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

[Serge E. Hallyn]

Quoting Daniel Lezcano (dlezcano at 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 :)

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.

thanks,
-serge
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