sched: group scheduler, fix fairness of cpu bandwidth allocation for task groups

The current load balancing scheme isn't good enough for precise
group fairness.

For example: on a 8-cpu system, I created 3 groups as under:

	a = 8 tasks (cpu.shares = 1024)
	b = 4 tasks (cpu.shares = 1024)
	c = 3 tasks (cpu.shares = 1024)

a, b and c are task groups that have equal weight. We would expect each
of the groups to receive 33.33% of cpu bandwidth under a fair scheduler.

This is what I get with the latest scheduler git tree:

Signed-off-by: Ingo Molnar <mingo@elte.hu>
--------------------------------------------------------------------------------
Col1  | Col2    | Col3  |  Col4
------|---------|-------|-------------------------------------------------------
a     | 277.676 | 57.8% | 54.1%  54.1%  54.1%  54.2%  56.7%  62.2%  62.8% 64.5%
b     | 116.108 | 24.2% | 47.4%  48.1%  48.7%  49.3%
c     |  86.326 | 18.0% | 47.5%  47.9%  48.5%
--------------------------------------------------------------------------------

Explanation of o/p:

Col1 -> Group name
Col2 -> Cumulative execution time (in seconds) received by all tasks of that
	group in a 60sec window across 8 cpus
Col3 -> CPU bandwidth received by the group in the 60sec window, expressed in
        percentage. Col3 data is derived as:
		Col3 = 100 * Col2 / (NR_CPUS * 60)
Col4 -> CPU bandwidth received by each individual task of the group.
		Col4 = 100 * cpu_time_recd_by_task / 60

[I can share the test case that produces a similar o/p if reqd]

The deviation from desired group fairness is as below:

	a = +24.47%
	b = -9.13%
	c = -15.33%

which is quite high.

After the patch below is applied, here are the results:

--------------------------------------------------------------------------------
Col1  | Col2    | Col3  |  Col4
------|---------|-------|-------------------------------------------------------
a     | 163.112 | 34.0% | 33.2%  33.4%  33.5%  33.5%  33.7%  34.4%  34.8% 35.3%
b     | 156.220 | 32.5% | 63.3%  64.5%  66.1%  66.5%
c     | 160.653 | 33.5% | 85.8%  90.6%  91.4%
--------------------------------------------------------------------------------

Deviation from desired group fairness is as below:

	a = +0.67%
	b = -0.83%
	c = +0.17%

which is far better IMO. Most of other runs have yielded a deviation within
+-2% at the most, which is good.

Why do we see bad (group) fairness with current scheuler?
=========================================================

Currently cpu's weight is just the summation of individual task weights.
This can yield incorrect results. For ex: consider three groups as below
on a 2-cpu system:

	CPU0	CPU1
---------------------------
	A (10)  B(5)
		C(5)
---------------------------

Group A has 10 tasks, all on CPU0, Group B and C have 5 tasks each all
of which are on CPU1. Each task has the same weight (NICE_0_LOAD =
1024).

The current scheme would yield a cpu weight of 10240 (10*1024) for each cpu and
the load balancer will think both CPUs are perfectly balanced and won't
move around any tasks. This, however, would yield this bandwidth:

	A = 50%
	B = 25%
	C = 25%

which is not the desired result.

What's changing in the patch?
=============================

	- How cpu weights are calculated when CONFIF_FAIR_GROUP_SCHED is
	  defined (see below)
	- API Change
		- Two tunables introduced in sysfs (under SCHED_DEBUG) to
		  control the frequency at which the load balance monitor
		  thread runs.

The basic change made in this patch is how cpu weight (rq->load.weight) is
calculated. Its now calculated as the summation of group weights on a cpu,
rather than summation of task weights. Weight exerted by a group on a
cpu is dependent on the shares allocated to it and also the number of
tasks the group has on that cpu compared to the total number of
(runnable) tasks the group has in the system.

Let,
	W(K,i)  = Weight of group K on cpu i
	T(K,i)  = Task load present in group K's cfs_rq on cpu i
	T(K)    = Total task load of group K across various cpus
	S(K) 	= Shares allocated to group K
	NRCPUS	= Number of online cpus in the scheduler domain to
	 	  which group K is assigned.

Then,
	W(K,i) = S(K) * NRCPUS * T(K,i) / T(K)

A load balance monitor thread is created at bootup, which periodically
runs and adjusts group's weight on each cpu. To avoid its overhead, two
min/max tunables are introduced (under SCHED_DEBUG) to control the rate
at which it runs.

