[PATCH] sched/fair: Force balancing on nohz balance if local group has capacity

[Brendan Jackman]

The "goto force_balance" here is intended to mitigate the fact that
avg_load calculations can result in bad placement decisions when
priority is asymmetrical. From the original commit (fab476228ba3
"sched: Force balancing on newidle balance if local group has
capacity") that adds it:

    Under certain situations, such as a niced down task (i.e. nice =
    -15) in the presence of nr_cpus NICE0 tasks, the niced task lands
    on a sched group and kicks away other tasks because of its large
    weight. This leads to sub-optimal utilization of the
    machine. Even though the sched group has capacity, it does not
    pull tasks because sds.this_load >> sds.max_load, and f_b_g()
    returns NULL.

A similar but inverted issue also affects ARM
big.LITTLE (asymmetrical CPU capacity) systems - consider 8
always-running, same-priority tasks on a system with 4 "big" and 4
"little" CPUs. Suppose that 5 of them end up on the "big" CPUs (which
will be represented by one sched_group in the DIE sched_domain) and 3
on the "little" (the other sched_group in DIE), leaving one CPU
unused. Because the "big" group has a higher group_capacity its
avg_load may not present an imbalance that would cause migrating a
task to the idle "little".

The force_balance case here solves the problem but currently only for
CPU_NEWLY_IDLE balances, which in theory might never happen on the
unused CPU. Including CPU_IDLE in the force_balance case means
there's an upper bound on the time before we can attempt to solve the
underutilization: after DIE's sd->balance_interval has passed the
next nohz balance kick will help us out.

Signed-off-by: Brendan Jackman <brendan.jackman@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
---
 kernel/sched/fair.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c95880e216f6..63eff3e881a0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7801,8 +7801,11 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
 	if (busiest->group_type == group_imbalanced)
 		goto force_balance;
 
-	/* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */
-	if (env->idle == CPU_NEWLY_IDLE && group_has_capacity(env, local) &&
+	/*
+	 * When dst_cpu is idle, prevent SMP nice and/or asymmetric group
+	 * capacities from resulting in underutilization due to avg_load.
+	 */
+	if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
 	    busiest->group_no_capacity)
 		goto force_balance;
 
-- 
2.13.0