sched/fair: Determine active load balance for SMT sched groups

sched/fair: Determine active load balance for SMT sched groups

On hybrid CPUs with scheduling cluster enabled, we will need to
consider balancing between SMT CPU cluster, and Atom core cluster.

Below shows such a hybrid x86 CPU with 4 big cores and 8 atom cores.
Each scheduling cluster span a L2 cache.

          --L2-- --L2-- --L2-- --L2-- ----L2---- -----L2------
          [0, 1] [2, 3] [4, 5] [5, 6] [7 8 9 10] [11 12 13 14]
          Big    Big    Big    Big    Atom       Atom
          core   core   core   core   Module     Module

If the busiest group is a big core with both SMT CPUs busy, we should
active load balance if destination group has idle CPU cores.  Such
condition is considered by asym_active_balance() in load balancing but not
considered when looking for busiest group and computing load imbalance.
Add this consideration in find_busiest_group() and calculate_imbalance().

In addition, update the logic determining the busier group when one group
is SMT and the other group is non SMT but both groups are partially busy
with idle CPU. The busier group should be the group with idle cores rather
than the group with one busy SMT CPU.  We do not want to make the SMT group
the busiest one to pull the only task off SMT CPU and causing the whole core to
go empty.

Otherwise suppose in the search for the busiest group, we first encounter
an SMT group with 1 task and set it as the busiest.  The destination
group is an atom cluster with 1 task and we next encounter an atom
cluster group with 3 tasks, we will not pick this atom cluster over the
SMT group, even though we should.  As a result, we do not load balance
the busier Atom cluster (with 3 tasks) towards the local atom cluster
(with 1 task).  And it doesn't make sense to pick the 1 task SMT group
as the busier group as we also should not pull task off the SMT towards
the 1 task atom cluster and make the SMT core completely empty.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/e24f35d142308790f69be65930b82794ef6658a2.1688770494.git.tim.c.chen@linux.intel.com