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

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Date:	Wed, 15 Oct 2008 17:13:21 +0200
From:	Cedric Le Goater <clg@...ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
CC:	Ingo Molnar <mingo@...e.hu>, Dave Hansen <dave@...ux.vnet.ibm.com>,
	jeremy@...p.org, arnd@...db.de,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrey Mirkin <major@...nvz.org>
Subject: Re: [RFC v6][PATCH 0/9] Kernel based checkpoint/restart

>> the self checkpoint and self restore syscalls, like Oren is proposing, are 
>> simpler but they require the process cooperation to be triggered. we could
>> image doing that in a special signal handler which would allow us to jump
>> in the right task context. 
> 
> This description is not accurate:
> 
> For checkpoint, both implementations use an "external" task to read the state
> from other tasks. (In my implementation that "other" task can be self).

which is good, since some applications want to checkpoint themselves and that's
a way to provide them a generic service.
 
> For restart, both implementation expect the restarting process to restore its
> own state. They differ in that Andrew's patchset also creates that process
> while mine (at the moment) relies on the existing (self) task.

hmm, 

It seems that your patchset relies on the fact that the tasks are checkpointed 
and restarted at a syscall boundary. right ? I'm might be completely wrong
on that :)

> In other words, none of them will require any cooperation on part of the
> checkpointed tasks, and both will require cooperation on part of the restarting
> tasks (the latter is easy since we create and fully control these tasks).

yes.

>> I don't have any preference but looking at the code of the different patchsets
>> there are some tricky areas and I'm wondering which path is easier, safer, 
>> and portable. 
> 
> I am thinking which path is preferred: create the processes in kernel space
> (like Andrew's patch does) or in user space (like Zap does). In the mini-summit
> we agreed in favor of kernel space, but I can still see arguments why user space
> may be better.

I'm more familiar with the second algorithm, restarting the process tree in
user space and let each task restart itself with the sys_restart syscall. But
that's because I've been working on a C/R framework which freezes tasks on 
a syscall boundary, which makes a developer's life easy for restart. 

But as you know, a restarted process resumes its execution where it was 
checkpointed. So i'm wondering what are the hidden issues with a in-kernel 
checkpoint and in-kernel restart. To be more precise, why Andrey needs a 
i386_ret_from_resume  trampoline in : 

	http://lkml.org/lkml/2008/9/3/181

and why don't you ? 

> (note: I refer strictly to the creation of the processes during restart, not 
>  how their state is restored).

OK 

> any thoughts ?

thanks Oren,

C.
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