[v9,10/10] sched: move cfs task on a CPU with higher capacity

Message ID 1415033687-23294-11-git-send-email-vincent.guittot@linaro.org
State New
Headers show

Commit Message

Vincent Guittot Nov. 3, 2014, 4:54 p.m.
When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining
capacity for CFS tasks can be significantly reduced. Once we detect such
situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle
load balance to check if it's worth moving its tasks on an idle CPU.

Once the idle load_balance has selected the busiest CPU, it will look for an
active load balance for only two cases :
- there is only 1 task on the busiest CPU.
- we haven't been able to move a task of the busiest rq.

A CPU with a reduced capacity is included in the 1st case, and it's worth to
actively migrate its task if the idle CPU has got full capacity. This test has
been added in need_active_balance.

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

env.src_cpu and env.src_rq must be set unconditionnally because they are used
in need_active_balance which is called even if busiest->nr_running equals 1

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 53 insertions(+), 21 deletions(-)

Comments

Morten Rasmussen Nov. 21, 2014, 12:37 p.m. | #1
On Mon, Nov 03, 2014 at 04:54:47PM +0000, Vincent Guittot wrote:
> When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining
> capacity for CFS tasks can be significantly reduced. Once we detect such
> situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle
> load balance to check if it's worth moving its tasks on an idle CPU.
> 
> Once the idle load_balance has selected the busiest CPU, it will look for an
> active load balance for only two cases :
> - there is only 1 task on the busiest CPU.
> - we haven't been able to move a task of the busiest rq.
> 
> A CPU with a reduced capacity is included in the 1st case, and it's worth to
> actively migrate its task if the idle CPU has got full capacity. This test has
> been added in need_active_balance.
> 
> 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
> 
> env.src_cpu and env.src_rq must be set unconditionnally because they are used
> in need_active_balance which is called even if busiest->nr_running equals 1
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 53 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index db392a6..02e8f7f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6634,6 +6634,28 @@ static int need_active_balance(struct lb_env *env)
>  			return 1;
>  	}
>  
> +	/*
> +	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
> +	 * It's worth migrating the task if the src_cpu's capacity is reduced
> +	 * because of other sched_class or IRQs whereas capacity stays
> +	 * available on dst_cpu.
> +	 */
> +	if ((env->idle != CPU_NOT_IDLE) &&
> +			(env->src_rq->cfs.h_nr_running == 1)) {
> +		unsigned long src_eff_capacity, dst_eff_capacity;
> +
> +		dst_eff_capacity = 100;
> +		dst_eff_capacity *= capacity_of(env->dst_cpu);
> +		dst_eff_capacity *= capacity_orig_of(env->src_cpu);
> +
> +		src_eff_capacity = sd->imbalance_pct;
> +		src_eff_capacity *= capacity_of(env->src_cpu);
> +		src_eff_capacity *= capacity_orig_of(env->dst_cpu);

Do we need to scale by capacity_orig? Shouldn't the absolute capacity be
better?

if (capacity_of(env->src) * sd->imbalance_pct < capacity_of(env->dst) *
100) ?

Isn't it the absolute available capacity that matters? For SMP
capacity_orig is the same and cancels out and doesn't change anything.
For big.LITTLE we would rather have the task run on a big where rt/irq
eats 30% than a little cpu where rq/irq eats 5%, assuming big capacity
is much bigger than little capacity so the absolute available capacity
(~cycles/time) is larger on the big cpu.
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Vincent Guittot Nov. 24, 2014, 2:45 p.m. | #2
On 21 November 2014 at 13:37, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> On Mon, Nov 03, 2014 at 04:54:47PM +0000, Vincent Guittot wrote:

>>
>> +     /*
>> +      * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
>> +      * It's worth migrating the task if the src_cpu's capacity is reduced
>> +      * because of other sched_class or IRQs whereas capacity stays
>> +      * available on dst_cpu.
>> +      */
>> +     if ((env->idle != CPU_NOT_IDLE) &&
>> +                     (env->src_rq->cfs.h_nr_running == 1)) {
>> +             unsigned long src_eff_capacity, dst_eff_capacity;
>> +
>> +             dst_eff_capacity = 100;
>> +             dst_eff_capacity *= capacity_of(env->dst_cpu);
>> +             dst_eff_capacity *= capacity_orig_of(env->src_cpu);
>> +
>> +             src_eff_capacity = sd->imbalance_pct;
>> +             src_eff_capacity *= capacity_of(env->src_cpu);
>> +             src_eff_capacity *= capacity_orig_of(env->dst_cpu);
>
> Do we need to scale by capacity_orig? Shouldn't the absolute capacity be
> better?
>
> if (capacity_of(env->src) * sd->imbalance_pct < capacity_of(env->dst) *
> 100) ?

we don't want to compare absolute capacity between CPUs but to compare
the reduction of their capacity because we want to choose the CPU
which is less used  by RT tasks or irq

Regards,
Vincent
>
> Isn't it the absolute available capacity that matters? For SMP
> capacity_orig is the same and cancels out and doesn't change anything.
> For big.LITTLE we would rather have the task run on a big where rt/irq
> eats 30% than a little cpu where rq/irq eats 5%, assuming big capacity
> is much bigger than little capacity so the absolute available capacity
> (~cycles/time) is larger on the big cpu.
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
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Patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index db392a6..02e8f7f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6634,6 +6634,28 @@  static int need_active_balance(struct lb_env *env)
 			return 1;
 	}
 
+	/*
+	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
+	 * It's worth migrating the task if the src_cpu's capacity is reduced
+	 * because of other sched_class or IRQs whereas capacity stays
+	 * available on dst_cpu.
+	 */
+	if ((env->idle != CPU_NOT_IDLE) &&
+			(env->src_rq->cfs.h_nr_running == 1)) {
+		unsigned long src_eff_capacity, dst_eff_capacity;
+
+		dst_eff_capacity = 100;
+		dst_eff_capacity *= capacity_of(env->dst_cpu);
+		dst_eff_capacity *= capacity_orig_of(env->src_cpu);
+
+		src_eff_capacity = sd->imbalance_pct;
+		src_eff_capacity *= capacity_of(env->src_cpu);
+		src_eff_capacity *= capacity_orig_of(env->dst_cpu);
+
+		if (src_eff_capacity < dst_eff_capacity)
+			return 1;
+	}
+
 	return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
 }
 
@@ -6733,6 +6755,9 @@  static int load_balance(int this_cpu, struct rq *this_rq,
 
 	schedstat_add(sd, lb_imbalance[idle], env.imbalance);
 
+	env.src_cpu = busiest->cpu;
+	env.src_rq = busiest;
+
 	ld_moved = 0;
 	if (busiest->nr_running > 1) {
 		/*
@@ -6742,8 +6767,6 @@  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);
 
 more_balance:
@@ -7443,22 +7466,25 @@  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.
  */
-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_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
@@ -7472,38 +7498,44 @@  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) {
 		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) &&
+				check_cpu_capacity(rq, sd)) {
+			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) { }