sched: Fix the group_imb logic

On a 2*6*2 machine something like:

 taskset -c 3-11 bash -c 'for ((i=0;i<9;i++)) do while :; do :; done & done'

_should_ result in 9 busy CPUs, each running 1 task.

However it didn't quite work reliably, most of the time one cpu of the
second socket (6-11) would be idle and one cpu of the first socket
(0-5) would have two tasks on it.

The group_imb logic is supposed to deal with this and detect when a
particular group is imbalanced (like in our case, 0-2 are idle but 3-5
will have 4 tasks on it).

The detection phase needed a bit of a tweak as it was too weak and
required more than 2 avg weight tasks difference between idle and busy
cpus in the group which won't trigger for our test-case. So cure that
to be one or more avg task weight difference between cpus.

Once the detection phase worked, it was then defeated by the f_b_g()
tests trying to avoid ping-pongs. In particular, this_load >= max_load
triggered because the pulling cpu (the (first) idle cpu in on the
second socket, say 6) would find this_load to be 5 and max_load to be
4 (there'd be 5 tasks running on our socket and only 4 on the other
socket).
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nikhil Rao <ncrao@google.com>
Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -2743,7 +2743,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
 
 	/*
 	 * Consider the group unbalanced when the imbalance is larger
-	 * than the average weight of two tasks.
+	 * than the average weight of a task.
 	 *
 	 * APZ: with cgroup the avg task weight can vary wildly and
 	 *      might not be a suitable number - should we keep a
@@ -2753,7 +2753,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
 	if (sgs->sum_nr_running)
 		avg_load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
 
-	if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task && max_nr_running > 1)
+	if ((max_cpu_load - min_cpu_load) >= avg_load_per_task && max_nr_running > 1)
 		sgs->group_imb = 1;
 
 	sgs->group_capacity = DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
@@ -3128,6 +3128,14 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 	if (!sds.busiest || sds.busiest_nr_running == 0)
 		goto out_balanced;
 
+	/*
+	 * If the busiest group is imbalanced the below checks don't
+	 * work because they assumes all things are equal, which typically
+	 * isn't true due to cpus_allowed constraints and the like.
+	 */
+	if (sds.group_imb)
+		goto force_balance;
+
 	/* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */
 	if (idle == CPU_NEWLY_IDLE && sds.this_has_capacity &&
 			!sds.busiest_has_capacity)