Re: [announce] [patch] ultra-scalable O(1) SMP and UP scheduler

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Jan 04, 2002 at 12:44:57PM +0100, Ingo Molnar wrote:
> 
> On Fri, 4 Jan 2002, David Lang wrote:
> 
> > Ingo,
> > back in the 2.4.4-2.4.5 days when we experimented with the
> > child-runs-first scheduling patch we ran into quite a few programs that
> > died or locked up due to this. (I had a couple myself and heard of others)
> 
> hm, Andrea said that the only serious issue was in the sysvinit code,
> which should be fixed in any recent distro. Andrea?

correct. I run child-run-first always on all my boxes. And
those races could trigger also before, so it's better to make them more
easily reproducible anyways.

I always run with this patch applied:

diff -urN parent-timeslice/include/linux/sched.h child-first/include/linux/sched.h
--- parent-timeslice/include/linux/sched.h	Thu May  3 18:17:56 2001
+++ child-first/include/linux/sched.h	Thu May  3 18:19:44 2001
@@ -301,7 +301,7 @@
  * all fields in a single cacheline that are needed for
  * the goodness() loop in schedule().
  */
-	int counter;
+	volatile int counter;
 	int nice;
 	unsigned int policy;
 	struct mm_struct *mm;
diff -urN parent-timeslice/kernel/fork.c child-first/kernel/fork.c
--- parent-timeslice/kernel/fork.c	Thu May  3 18:18:31 2001
+++ child-first/kernel/fork.c	Thu May  3 18:20:40 2001
@@ -665,15 +665,18 @@
 	p->pdeath_signal = 0;
 
 	/*
-	 * "share" dynamic priority between parent and child, thus the
-	 * total amount of dynamic priorities in the system doesnt change,
-	 * more scheduling fairness. This is only important in the first
-	 * timeslice, on the long run the scheduling behaviour is unchanged.
+	 * Scheduling the child first is especially useful in avoiding a
+	 * lot of copy-on-write faults if the child for a fork() just wants
+	 * to do a few simple things and then exec().
 	 */
-	p->counter = (current->counter + 1) >> 1;
-	current->counter >>= 1;
-	if (!current->counter)
+	{
+		int counter = current->counter;
+		p->counter = (counter + 1) >> 1;
+		current->counter = counter >> 1;
+		p->policy &= ~SCHED_YIELD;
+		current->policy |= SCHED_YIELD;
 		current->need_resched = 1;
+	}
 
 	/* Tell the parent if it can get back its timeslice when child exits */
 	p->get_child_timeslice = 1;

> 
> > try switching this back to the current behaviour and see if the
> > lockups still happen.
> 
> there must be some other bug as well, the child-runs-first scheduling can
> cause lockups, but it shouldnt cause oopes.

definitely. My above implementation works fine.

Andrea