sched/fair: Improve the ->group_imb logic

Group imbalance is meant to deal with situations where affinity masks
and sched domains don't align well, such as 3 cpus from one group and
6 from another. In this case the domain based balancer will want to
put an equal amount of tasks on each side even though they don't have
equal cpus.

Currently group_imb is set whenever two cpus of a group have a weight
difference of at least one avg task and the heaviest cpu has at least
two tasks. A group with imbalance set will always be picked as busiest
and a balance pass will be forced.

The problem is that even if there are no affinity masks this stuff can
trigger and cause weird balancing decisions, eg. the observed
behaviour was that of 6 cpus, 5 had 2 and 1 had 3 tasks, due to the
difference of 1 avg load (they all had the same weight) and nr_running
being >1 the group_imbalance logic triggered and did the weird thing
of pulling more load instead of trying to move the 1 excess task to
the other domain of 6 cpus that had 5 cpu with 2 tasks and 1 cpu with
1 task.

Curb the group_imbalance stuff by making the nr_running condition
weaker by also tracking the min_nr_running and using the difference in
nr_running over the set instead of the absolute max nr_running.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-9s7dedozxo8kjsb9kqlrukkf@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
sched: Reset loop counters if all tasks are pinned and we need to redo load balance

While load balancing, if all tasks on the source runqueue are pinned,
we retry after excluding the corresponding source cpu. However, loop counters
env.loop and env.loop_break are not reset before retrying, which can lead
to failure in moving the tasks. In this patch we reset env.loop and
env.loop_break to their inital values before we retry.

Signed-off-by: Prashanth Nageshappa <prashanth@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
sched: Improve balance_cpu() to consider other cpus in its group as target of (pinned) task

Current load balance scheme requires only one cpu in a
sched_group (balance_cpu) to look at other peer sched_groups for
imbalance and pull tasks towards itself from a busy cpu. Tasks
thus pulled by balance_cpu could later get picked up by cpus
that are in the same sched_group as that of balance_cpu.

This scheme however fails to pull tasks that are not allowed to
run on balance_cpu (but are allowed to run on other cpus in its
sched_group). That can affect fairness and in some worst case
scenarios cause starvation.

Consider a two core (2 threads/core) system running tasks as
below:

          Core0            Core1
         /     \          /     \
	C0     C1	 C2     C3
        |      |         |      |
        v      v         v      v
	F0     T1        F1     [idle]
			 T2

 F0 = SCHED_FIFO task (pinned to C0)
 F1 = SCHED_FIFO task (pinned to C2)
 T1 = SCHED_OTHER task (pinned to C1)
 T2 = SCHED_OTHER task (pinned to C1 and C2)

F1 could become a cpu hog, which will starve T2 unless C1 pulls
it. Between C0 and C1 however, C0 is required to look for
imbalance between cores, which will fail to pull T2 towards
Core0. T2 will starve eternally in this case. The same scenario
can arise in presence of non-rt tasks as well (say we replace F1
with high irq load).

We tackle this problem by having balance_cpu move pinned tasks
to one of its sibling cpus (where they can run). We first check
if load balance goal can be met by ignoring pinned tasks,
failing which we retry move_tasks() with a new env->dst_cpu.

This patch modifies load balance semantics on who can move load
towards a given cpu in a given sched_domain.

Before this patch, a given_cpu or a ilb_cpu acting on behalf of
an idle given_cpu is responsible for moving load to given_cpu.

With this patch applied, balance_cpu can in addition decide on
moving some load to a given_cpu.

There is a remote possibility that excess load could get moved
as a result of this (balance_cpu and given_cpu/ilb_cpu deciding
*independently* and at *same* time to move some load to a
given_cpu). However we should see less of such conflicting
decisions in practice and moreover subsequent load balance
cycles should correct the excess load moved to given_cpu.

Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Prashanth Nageshappa <prashanth@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/4FE06CDB.2060605@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>