[Devel] Re: [PATCH 0/6] /proc/pid/checkpointable

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> 
> 
> Sukadev Bhattiprolu wrote:
> > From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> > Date: Fri, 13 Mar 2009 17:25:42 -0700
> > Subject: [PATCH 5/6] Define and use proc_pid_checkpointable()
> > 
> > Create a proc file, /proc/pid/checkpointable, which shows '1' if
> > task is checkpointable and '0' if it is not.
> > 
> > To determine whether a task is checkpointable, the handler for this
> > new proc file, shares the same code with sys_checkpoint().

Hey Oren,

3 counter-points:

> I still don't understand why we would like to do it this way.
> 
> First, it makes little sense to do it per-task, because we are supposed
> to checkpoint an entire container.

Yes we need per-container info too.  Actually, per-checkpoint-job-init,
so if we send pids in for that, it should return false if we send in the
pid of a task which isn't a proper checkpoint-job-init.

But we also want the info per-task, for debugging info.

I don't know how to represent those two cases, though.

Also debugfs may be a more appropriate medium.

> Second, what's wrong with doing a "dry" checkpoint on the container (or
> if you prefer, the task, for what it's worth), that will not buffer nor
> write out any data - just say "yes" or "no" ?

You can't get a text explanation like 'fd 4 (/sys/class/net/eth0) is not
checkpointable'.  That's what's wrong with it.

> (we could use a flag "CR_CTX_DRYRUN" when calling sys_checkpoint() for
> this, and test for this flag in, say, kwrite/kread).
> 
> After all, we don't expect applications or users to continuously and
> repeatedly test if they can checkpoint, so it isn't performance critical.
> So we simply reuse the existing code.

No, but we do expect someone trying to checkpoint their job and failing
to be curious as to why.

> This would also catch cases where we can't checkpoint because the kernel
> is low on memory - which wouldn't show up otherwise.
> 
> And in any case, this is orthogonal to what Dave is pushing, following
> Ingo's comment, to know when a task _becomes_ not-checkpointable. (And
> in any case, I think our time is better spent on adding functionality
> instead).

I think Suka's patch is small enough that there's not a lot of pursuing
going on.  Dave's may_checkpoint is a lot more ambitious.

Just to be clear - are you saying you think both this patch and Dave's
may_checkpoint patchsets ought to be delayed, or just Suka's, or just
Dave's?  :)

-serge
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