Subject: [PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity

From: [email-masked] (Preeti U Murthy)
Date: Sat, 30 Aug 2014 23:20:56 +0530

Hi Vincent,

On 08/26/2014 04:36 PM, Vincent Guittot wrote:
> If the CPU is used for handling lot of IRQs, trig a load balance to check if
> it's worth moving its tasks on another CPU that has more capacity.
> 
> As a sidenote, this will note generate more spurious ilb because we already
> trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that
> has a task, we will trig the ilb once for migrating the task.
> 
> The nohz_kick_needed function has been cleaned up a bit while adding the new
> test
> 
> Signed-off-by: Vincent Guittot <[email-masked]>
> ---
>  kernel/sched/fair.c | 69 +++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 49 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 18db43e..60ae1ce 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6049,6 +6049,14 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  			return true;
>  	}
> 
> +	/*
> +	 * The group capacity is reduced probably because of activity from other
> +	 * sched class or interrupts which use part of the available capacity
> +	 */
> +	if ((sg->sgc->capacity_orig * 100) > (sgs->group_capacity *
> +				env->sd->imbalance_pct))

Wouldn't the check on avg_load let us know if we are packing more tasks
in this group than its capacity ? Isn't that the metric we are more
interested in?

> +		return true;
> +
>  	return false;
>  }
> 
> @@ -6534,13 +6542,23 @@ static int need_active_balance(struct lb_env *env)
>  	struct sched_domain *sd = env->sd;
> 
>  	if (env->idle == CPU_NEWLY_IDLE) {
> +		int src_cpu = env->src_cpu;
> 
>  		/*
>  		 * ASYM_PACKING needs to force migrate tasks from busy but
>  		 * higher numbered CPUs in order to pack all tasks in the
>  		 * lowest numbered CPUs.
>  		 */
> -		if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
> +		if ((sd->flags & SD_ASYM_PACKING) && src_cpu > env->dst_cpu)
> +			return 1;
> +
> +		/*
> +		 * If the CPUs share their cache and the src_cpu's capacity is
> +		 * reduced because of other sched_class or IRQs, we trig an
> +		 * active balance to move the task
> +		 */
> +		if ((capacity_orig_of(src_cpu) * 100) > (capacity_of(src_cpu) *
> +				sd->imbalance_pct))
>  			return 1;
>  	}
> 
> @@ -6643,6 +6661,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> 
>  	schedstat_add(sd, lb_imbalance[idle], env.imbalance);
> 
> +	env.src_cpu = busiest->cpu;
> +
>  	ld_moved = 0;
>  	if (busiest->nr_running > 1) {
>  		/*
> @@ -6652,7 +6672,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>  		 * correctly treated as an imbalance.
>  		 */
>  		env.flags |= LBF_ALL_PINNED;
> -		env.src_cpu   = busiest->cpu;
>  		env.src_rq    = busiest;
>  		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
> 
> @@ -7359,10 +7378,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> 
>  /*
>   * Current heuristic for kicking the idle load balancer in the presence
> - * of an idle cpu is the system.
> + * of an idle cpu in the system.
>   *   - This rq has more than one task.
> - *   - At any scheduler domain level, this cpu's scheduler group has multiple
> - *     busy cpu's exceeding the group's capacity.
> + *   - This rq has at least one CFS task and the capacity of the CPU is
> + *     significantly reduced because of RT tasks or IRQs.
> + *   - At parent of LLC scheduler domain level, this cpu's scheduler group has
> + *     multiple busy cpu.
>   *   - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler
>   *     domain span are idle.
>   */
> @@ -7372,9 +7393,10 @@ static inline int nohz_kick_needed(struct rq *rq)
>  	struct sched_domain *sd;
>  	struct sched_group_capacity *sgc;
>  	int nr_busy, cpu = rq->cpu;
> +	bool kick = false;
> 
>  	if (unlikely(rq->idle_balance))
> -		return 0;
> +		return false;
> 
>         /*
>  	* We may be recently in ticked or tickless idle mode. At the first
> @@ -7388,38 +7410,45 @@ static inline int nohz_kick_needed(struct rq *rq)
>  	 * balancing.
>  	 */
>  	if (likely(!atomic_read(&nohz.nr_cpus)))
> -		return 0;
> +		return false;
> 
>  	if (time_before(now, nohz.next_balance))
> -		return 0;
> +		return false;
> 
>  	if (rq->nr_running >= 2)

Will this check ^^ not catch those cases which this patch is targeting?

Regards
Preeti U Murthy

> -		goto need_kick;
> +		return true;
> 
>  	rcu_read_lock();
>  	sd = rcu_dereference(per_cpu(sd_busy, cpu));
> -
>  	if (sd) {
>  		sgc = sd->groups->sgc;
>  		nr_busy = atomic_read(&sgc->nr_busy_cpus);
> 
> -		if (nr_busy > 1)
> -			goto need_kick_unlock;
> +		if (nr_busy > 1) {
> +			kick = true;
> +			goto unlock;
> +		}
> +
>  	}
> 
> -	sd = rcu_dereference(per_cpu(sd_asym, cpu));
> +	sd = rcu_dereference(rq->sd);
> +	if (sd) {
> +		if ((rq->cfs.h_nr_running >= 1) &&
> +				((rq->cpu_capacity * sd->imbalance_pct) <
> +				(rq->cpu_capacity_orig * 100))) {
> +			kick = true;
> +			goto unlock;
> +		}
> +	}
> 
> +	sd = rcu_dereference(per_cpu(sd_asym, cpu));
>  	if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
>  				  sched_domain_span(sd)) < cpu))
> -		goto need_kick_unlock;
> +		kick = true;
> 
> +unlock:
>  	rcu_read_unlock();
> -	return 0;
> -
> -need_kick_unlock:
> -	rcu_read_unlock();
> -need_kick:
> -	return 1;
> +	return kick;
>  }
>  #else
>  static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) { }
>