding pagetable updates pending). In other
words, the freezer as it currently stands is what I want, modulo
some of these issues where it gets caught up unexpectedly. If
threads end up getting frozen anywhere preempt isn't explicitly
disabled, it wouldn't work for me.
The problem with "one freezer" is that "known consistent state" means
something completely different to every single driver and subsystem.
Xen wants it to mean "No pending page table updates and no more
updates from this point forward". A network driver wants it to mean
"All pending network packets DMAed out or in and the device shut down
with all remaining packets queued. A SATA controller wants it to
mean "All DMA quiesced and no more commands", etc.
The only way to have that work is to put minimal definitions of what
state you care about in the drivers themselves. For Xen this means
that you need to have an appropriately-timed suspend handler which
hooks into Xen code very precisely to create and preserve the "No
pending page table updates" state that you care about. It will be
more work in the short term but it's the only maintainable solution
in the long term IMO.
Cheers,
Kyle Moffett

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