sched: Remove unnecessary iteration over sched domains to update nr_busy_cpus

commit 37dc6b50 upstream.

nr_busy_cpus parameter is used by nohz_kick_needed() to find out the
number of busy cpus in a sched domain which has SD_SHARE_PKG_RESOURCES
flag set.  Therefore instead of updating nr_busy_cpus at every level
of sched domain, since it is irrelevant, we can update this parameter
only at the parent domain of the sd which has this flag set. Introduce
a per-cpu parameter sd_busy which represents this parent domain.

In nohz_kick_needed() we directly query the nr_busy_cpus parameter
associated with the groups of sd_busy.

By associating sd_busy with the highest domain which has
SD_SHARE_PKG_RESOURCES flag set, we cover all lower level domains
which could have this flag set and trigger nohz_idle_balancing if any
of the levels have more than one busy cpu.

sd_busy is irrelevant for asymmetric load balancing. However sd_asym
has been introduced to represent the highest sched domain which has
SD_ASYM_PACKING flag set so that it can be queried directly when
required.

While we are at it, we might as well change the nohz_idle parameter to
be updated at the sd_busy domain level alone and not the base domain
level of a CPU.  This will unify the concept of busy cpus at just one
level of sched domain where it is currently used.

Signed-off-by: Preeti U Murthy<preeti@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: svaidy@linux.vnet.ibm.com
Cc: vincent.guittot@linaro.org
Cc: bitbucket@online.de
Cc: benh@kernel.crashing.org
Cc: anton@samba.org
Cc: Morten.Rasmussen@arm.com
Cc: pjt@google.com
Cc: peterz@infradead.org
Cc: mikey@neuling.org
Link: http://lkml.kernel.org/r/20131030031252.23426.4417.stgit@preeti.in.ibm.comSigned-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

kernel/sched/core.c

@@ -5119,6 +5119,8 @@ static void destroy_sched_domains(struct sched_domain *sd, int cpu)
 DEFINE_PER_CPU(struct sched_domain *, sd_llc);
 DEFINE_PER_CPU(int, sd_llc_size);
 DEFINE_PER_CPU(int, sd_llc_id);
+DEFINE_PER_CPU(struct sched_domain *, sd_busy);
+DEFINE_PER_CPU(struct sched_domain *, sd_asym);
 
 static void update_top_cache_domain(int cpu)
 {
@@ -5130,11 +5132,15 @@ static void update_top_cache_domain(int cpu)
 	if (sd) {
 		id = cpumask_first(sched_domain_span(sd));
 		size = cpumask_weight(sched_domain_span(sd));
+		rcu_assign_pointer(per_cpu(sd_busy, cpu), sd->parent);
 	}
 
 	rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
 	per_cpu(sd_llc_size, cpu) = size;
 	per_cpu(sd_llc_id, cpu) = id;
+
+	sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
+	rcu_assign_pointer(per_cpu(sd_asym, cpu), sd);
 }
 
 /*

kernel/sched/fair.c

@@ -5598,15 +5598,15 @@ static inline void nohz_balance_exit_idle(int cpu)
 static inline void set_cpu_sd_state_busy(void)
 {
 	struct sched_domain *sd;
+	int cpu = smp_processor_id();
 
 	rcu_read_lock();
-	sd = rcu_dereference_check_sched_domain(this_rq()->sd);
+	sd = rcu_dereference(per_cpu(sd_busy, cpu));
 
 	if (!sd || !sd->nohz_idle)
 		goto unlock;
 	sd->nohz_idle = 0;
 
-	for (; sd; sd = sd->parent)
 	atomic_inc(&sd->groups->sgp->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
@@ -5615,15 +5615,15 @@ static inline void set_cpu_sd_state_busy(void)
 void set_cpu_sd_state_idle(void)
 {
 	struct sched_domain *sd;
+	int cpu = smp_processor_id();
 
 	rcu_read_lock();
-	sd = rcu_dereference_check_sched_domain(this_rq()->sd);
+	sd = rcu_dereference(per_cpu(sd_busy, cpu));
 
 	if (!sd || sd->nohz_idle)
 		goto unlock;
 	sd->nohz_idle = 1;
 
-	for (; sd; sd = sd->parent)
 	atomic_dec(&sd->groups->sgp->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
@@ -5807,6 +5807,8 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
 {
 	unsigned long now = jiffies;
 	struct sched_domain *sd;
+	struct sched_group_power *sgp;
+	int nr_busy;
 
 	if (unlikely(idle_cpu(cpu)))
 		return 0;
@@ -5832,22 +5834,22 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
 		goto need_kick;
 
 	rcu_read_lock();
-	for_each_domain(cpu, sd) {
-		struct sched_group *sg = sd->groups;
-		struct sched_group_power *sgp = sg->sgp;
-		int nr_busy = atomic_read(&sgp->nr_busy_cpus);
+	sd = rcu_dereference(per_cpu(sd_busy, cpu));
+
+	if (sd) {
+		sgp = sd->groups->sgp;
+		nr_busy = atomic_read(&sgp->nr_busy_cpus);
 
-		if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
+		if (nr_busy > 1)
 			goto need_kick_unlock;
+	}
+
+	sd = rcu_dereference(per_cpu(sd_asym, cpu));
 
-		if (sd->flags & SD_ASYM_PACKING
-		    && (cpumask_first_and(nohz.idle_cpus_mask,
+	if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
 				  sched_domain_span(sd)) < cpu))
 		goto need_kick_unlock;
 
-		if (!(sd->flags & (SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING)))
-			break;
-	}
 	rcu_read_unlock();
 	return 0;
 

kernel/sched/sched.h

@@ -596,6 +596,8 @@ static inline struct sched_domain *highest_flag_domain(int cpu, int flag)
 DECLARE_PER_CPU(struct sched_domain *, sd_llc);
 DECLARE_PER_CPU(int, sd_llc_size);
 DECLARE_PER_CPU(int, sd_llc_id);
+DECLARE_PER_CPU(struct sched_domain *, sd_busy);
+DECLARE_PER_CPU(struct sched_domain *, sd_asym);
 
 struct sched_group_power {
 	atomic_t ref;