Re: sched: Improve load balancing in the presence of idle CPUs

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Jason Low <jason.low2@hp.com> [2015-04-07 17:07:46]:

> 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.
> 
> 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;

Why are we updating nohz.next_balance here?

> +	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 ||

Would it make sense to add need_resched here like
http://mid.gmane.org/1427442750-8112-1-git-send-email-wanpeng.li@linux.intel.com

> -	    !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))

<snipped > 

> @@ -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;


The only other place that we use idle_balance is
run_rebalance_domains(). Would it make sense to just use idle_cpu() in
run_rebalance_domains() and remove rq->idle_balance?

> 
>         /*
> @@ -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);
>  }
> 
>  /*
> 
> 

-- 
Thanks and Regards
Srikar Dronamraju