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

* Dave Hansen <dave@linux.vnet.ibm.com> wrote:

> On Thu, 2008-10-09 at 14:46 +0200, Ingo Molnar wrote:
> > * Oren Laadan <orenl@cs.columbia.edu> wrote:
> >
> > > These patches implement basic checkpoint-restart [CR]. This version
> > > (v6) supports basic tasks with simple private memory, and open files
> > > (regular files and directories only). Changes mainly cleanups. See
> > > original announcements below.
> >
> > i'm wondering about the following productization aspect: it would be
> > very useful to applications and users if they knew whether it is safe to
> > checkpoint a given app. I.e. whether that app has any state that cannot
> > be stored/restored yet.
>
> Absolutely!
>
> My first inclination was to do this at checkpoint time: detect and
> tell users why an app or container can't actually be checkpointed.
> But, if I get you right, you're talking about something that happens
> more during the runtime of the app than during the checkpoint. This
> sounds like a wonderful approach to me, and much better than what I
> was thinking of.
>
> What kind of mechanism do you have in mind?
>
> int sys_remap_file_pages(...)
> {
> ...
> oh_crap_we_dont_support_this_yet(current);
> }
>
> Then the oh_crap..() function sets a task flag or something?

yeah, something like that. A key aspect of it is that is has to be very
low-key on the source code level - we dont want to sprinkle the kernel
with anything ugly. Perhaps something pretty explicit:

current->flags |= PF_NOCR;
as we do the same thing today for certain facilities:

current->flags |= PF_NOFREEZE;
you probably want to hide it behind:

set_current_nocr();

and have a set_task_nocr() as well, in case there's some proxy state
installed by another task.

Via such wrappers there's no overhead at all in the
!CONFIG_CHECKPOINT_RESTART case.

Plus you could drive the debug mechanism via it as well, by using a
trivial extension of the facility:

set_current_nocr("CR: sys_remap_file_pages not supported yet.");
...
set_task_nocr(t, "CR: PI futexes not supported yet.");

Ingo


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