linux-kernel - Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity

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Message-ID: <20140529095024.GF11074@laptop.programming.kicks-ass.net>
Date:	Thu, 29 May 2014 11:50:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com, efault@....de,
	nicolas.pitre@...aro.org, linaro-kernel@...ts.linaro.org,
	daniel.lezcano@...aro.org
Subject: Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher
 capacity

On Fri, May 23, 2014 at 05:53:04PM +0200, 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
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e8a30f9..2501e49 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5948,6 +5948,13 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  	if (sgs->sum_nr_running > sgs->group_capacity)
>  		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->sgp->power_orig * 100) > (sgs->group_power * env->sd->imbalance_pct))
> +		return true;
> +
>  	if (sgs->group_imb)
>  		return true;
>  

But we should already do this because the load numbers are scaled with
the power/capacity figures. If one CPU gets significant less time to run
fair tasks, its effective load would spike and it'd get to be selected
here anyway.

Or am I missing something?

> @@ -7282,6 +7289,12 @@ static inline int nohz_kick_needed(struct rq *rq)
>  
>  		if (nr_busy > 1)
>  			goto need_kick_unlock;
> +
> +		if ((rq->cfs.h_nr_running >= 1)
> +		 && ((rq->cpu_power * sd->imbalance_pct) <
> +					(rq->cpu_power_orig * 100)))
> +			goto need_kick_unlock;
> +
>  	}
>  
>  	sd = rcu_dereference(per_cpu(sd_asym, cpu));

OK, so there you're kicking the idle balancer to try and get another CPU
to pull some load? That makes sense I suppose.

That function is pretty horrible though; how about something like this
first?

---
 kernel/sched/fair.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c9617b73bcc0..47fb96e6fa83 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7215,15 +7215,16 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
  *   - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler
  *     domain span are idle.
  */
-static inline int nohz_kick_needed(struct rq *rq)
+static inline bool nohz_kick_needed(struct rq *rq)
 {
 	unsigned long now = jiffies;
 	struct sched_domain *sd;
 	struct sched_group_power *sgp;
 	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
@@ -7237,38 +7238,34 @@ 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)
-		goto need_kick;
+		return true;
 
 	rcu_read_lock();
 	sd = rcu_dereference(per_cpu(sd_busy, cpu));
-
 	if (sd) {
 		sgp = sd->groups->sgp;
 		nr_busy = atomic_read(&sgp->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));
-
 	if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
 				  sched_domain_span(sd)) < cpu))
-		goto need_kick_unlock;
-
-	rcu_read_unlock();
-	return 0;
+		kick = true;
 
-need_kick_unlock:
+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) { }
--
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