Re: [PATCH 0/9] OpenVZ kernel based checkpointing/restart

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

Serge E. Hallyn wrote:
> Quoting Daniel Lezcano (dlezcano@fr.ibm.com):
>> Oren Laadan wrote:
>>> Daniel Lezcano wrote:
>>>> Louis Rilling wrote:
>>>>> On Fri, Oct 17, 2008 at 04:33:03PM -0700, Dave Hansen wrote:
>>>>>> On Wed, 2008-09-03 at 14:57 +0400, Andrey Mirkin wrote:
>>>>>>> This patchset introduces kernel based checkpointing/restart as it is
>>>>>>> implemented in OpenVZ project. This patchset has limited functionality and
>>>>>>> are able to checkpoint/restart only single process. Recently Oren Laaden
>>>>>>> sent another kernel based implementation of checkpoint/restart. The main
>>>>>>> differences between this patchset and Oren's patchset are:
>>>>>> Hi Andrey,
>>>>>>
>>>>>> I'm curious what you want to happen with this patch set.  Is there
>>>>>> something specific in Oren's set that deficient which you need
>>>>>> implemented?  Are there some technical reasons you prefer this code?
>>>>> To be fair, and since (IIRC) the initial intent was to start with OpenVZ's
>>>>> approach, shouldn't Oren answer the same questions with respect to Andrey's
>>>>> patchset?
>>>>>
>>>>> I'm afraid that we are forgetting to take the best from both approaches...
>>>> I agree with Louis.
>>>>
>>>> I played with Oren's patchset and tryed to port it on x86_64. I was able 
>>>> to sys_checkpoint/sys_restart but if you remove the restoring of the 
>>>> general registers, the restart still works. I am not an expert on asm, 
>>>> but my hypothesis is when we call sys_checkpoint the registers are saved 
>>>> on the stack by the syscall and when we restore the memory of the 
>>>> process, we restore the stack and the stacked registers are restored 
>>>> when exiting the sys_restart. That make me feel there is an important 
>>>> gap between external checkpoint and internal checkpoint.
>>> This is a misconception: my patches are not "internal checkpoint". My
>>> patches are basically "external checkpoint" by design, which *also*
>>> accommodates self-checkpointing (aka internal). The same holds for the
>>> restart. The implementation is demonstrated with "self-checkpoint" to
>>> avoid complicating things at this early stage of proof-of-concept.
>> Yep, I read your patchset :)
>>
>> I just want to clarify what we want to demonstrate with this patchset 
>> for the proof-of-concept ? A self CR does not show what are the 
>> complicate parts of the CR, we are just showing we can dump the memory 
>> from the kernel and do setcontext/getcontext.
>>
>> We state at the container mini-summit on an approach:
>>
>>     1. Pre-dump
>>     2. Freeze the container
>>     3. Dump
>>     4. Thaw/Kill the container
>>     5. Post-dump
>>
>> We already have the freezer, and we can forget for now pre-dump and 
>> post-dump.
>>
>> IMHO, for the proof-of-concept we should do a minimal CR (like you did), 
>> but conforming with these 5 points, but that means we have to do an 
>> external checkpoint.
> 
> Right, Oren, iiuc you are insisting that 'external checkpoint' and
> 'multiple task checkpoint' are the same thing.  But they aren't.
> Rather, I think that what we say is 'multiple tasks c/r' is what you say
> should be done from user-space :)

Then I don't explain myself clearly :)

The only thing I consider doing in user space is the creation of
the container, the namespaces and the processes.

I argue that "external checkpoint of a single process" is very few
lines of code away from "self checkpoint" that is in v7.

I'm not sure how you define "external restart" ?  eventually, the
processes restart themselves. It is a question of how the processes
are created to begin with.

> 
> So particularly given that your patchset seems to be in good shape,
> I'd like to see external checkpoint explicitly supported.  Please
> just call me a dunce if v7 already works for that.
> 

It seems like you want a single process to checkpoint a single (other)
process, and then a single process to start a single (other) process.

I tried to explicitly avoid dealing with the container (user space ?
kernel space ?) and with creating new processes (user space ? kernel
space ?).

Nevertheless, it's the _same_ code. All that is needed is to make the
checkpoint syscall refer to the other task instead of self, and the
restart should create a container and fork there, then call sys_restart.

I guess instead of repeating this argument over, I will go ahead and
post a patch on top of v7 to demonstrate this (without a container,
therefore without preserving the original pid).

Oren.