From: Brendan Jackman Date: Thu, 5 Oct 2017 10:58:54 +0000 (+0100) Subject: sched/fair: Force balancing on NOHZ balance if local group has capacity X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=7fdab2bfd48e5000baea51f3e08bafc02eeb6f21;p=platform%2Fkernel%2Flinux-exynos.git sched/fair: Force balancing on NOHZ balance if local group has capacity 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. The original commit that adds it: fab476228ba3 ("sched: Force balancing on newidle balance if local group has capacity") explains: 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 Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Morten Rasmussen Cc: Paul Turner Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20170807163900.25180-1-brendan.jackman@arm.com Signed-off-by: Ingo Molnar Signed-off-by: Lukasz Luba --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7dfb70202e08..f0dec8c3ceee 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8187,8 +8187,11 @@ 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;