Fixes from: Peter Zijlstra <a.p.zijlstra@chello.nl>

- don't start the load_balance_monitor when there is only a single cpu.
- rename the kthread because its currently longer than TASK_COMM_LEN

Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 53 insertions, 31 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 30ae9c2a2861..5c208e090ae4 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -707,6 +707,8 @@ static inline struct sched_entity *parent_entity(struct sched_entity *se)
 	return se->parent;
 }
 
+#define GROUP_IMBALANCE_PCT	20
+
 #else	/* CONFIG_FAIR_GROUP_SCHED */
 
 #define for_each_sched_entity(se) \
@@ -967,25 +969,6 @@ static struct task_struct *load_balance_next_fair(void *arg)
 	return __load_balance_iterator(cfs_rq, cfs_rq->rb_load_balance_curr);
 }
 
-#ifdef CONFIG_FAIR_GROUP_SCHED
-static int cfs_rq_best_prio(struct cfs_rq *cfs_rq)
-{
-	struct sched_entity *curr;
-	struct task_struct *p;
-
-	if (!cfs_rq->nr_running)
-		return MAX_PRIO;
-
-	curr = cfs_rq->curr;
-	if (!curr)
-		curr = __pick_next_entity(cfs_rq);
-
-	p = task_of(curr);
-
-	return p->prio;
-}
-#endif
-
 static unsigned long
 load_balance_fair(struct rq *this_rq, int this_cpu, struct rq *busiest,
 		  unsigned long max_load_move,
@@ -995,28 +978,45 @@ load_balance_fair(struct rq *this_rq, int this_cpu, struct rq *busiest,
 	struct cfs_rq *busy_cfs_rq;
 	long rem_load_move = max_load_move;
 	struct rq_iterator cfs_rq_iterator;
+	unsigned long load_moved;
 
 	cfs_rq_iterator.start = load_balance_start_fair;
 	cfs_rq_iterator.next = load_balance_next_fair;
 
 	for_each_leaf_cfs_rq(busiest, busy_cfs_rq) {
 #ifdef CONFIG_FAIR_GROUP_SCHED
-		struct cfs_rq *this_cfs_rq;
-		long imbalance;
-		unsigned long maxload;
+		struct cfs_rq *this_cfs_rq = busy_cfs_rq->tg->cfs_rq[this_cpu];
+		unsigned long maxload, task_load, group_weight;
+		unsigned long thisload, per_task_load;
+		struct sched_entity *se = busy_cfs_rq->tg->se[busiest->cpu];
 
-		this_cfs_rq = cpu_cfs_rq(busy_cfs_rq, this_cpu);
+		task_load = busy_cfs_rq->load.weight;
+		group_weight = se->load.weight;
 
-		imbalance = busy_cfs_rq->load.weight - this_cfs_rq->load.weight;
-		/* Don't pull if this_cfs_rq has more load than busy_cfs_rq */
-		if (imbalance <= 0)
+		/*
+		 * 'group_weight' is contributed by tasks of total weight
+		 * 'task_load'. To move 'rem_load_move' worth of weight only,
+		 * we need to move a maximum task load of:
+		 *
+		 * 	maxload = (remload / group_weight) * task_load;
+		 */
+		maxload = (rem_load_move * task_load) / group_weight;
+
+		if (!maxload || !task_load)
 			continue;
 
-		/* Don't pull more than imbalance/2 */
-		imbalance /= 2;
-		maxload = min(rem_load_move, imbalance);
+		per_task_load = task_load / busy_cfs_rq->nr_running;
+		/*
+		 * balance_tasks will try to forcibly move atleast one task if
+		 * possible (because of SCHED_LOAD_SCALE_FUZZ). Avoid that if
+		 * maxload is less than GROUP_IMBALANCE_FUZZ% the per_task_load.
+		 */
+		 if (100 * maxload < GROUP_IMBALANCE_PCT * per_task_load)
+			continue;
 
-		*this_best_prio = cfs_rq_best_prio(this_cfs_rq);
+		/* Disable priority-based load balance */
+		*this_best_prio = 0;
+		thisload = this_cfs_rq->load.weight;
 #else
 # define maxload rem_load_move
 #endif
@@ -1025,11 +1025,33 @@ load_balance_fair(struct rq *this_rq, int this_cpu, struct rq *busiest,
 		 * load_balance_[start|next]_fair iterators
 		 */
 		cfs_rq_iterator.arg = busy_cfs_rq;
-		rem_load_move -= balance_tasks(this_rq, this_cpu, busiest,
+		load_moved = balance_tasks(this_rq, this_cpu, busiest,
 					       maxload, sd, idle, all_pinned,
 					       this_best_prio,
 					       &cfs_rq_iterator);
 
+#ifdef CONFIG_FAIR_GROUP_SCHED
+		/*
+		 * load_moved holds the task load that was moved. The
+		 * effective (group) weight moved would be:
+		 * 	load_moved_eff = load_moved/task_load * group_weight;
+		 */
+		load_moved = (group_weight * load_moved) / task_load;
+
+		/* Adjust shares on both cpus to reflect load_moved */
+		group_weight -= load_moved;
+		set_se_shares(se, group_weight);
+
+		se = busy_cfs_rq->tg->se[this_cpu];
+		if (!thisload)
+			group_weight = load_moved;
+		else
+			group_weight = se->load.weight + load_moved;
+		set_se_shares(se, group_weight);
+#endif
+
+		rem_load_move -= load_moved;
+
 		if (rem_load_move <= 0)
 			break;
 	}