Re: How to notify app of changed cpu/mem/io node configuration?

[Dave Hansen <haveblue@us.ibm.com>]

On Mon, 2004-06-28 at 23:24, Greg KH wrote: 
> On Mon, Jun 28, 2004 at 05:38:08PM -0700, Paul Jackson wrote:
> > I can imagine using for notification a new signal, that could be sent by
> > an administrator or system service (batch manager, perhaps) to tasks if
> > their allowed CPUs or Memory Nodes or other such had changed.
> 
> I recall some initial proposals for when CPUs were offlined, to send the
> applications that were bound to those CPUs a specific signal, but I do
> not know if that got implemented or not.  Anyone know?

I don't see anything in the various signal.h files that looks likely.  

But, we'll probably need some kind of synchronous process notification
at some point.  If an app gets itself in a bad state such that it has no
possibility of being scheduled, or gets itself into some other hopeless
state due to a hotplug action, the kernel will likely have the option to
kill it.  

I think the problem for the kernel comes when we consider how to notify
the app.  Doing a hotplug event and having the scripts send an
appropriate signal isn't really a viable option because the kernel
wouldn't really know when each app had a chance to handle the signal. 
Sleeping for 5 seconds and hoping for the best probably isn't the best
option, either. :)

The other option would be to have the kernel send signals to the
processes, and recheck the "impossible state" after the signal has been
handled.

I don't really remember how the discussions about CPU hotplug ended, but
I wonder if a much more generic signal could be of more use than a
single task CPU hotplug signal.

What about a SIGHOTPLUG that can be used whenever the kernel notices
that a task is using a resource that's being hotplugged?


-- Dave



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