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SubjectRe: sched: Improve load balancing in the presence of idle CPUs
Hi Jason,

On 04/08/2015 05:37 AM, Jason Low wrote:
> On Tue, 2015-04-07 at 16:28 -0700, Jason Low wrote:
>
>> Okay, so perhaps we can also try continuing nohz load balancing if we
>> find that there are overloaded CPUs in the system.

Sorry about the delay. Ok I will test out the below patch and share the
results.

Regards
Preeti U Murthy
>
> Something like the following.
>
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fdae26e..d636bf7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7620,6 +7620,16 @@ out:
> }
>
> #ifdef CONFIG_NO_HZ_COMMON
> +static inline bool nohz_kick_needed(struct rq *rq);
> +
> +static inline void pass_nohz_balance(struct rq *this_rq, int this_cpu)
> +{
> + clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> + nohz.next_balance = jiffies;
> + if (nohz_kick_needed(this_rq))
> + nohz_balancer_kick();
> +}
> +
> /*
> * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
> * rebalancing for all the cpus for whom scheduler ticks are stopped.
> @@ -7631,8 +7641,10 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> int balance_cpu;
>
> if (idle != CPU_IDLE ||
> - !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> - goto end;
> + !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu))) {
> + pass_nohz_balance(this_rq, this_cpu);
> + return;
> + }
>
> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
> @@ -7643,8 +7655,10 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> * work being done for other cpus. Next load
> * balancing owner will pick it up.
> */
> - if (need_resched())
> - break;
> + if (need_resched()) {
> + pass_nohz_balance(this_rq, this_cpu);
> + return;
> + }
>
> rq = cpu_rq(balance_cpu);
>
> @@ -7664,7 +7678,6 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> this_rq->next_balance = rq->next_balance;
> }
> nohz.next_balance = this_rq->next_balance;
> -end:
> clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> }
>
> @@ -7687,7 +7700,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
> int nr_busy, cpu = rq->cpu;
> bool kick = false;
>
> - if (unlikely(rq->idle_balance))
> + if (unlikely(idle_cpu(cpu)))
> return false;
>
> /*
> @@ -7707,7 +7720,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
> if (time_before(now, nohz.next_balance))
> return false;
>
> - if (rq->nr_running >= 2)
> + if (rq->nr_running >= 2 || rq->rd->overload)
> return true;
>
> rcu_read_lock();
> @@ -7757,16 +7770,14 @@ static void run_rebalance_domains(struct softirq_action *h)
> enum cpu_idle_type idle = this_rq->idle_balance ?
> CPU_IDLE : CPU_NOT_IDLE;
>
> + rebalance_domains(this_rq, idle);
> +
> /*
> * If this cpu has a pending nohz_balance_kick, then do the
> * balancing on behalf of the other idle cpus whose ticks are
> - * stopped. Do nohz_idle_balance *before* rebalance_domains to
> - * give the idle cpus a chance to load balance. Else we may
> - * load balance only within the local sched_domain hierarchy
> - * and abort nohz_idle_balance altogether if we pull some load.
> + * stopped.
> */
> nohz_idle_balance(this_rq, idle);
> - rebalance_domains(this_rq, idle);
> }
>
> /*
>
>



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