Re: [rfc][patch] sched: remove smpnice

[Peter Williams <pwil3058@bigpond.net.au>]

Andrew Morton wrote:
> Peter Williams <pwil3058@bigpond.net.au> wrote:
>
> > I don't think either of these issues warrant abandoning smpnice.  The 
> > first is highly unlikely to occur on real systems and the second is just 
> > an example of the patch doing its job (maybe too officiously).  I don't 
> > think users would notice either on real systems.
> >
> > Even if you pull it from 2.6.16 rather than upgrading it with my patch 
> > can you please leave both in -mm?
>
>
> Yes, I have done that.  I currently have:
>
> sched-restore-smpnice.patch
> sched-modified-nice-support-for-smp-load-balancing.patch
> sched-cleanup_task_activated.patch
> sched-alter_uninterruptible_sleep_interactivity.patch
> sched-make_task_noninteractive_use_sleep_type.patch
> sched-dont_decrease_idle_sleep_avg.patch
> sched-include_noninteractive_sleep_in_idle_detect.patch
> sched-new-sched-domain-for-representing-multi-core.patch
> sched-fix-group-power-for-allnodes_domains.patch

OK.  Having slept on these problems I am now of the opinion that the 
problems are caused by the use of NICE_TO_BIAS_PRIO(0) to set *imbalance 
inside the (*imbalance < SCHED_LOAD_SCALE) if statement in 
find_busiest_group().  What is happening here is that even though the 
imbalance is less than one (average) task sometimes the decision is made 
to move a task anyway but with the current version this decision can be 
subverted in two ways: 1) if the task to be moved has a nice value less 
than zero the value of *imbalance that is set will be too small for 
move_tasks() to move it; and 2) if there are a number of tasks with nice 
values greater than zero on the "busiest" more than one of them may be 
moved as the value of *imbalance that is set may be big enough to 
include more than one of these tasks.

The fix for this problem is to replace NICE_TO_BIAS_PRIO(0) with the 
"average bias prio per runnable task" on "busiest".  This will 
(generally) result in a larger value for *imbalance in case 1. above and 
a smaller one in case 2. and alleviate both problems.  A patch to apply 
this fix is attached.

Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>

Could you please add this patch to -mm so that it can be tested?

Thanks
Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce

Index: MM-2.6.X/kernel/sched.c
===================================================================
--- MM-2.6.X.orig/kernel/sched.c	2006-02-12 11:24:48.000000000 +1100
+++ MM-2.6.X/kernel/sched.c	2006-02-12 11:35:40.000000000 +1100
@@ -735,6 +735,19 @@ static inline unsigned long biased_load(
 {
 	return (wload * NICE_TO_BIAS_PRIO(0)) / SCHED_LOAD_SCALE;
 }
+
+/* get the average biased load per runnable task for a run queue */
+static inline unsigned long avg_biased_load(runqueue_t *rq)
+{
+	/*
+	 * When I'm convinced that this won't be called with a zero nr_running
+	 * and that it can't change during the call this can be simplified.
+	 * For the time being and for proof of concept let's paly it safe.
+	 */
+	unsigned long n = rq->nr_running;
+
+	return n ? rq->prio_bias / n : 0;
+}
 #else
 static inline void set_bias_prio(task_t *p)
 {
@@ -2116,7 +2129,7 @@ find_busiest_group(struct sched_domain *
 		unsigned long tmp;
 
 		if (max_load - this_load >= SCHED_LOAD_SCALE*2) {
-			*imbalance = NICE_TO_BIAS_PRIO(0);
+			*imbalance = avg_biased_load(busiest);
 			return busiest;
 		}
 
@@ -2149,7 +2162,7 @@ find_busiest_group(struct sched_domain *
 		if (pwr_move <= pwr_now)
 			goto out_balanced;
 
-		*imbalance = NICE_TO_BIAS_PRIO(0);
+		*imbalance = avg_biased_load(busiest);
 		return busiest;
 	}