sched/fair: Reduce minimal imbalance threshold

The 25% default imbalance threshold for DIE and NUMA domain is large
enough to generate significant unfairness between threads. A typical
example is the case of 11 threads running on 2x4 CPUs. The imbalance of
20% between the 2 groups of 4 cores is just low enough to not trigger
the load balance between the 2 groups. We will have always the same 6
threads on one group of 4 CPUs and the other 5 threads on the other
group of CPUS. With a fair time sharing in each group, we ends up with
+20% running time for the group of 5 threads.

Consider decreasing the imbalance threshold for overloaded case where we
use the load to balance task and to ensure fair time sharing.

Signed-off-by: Vincent Guittot <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Phil Auld <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: vingu-linaro

kernel/sched/topology.c

@@ -1349,7 +1349,7 @@ sd_init(struct sched_domain_topology_level *tl,
 		.min_interval		= sd_weight,
 		.max_interval		= 2*sd_weight,
 		.busy_factor		= 32,
-		.imbalance_pct		= 125,
+		.imbalance_pct		= 117,
 
 		.cache_nice_tries	= 0,