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Subject[PATCH] sched/fair: Force balancing on nohz balance if local group has capacity
Date
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@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Turner <pjt@google.com>
---
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
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