Re: freeze vs freezer

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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