Messages in this thread | | | Date | Thu, 16 Oct 2008 09:49:40 -0400 | From | Oren Laadan <> | Subject | Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart |
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Daniel Lezcano wrote: > Oren Laadan wrote: >> Cedric Le Goater wrote: >>> Dave Hansen wrote: >>>> On Mon, 2008-10-13 at 10:13 +0200, Cedric Le Goater wrote: >>>>> hmm, that's rather complex, because we have to take into account >>>>> the kernel stack, no ? This is what Andrey was trying to solve in >>>>> his patchset back in September : >>>>> >>>>> http://lkml.org/lkml/2008/9/3/96 >>>>> >>>>> the restart phase simulates a clone and switch_to to (not) restore >>>>> the kernel stack. right ? >>>> Do we ever have to worry about the kernel stack if we simply say that >>>> tasks have to be *in* userspace when we checkpoint them. >>> at a syscall boundary for example. that would make our life easier >>> definitely. >> >> The ideal situation is never worry about kernel stack: either we catch >> the task in user space or at a syscall boundary. This is taken care of >> by freezing the tasks prior to checkpoint. >> >> The one exception (and it is a tedious one !) are states in which the >> task is already frozen by definition: any ptrace blocking point where >> the tracee waits for the tracer to grant permission to proceed with >> its execution. Another example is in vfork(), waiting for completion. > > I would say these are perfect places for "may be non-checkpointable" :)
For now, yes. But we definitely want this capability in the long run; otherwise we won't be able to checkpoint a kernel compile ('make' uses vfork), or anything with 'gdb' running inside, or 'strace', and other goodies.
> >> In both cases, there will be a kernel stack and we cannot avoid it. >> The bad news is that it may be a bit tedious to restart these cases. >> The good news, however, is that they are very well defined locations >> with well defined semantics. So upon restart all that is needed is >> to emulate the expected behavior had we not been checkpointed. This, >> luckily, does not require rebuilding the kernel stack, but instead >> some smart glue code for a finite set of special cases.
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