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

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Date:	Thu, 16 Oct 2008 09:49:40 -0400
From:	Oren Laadan <orenl@...columbia.edu>
To:	Daniel Lezcano <dlezcano@...ibm.com>
CC:	Cedric Le Goater <clg@...ibm.com>, jeremy@...p.org, arnd@...db.de,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrey Mirkin <major@...nvz.org>
Subject: Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart



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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