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

[Peter Williams]

Peter Williams wrote:
> Peter Williams wrote:
>
> > Peter Williams wrote:
> >
> > > Andrew Morton wrote:
> > >
> > > > Peter Williams <pwil3058@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > >
> > > Could you please add this patch to -mm so that it can be tested?
> > >
> > > [old patch removed]
> >
> >
> >
> > Can we pull this one, please?  I've mistakenly assumed that busiest 
> > was a run queue when it's actually a sched group. :-(
>
>
> Here's a fixed version that takes into account the fact that busiest is 
> a group not a run queue.  This patch also includes "improvements" to the 
> logic at the end of find_busiest_queue() to take account of the fact 
> that the loads are weighted.  Hopefully, the comments in the code 
> explain these changes.
>
> Signed-off-by: Peter Williams <pwil3058@xxxxxxxxxxxxxx>
>
> Because this is a more substantial change it may be advisable to wait 
> for comments from Ingo and Nick before including this in -mm for testing.
>
> Peter
>
>
> ------------------------------------------------------------------------
>
> Index: MM-2.6.X/kernel/sched.c
> ===================================================================
> --- MM-2.6.X.orig/kernel/sched.c	2006-02-13 10:23:12.000000000 +1100
> +++ MM-2.6.X/kernel/sched.c	2006-02-13 11:46:41.000000000 +1100
> @@ -2033,9 +2033,11 @@ find_busiest_group(struct sched_domain *
>  	struct sched_group *busiest = NULL, *this = NULL, *group = sd->groups;
>  	unsigned long max_load, avg_load, total_load, this_load, total_pwr;
>  	unsigned long max_pull;
> +	unsigned long avg_bias_prio, busiest_prio_bias, busiest_nr_running;
>  	int load_idx;
>  
>  	max_load = this_load = total_load = total_pwr = 0;
> +	busiest_prio_bias = busiest_nr_running = 0;
>  	if (idle == NOT_IDLE)
>  		load_idx = sd->busy_idx;
>  	else if (idle == NEWLY_IDLE)
> @@ -2047,13 +2049,16 @@ find_busiest_group(struct sched_domain *
>  		unsigned long load;
>  		int local_group;
>  		int i;
> +		unsigned long sum_prio_bias, sum_nr_running;
>  
>  		local_group = cpu_isset(this_cpu, group->cpumask);
>  
>  		/* Tally up the load of all CPUs in the group */
> -		avg_load = 0;
> +		sum_prio_bias = sum_nr_running = avg_load = 0;
>  
>  		for_each_cpu_mask(i, group->cpumask) {
> +			runqueue_t *rq = cpu_rq(i);
> +
>  			if (*sd_idle && !idle_cpu(i))
>  				*sd_idle = 0;
>  
> @@ -2064,6 +2069,8 @@ find_busiest_group(struct sched_domain *
>  				load = source_load(i, load_idx);
>  
>  			avg_load += load;
> +			sum_prio_bias += rq->prio_bias;
> +			sum_nr_running += rq->nr_running;
>  		}
>  
>  		total_load += avg_load;
> @@ -2078,6 +2085,8 @@ find_busiest_group(struct sched_domain *
>  		} else if (avg_load > max_load) {
>  			max_load = avg_load;
>  			busiest = group;
> +			busiest_prio_bias = sum_prio_bias;
> +			busiest_nr_running = sum_nr_running;
>  		}
>  		group = group->next;
>  	} while (group != sd->groups);
> @@ -2111,12 +2120,25 @@ find_busiest_group(struct sched_domain *
>  				(avg_load - this_load) * this->cpu_power)
>  			/ SCHED_LOAD_SCALE;
>  
> -	if (*imbalance < SCHED_LOAD_SCALE) {
> +	/* assume that busiest_nr_running > 0 */
> +	avg_bias_prio = busiest_prio_bias / busiest_nr_running;
> +	/*
> +	 * Get rid of the scaling factor, rounding down as we divide and
> +	 * converting to biased load for use by move_tasks()
> +	 */
> +	*imbalance = biased_load(*imbalance);
> +	/*
> +	 * if *imbalance is less than the average runnable task biased prio
> +	 * there is no gaurantee that any tasks will be moved so we'll have
> +	 * a think about bumping its value to force at least one task to be
> +	 * moved
> +	 */
> +	if (*imbalance < avg_bias_prio) {
>  		unsigned long pwr_now = 0, pwr_move = 0;
>  		unsigned long tmp;
>  
> -		if (max_load - this_load >= SCHED_LOAD_SCALE*2) {
> -			*imbalance = NICE_TO_BIAS_PRIO(0);
> +		if (biased_load(max_load - this_load) >= avg_bias_prio*2) {
> +			*imbalance = avg_bias_prio;
>  			return busiest;
>  		}
>  
> @@ -2146,18 +2168,19 @@ find_busiest_group(struct sched_domain *
>  		pwr_move /= SCHED_LOAD_SCALE;
>  
>  		/* Move if we gain throughput */
> -		if (pwr_move <= pwr_now)
> -			goto out_balanced;
> +		if (pwr_move <= pwr_now) {
> +			/* or if there's a reasonable chance that *imbalance
> +			 * is big enough to cause a move
> +			 */
> +			if (*imbalance <= avg_bias_prio / 2)
> +				goto out_balanced;
> +			else
> +				return busiest;
> +		}
>  
> -		*imbalance = NICE_TO_BIAS_PRIO(0);
> -		return busiest;
> +		*imbalance = avg_bias_prio;
>  	}
>  
> -	/*
> -	 * Get rid of the scaling factor, rounding down as we divide and
> -	 * converting to biased load for use by move_tasks()
> -	 */
> -	*imbalance = biased_load(*imbalance);
>  	return busiest;
>  
>  out_balanced:

Attached is an alternative patch for this problem that adds less 
overhead.  It takes advantage of the fact that biased_load(max_load) for 
busiest group is APPROXIMATELY equal to the sum of the prio_biases for 
the run queues in the group.  This has a number of advantages:

1. less work is done in the do loop
2. there is no need to convert *imbalance to biased load before 
comparison to avg_bias_prio as they are in the same units.

There is one possible disadvantage and that is that the approximation is 
sufficiently bad to break the functionality.

In summary, if this version works it should be used in preference to v2 
as it involves less overhead.

Peter
--
Peter Williams                                   pwil3058@xxxxxxxxxxxxxx

"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	2006-02-13 10:23:12.000000000 +1100
+++ MM-2.6.X/kernel/sched.c	2006-02-14 09:47:17.000000000 +1100
@@ -2033,9 +2033,11 @@ find_busiest_group(struct sched_domain *
 	struct sched_group *busiest = NULL, *this = NULL, *group = sd->groups;
 	unsigned long max_load, avg_load, total_load, this_load, total_pwr;
 	unsigned long max_pull;
+	unsigned long avg_load_per_task, busiest_nr_running;
 	int load_idx;
 
 	max_load = this_load = total_load = total_pwr = 0;
+	busiest_nr_running = 0;
 	if (idle == NOT_IDLE)
 		load_idx = sd->busy_idx;
 	else if (idle == NEWLY_IDLE)
@@ -2047,11 +2049,12 @@ find_busiest_group(struct sched_domain *
 		unsigned long load;
 		int local_group;
 		int i;
+		unsigned long sum_nr_running;
 
 		local_group = cpu_isset(this_cpu, group->cpumask);
 
 		/* Tally up the load of all CPUs in the group */
-		avg_load = 0;
+		sum_nr_running = avg_load = 0;
 
 		for_each_cpu_mask(i, group->cpumask) {
 			if (*sd_idle && !idle_cpu(i))
@@ -2064,6 +2067,7 @@ find_busiest_group(struct sched_domain *
 				load = source_load(i, load_idx);
 
 			avg_load += load;
+			sum_nr_running += cpu_rq(i)->nr_running;
 		}
 
 		total_load += avg_load;
@@ -2078,6 +2082,7 @@ find_busiest_group(struct sched_domain *
 		} else if (avg_load > max_load) {
 			max_load = avg_load;
 			busiest = group;
+			busiest_nr_running = sum_nr_running;
 		}
 		group = group->next;
 	} while (group != sd->groups);
@@ -2111,12 +2116,20 @@ find_busiest_group(struct sched_domain *
 				(avg_load - this_load) * this->cpu_power)
 			/ SCHED_LOAD_SCALE;
 
-	if (*imbalance < SCHED_LOAD_SCALE) {
+	/* Don't assume that busiest_nr_running > 0 */
+	avg_load_per_task = busiest_nr_running ? max_load / busiest_nr_running : avg_load;
+	/*
+	 * if *imbalance is less than the average load per runnable task
+	 * there is no gaurantee that any tasks will be moved so we'll have
+	 * a think about bumping its value to force at least one task to be
+	 * moved
+	 */
+	if (*imbalance < avg_load_per_task) {
 		unsigned long pwr_now = 0, pwr_move = 0;
 		unsigned long tmp;
 
-		if (max_load - this_load >= SCHED_LOAD_SCALE*2) {
-			*imbalance = NICE_TO_BIAS_PRIO(0);
+		if (max_load - this_load >= avg_load_per_task*2) {
+			*imbalance = biased_load(avg_load_per_task);
 			return busiest;
 		}
 
@@ -2146,11 +2159,14 @@ find_busiest_group(struct sched_domain *
 		pwr_move /= SCHED_LOAD_SCALE;
 
 		/* Move if we gain throughput */
-		if (pwr_move <= pwr_now)
-			goto out_balanced;
-
-		*imbalance = NICE_TO_BIAS_PRIO(0);
-		return busiest;
+		if (pwr_move <= pwr_now) {
+			/* or if there's a reasonable chance that *imbalance
+			 * is big enough to cause a move
+			 */
+			if (*imbalance <= avg_load_per_task / 2)
+				goto out_balanced;
+		} else
+			*imbalance = avg_load_per_task;
 	}
 
 	/*