Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

[Oren Laadan <orenl@cs.columbia.edu>]

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.