Re: change in sched cpu_power causing regressions with SCHED_MC

[Suresh Siddha]

On Sat, 2010-02-13 at 02:36 -0800, Peter Zijlstra wrote:
> On Fri, 2010-02-12 at 17:31 -0800, Suresh Siddha wrote:
> > 
> > We have one more problem that Yanmin and Ling Ma reported. On a dual
> > socket quad-core platforms (for example platforms based on NHM-EP), we
> > are seeing scenarios where one socket is completely busy (with all the 4
> > cores running with 4 tasks) and another socket is completely idle.
> > 
> > This causes performance issues as those 4 tasks share the memory
> > controller, last-level cache bandwidth etc. Also we won't be taking
> > advantage of turbo-mode as much as we like. We will have all these
> > benefits if we move two of those tasks to the other socket. Now both the
> > sockets can potentially go to turbo etc and improve performance.
> > 
> > In short, your recent change (shown below) broke this behavior. In the
> > kernel summit you mentioned you made this change with out affecting the
> > behavior of SMT/MC. And my testing immediately after kernel-summit also
> > didn't show the problem (perhaps my test didn't hit this specific
> > change). But apparently we are having performance issues with this patch
> > (Ling Ma's bisect pointed to this patch). I will look more detailed into
> > this after the long weekend (to see if we can catch this scenario in
> > fix_small_imbalance() etc). But wanted to give you a quick heads up.
> > Thanks.
> 
> Right, so the behaviour we want should be provided by SD_PREFER_SIBLING,
> it provides the capacity==1 thing the cpu_power games used to provide.
> 
> Not saying it's not broken, but that's where the we should be looking to
> fix it.

Peter, Some portions of code in fix_small_imbalance() and
calculate_imbalance() are comparing max_load and busiest_load_per_task.
Some of these comparisons are ok but some of them are broken. Broken
comparisons are assuming that the cpu_power is SCHED_LOAD_SCALE. Also
there is one check which still assumes that the world is balanced when
max_load <= busiest_load_per_task. This is wrong with the recent changes
(as cpu power no longer reflects the group capacity that is needed to
implement SCHED_MC/SCHED_SMT).

The appended patch works for me and fixes the SCHED_MC performance
behavior. I am sending this patch out for a quick review and I will do
bit more testing tomorrow and If you don't follow what I am doing in
this patch and why, then stay tuned for a patch with complete changelog
that I will send tomorrow. Good night. Thanks.
---

Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>

diff --git a/kernel/sched.c b/kernel/sched.c
index 3a8fb30..2f4cac0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3423,6 +3423,7 @@ struct sd_lb_stats {
 	unsigned long max_load;
 	unsigned long busiest_load_per_task;
 	unsigned long busiest_nr_running;
+	unsigned long busiest_group_capacity;
 
 	int group_imb; /* Is there imbalance in this sd */
 #if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
@@ -3880,6 +3881,7 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
 			sds->max_load = sgs.avg_load;
 			sds->busiest = group;
 			sds->busiest_nr_running = sgs.sum_nr_running;
+			sds->busiest_group_capacity = sgs.group_capacity;
 			sds->busiest_load_per_task = sgs.sum_weighted_load;
 			sds->group_imb = sgs.group_imb;
 		}
@@ -3902,6 +3904,7 @@ static inline void fix_small_imbalance(struct sd_lb_stats *sds,
 {
 	unsigned long tmp, pwr_now = 0, pwr_move = 0;
 	unsigned int imbn = 2;
+	unsigned long scaled_busy_load_per_task;
 
 	if (sds->this_nr_running) {
 		sds->this_load_per_task /= sds->this_nr_running;
@@ -3912,8 +3915,12 @@ static inline void fix_small_imbalance(struct sd_lb_stats *sds,
 		sds->this_load_per_task =
 			cpu_avg_load_per_task(this_cpu);
 
-	if (sds->max_load - sds->this_load + sds->busiest_load_per_task >=
-			sds->busiest_load_per_task * imbn) {
+	scaled_busy_load_per_task = sds->busiest_load_per_task
+						 * SCHED_LOAD_SCALE;
+	scaled_busy_load_per_task /= sds->busiest->cpu_power;
+
+	if (sds->max_load - sds->this_load + scaled_busy_load_per_task >=
+			scaled_busy_load_per_task * imbn) {
 		*imbalance = sds->busiest_load_per_task;
 		return;
 	}
@@ -3964,7 +3971,7 @@ static inline void fix_small_imbalance(struct sd_lb_stats *sds,
 static inline void calculate_imbalance(struct sd_lb_stats *sds, int this_cpu,
 		unsigned long *imbalance)
 {
-	unsigned long max_pull;
+	unsigned long max_pull, load_above_capacity = ~0UL;
 	/*
 	 * In the presence of smp nice balancing, certain scenarios can have
 	 * max load less than avg load(as we skip the groups at or below
@@ -3975,9 +3982,30 @@ static inline void calculate_imbalance(struct sd_lb_stats *sds, int this_cpu,
 		return fix_small_imbalance(sds, this_cpu, imbalance);
 	}
 
-	/* Don't want to pull so many tasks that a group would go idle */
-	max_pull = min(sds->max_load - sds->avg_load,
-			sds->max_load - sds->busiest_load_per_task);
+	if (!sds->group_imb) {
+		/*
+ 	 	 * Don't want to pull so many tasks that a group would go idle.
+	 	 */
+		load_above_capacity = (sds->busiest_nr_running - 
+						sds->busiest_group_capacity);
+
+		load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_LOAD_SCALE);
+	
+		load_above_capacity /= sds->busiest->cpu_power;
+	}
+
+	/*
+	 * We're trying to get all the cpus to the average_load, so we don't
+	 * want to push ourselves above the average load, nor do we wish to
+	 * reduce the max loaded cpu below the average load, as either of these
+	 * actions would just result in more rebalancing later, and ping-pong
+	 * tasks around. Thus we look for the minimum possible imbalance.
+	 * Negative imbalances (*we* are more loaded than anyone else) will
+	 * be counted as no imbalance for these purposes -- we can't fix that
+	 * by pulling tasks to us. Be careful of negative numbers as they'll
+	 * appear as very large values with unsigned longs.
+	 */
+	max_pull = min(sds->max_load - sds->avg_load, load_above_capacity);
 
 	/* How much load to actually move to equalise the imbalance */
 	*imbalance = min(max_pull * sds->busiest->cpu_power,
@@ -4069,19 +4097,6 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 		sds.busiest_load_per_task =
 			min(sds.busiest_load_per_task, sds.avg_load);
 
-	/*
-	 * We're trying to get all the cpus to the average_load, so we don't
-	 * want to push ourselves above the average load, nor do we wish to
-	 * reduce the max loaded cpu below the average load, as either of these
-	 * actions would just result in more rebalancing later, and ping-pong
-	 * tasks around. Thus we look for the minimum possible imbalance.
-	 * Negative imbalances (*we* are more loaded than anyone else) will
-	 * be counted as no imbalance for these purposes -- we can't fix that
-	 * by pulling tasks to us. Be careful of negative numbers as they'll
-	 * appear as very large values with unsigned longs.
-	 */
-	if (sds.max_load <= sds.busiest_load_per_task)
-		goto out_balanced;
 
 	/* Looks like there is an imbalance. Compute it */
 	calculate_imbalance(&sds, this_cpu, imbalance);


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