Re: bugs with ckpt-v15-dev

[Matt Helsley <matthltc-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Wed, May 20, 2009 at 08:14:57AM -0500, Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org):
> > 
> > 
> > Matt Helsley wrote:
> > > On Mon, May 18, 2009 at 06:21:22PM -0500, Nathan Lynch wrote:
> > >> Matt Helsley <matthltc-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> writes:
> > >>
> > >>> On Mon, May 18, 2009 at 04:36:11PM -0500, Nathan Lynch wrote:
> > >>>> [1] Should CONFIG_CHECKPOINT depend on CONFIG_CGROUPS and/or
> > >>>> CONFIG_CGROUPS_FREEZER?  We require tasks to be put in frozen state
> > >>>> before checkpoint, is there any mechanism apart from
> > >>>> cgroup/freezer.state to do this?
> > >>> Have you tried sending all of the tasks SIGSTOP? It won't 100% freeze
> > >>> the tasks -- they'd still be capable of responding to some signals
> > >>> (CONT, TERM..). Also they'd presumably be placed in the stopped state
> > >>> upon restart so a SIGCONT will be needed. In the case of bash, at
> > >>> least, that will technically change what happens upon restart. My
> > >>> guess is that in many cases it won't matter but there are some where
> > >>> it will.
> > >> Hmm, I'm having trouble understanding your suggestion.  The current
> > >> checkpoint implementation requires non-self tasks to be frozen (p->flags
> > >> & PF_FROZEN), which is not equivalent to stopped state (task->state &
> > >> __TASK_STOPPED).  That is, it would refuse to checkpoint tasks in
> > >> stopped state.  See may_checkpoint_task().
> > > 
> > > Oops. You're right. That would require changing may_checkpoint_task() to include
> > > __TASK_STOPPED -- not something we'd want in the final code. I had assumed
> > > you wanted to try a different mechanism for debugging purposes.
> > > 
> > 
> > Allowing checkpoint of stopped tasks is actually not such a bad
> > idea, IMHO.
> 
> Well, it might be bad for the same reason that Matt is pursuing the
> CHECKPOINTING freezer state:  the task might get kicked alive in
> the middle of the checkpoint.
> 
> So it might be ok so long as we still move the task to CHECKPOINTING
> state.  But I'm just not sure it's worth worrying about.

FYI: currently there is no CHECKPOINTING state. CHECKPOINTING is
specific to the freezer.state -- the tasks still appear "frozen" in the
D state. This works since nothing else unfreezes these tasks.

Cheers,
	-Matt Helsley