Re: [RFC][PATCH] Restricted hard realtime

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Sat, Oct 23, 2004 at 10:17:24PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@us.ibm.com> wrote:
> 
> > +	bool "Reserve a CPU for hard realtime processes"
> 
> this has been implemented in a clean way already: check out the
> "isolcpus=" boot option & scheduler feature (implemented by Dimitri
> Sivanich) which isolates a set of CPUs via sched-domains for precisely
> such purposes. The way to enter such a domain is via the affinity
> syscall - and balancing will leave such domains isolated.

Thanks again for the pointer, am slowly getting my head around this.
I haven't proven to myself that the isolcpus code gets rid of all of the
cross-runqueue lock acquisitions, but it certainly gets rid of a large
number of them.  It doesn't seem to do system-call or exception-handler
offload, but it does help me see how to do this sort of thing cleanly.

Dimitri, one nit so far...   Why is sched_domain_dummy under two
layers of #ifdef CONFIG_SMP?  Any reason why the attached patch
would not be in order?

						Thanx, Paul

diff -urpN -X ../dontdiff linux-2.5-2004.10.23/kernel/sched.c linux-2.5-2004.10.23-LBinf/kernel/sched.c
--- linux-2.5-2004.10.23/kernel/sched.c	Sat Oct 23 13:23:31 2004
+++ linux-2.5-2004.10.23-LBinf/kernel/sched.c	Sun Oct 24 10:50:12 2004
@@ -4437,14 +4437,12 @@ static void sched_domain_debug(void)
 #define sched_domain_debug() {}
 #endif
 
-#ifdef CONFIG_SMP
 /*
  * Initial dummy domain for early boot and for hotplug cpu. Being static,
  * it is initialized to zero, so all balancing flags are cleared which is
  * what we want.
  */
 static struct sched_domain sched_domain_dummy;
-#endif
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*