Re: CPU Hotplug: Hotplug Script And SIGPWR

[Nick Piggin]

Tim Hockin wrote:

> On Tue, Jan 20, 2004 at 06:45:37PM +1100, Nick Piggin wrote:
>
> > > I guess a hotplug script MAY fail.  I don't think it's a good idea to make
> > > your CPU hotplug script fail.  May and Misght are different.  It's up to 
> > > the
> > > implementor whether the script can get into a failure condition.
> > Sorry bad wording. The script may fail to be executed.
>
> Under what conditions?  Not arbitrary entropy, surely.  If a hotplug script
> is present and does not blow up, it should be safe to assume it will be run
> upon an event being delivered.  If not, we have a WAY bigger problem :)

That assumption is not safe. The main problems are of course process limits
and memory allocation failure.

> > > What if <which> process needs guaranteed scheduling latency?  Do we really
> > > _guarantee_ scheduling latency *anywhere*?
> > We do guarantee that a realtime task won't be blocked waiting for
> > a hotplug script to fault in and start it up again (which may not
> > happen). Not sure how important this issue is.
>
> We have a conflict of priority here.  If an RT task is affined to CPU A and
> CPU A gets yanked out, what do we do?
>
> Obviously the RT task can't keep running as it was.  It was affined to A.
> Maybe for a good reason.  I see we have a few choices here:
>
> * re-affine it automatically, thereby silently undoing the explicit
>  affinity.
> * violate it's RT scheduling by not running it until it has been re-affined
>  or CPU A returns to the pool/
>
> Sending it a SIGPWR means you have to run it on a different CPU that it was
> affined to, which is already a violation.

At least the task has the option to handle the problem.


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