[PATCH v2 6/8] sched/idle: Move busy_cpu accounting to idle callback

[Srikar Dronamraju]

Currently we account nr_busy_cpus in no_hz idle functions.
There is no reason why nr_busy_cpus should updated be in NO_HZ_COMMON
configs only. Also scheduler can mark a CPU as non-busy as soon as an
idle class task starts to run. Scheduler can then mark a CPU as busy
as soon as its woken up from idle or a new task is placed on it's
runqueue.

Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Cc: Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx>
Cc: Parth Shah <parth@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c     |  6 ++++--
 kernel/sched/idle.c     | 29 +++++++++++++++++++++++++++--
 kernel/sched/sched.h    |  1 +
 kernel/sched/topology.c |  2 ++
 4 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c30587631a24..4d3b0928fe98 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10394,7 +10394,10 @@ static void set_cpu_sd_state_busy(int cpu)
 		goto unlock;
 	sd->nohz_idle = 0;
 
-	atomic_inc(&sd->shared->nr_busy_cpus);
+	if (sd && per_cpu(is_idle, cpu)) {
+		atomic_add_unless(&sd->shared->nr_busy_cpus, 1, per_cpu(sd_llc_size, cpu));
+		per_cpu(is_idle, cpu) = 0;
+	}
 unlock:
 	rcu_read_unlock();
 }
@@ -10424,7 +10427,6 @@ static void set_cpu_sd_state_idle(int cpu)
 		goto unlock;
 	sd->nohz_idle = 1;
 
-	atomic_dec(&sd->shared->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
 }
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index cc828f3efe71..e624a05e48bd 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -425,12 +425,25 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
 
 static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
 {
-#ifdef CONFIG_SCHED_SMT
+#ifdef CONFIG_SMP
+	struct sched_domain_shared *sds;
 	int cpu = rq->cpu;
 
+#ifdef CONFIG_SCHED_SMT
 	if (static_branch_likely(&sched_smt_present))
 		set_core_busy(cpu);
 #endif
+
+	rcu_read_lock();
+	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+	if (sds) {
+		if (per_cpu(is_idle, cpu)) {
+			atomic_inc(&sds->nr_busy_cpus);
+			per_cpu(is_idle, cpu) = 0;
+		}
+	}
+	rcu_read_unlock();
+#endif
 }
 
 static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
@@ -442,9 +455,21 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
 struct task_struct *pick_next_task_idle(struct rq *rq)
 {
 	struct task_struct *next = rq->idle;
+#ifdef CONFIG_SMP
+	struct sched_domain_shared *sds;
+	int cpu = rq->cpu;
 
-	set_next_task_idle(rq, next, true);
+	rcu_read_lock();
+	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+	if (sds) {
+		atomic_add_unless(&sds->nr_busy_cpus, -1, 0);
+		per_cpu(is_idle, cpu) = 1;
+	}
 
+	rcu_read_unlock();
+#endif
+
+	set_next_task_idle(rq, next, true);
 	return next;
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 5c0bd4b0e73a..baf8d9a4cb26 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1483,6 +1483,7 @@ DECLARE_PER_CPU(int, sd_llc_id);
 #ifdef CONFIG_SCHED_SMT
 DECLARE_PER_CPU(int, smt_id);
 #endif
+DECLARE_PER_CPU(int, is_idle);
 DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
 DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
 DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8db40c8a6ad0..00e4669bb241 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -647,6 +647,7 @@ DEFINE_PER_CPU(int, sd_llc_id);
 #ifdef CONFIG_SCHED_SMT
 DEFINE_PER_CPU(int, smt_id);
 #endif
+DEFINE_PER_CPU(int, is_idle);
 DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
 DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
 DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
@@ -673,6 +674,7 @@ static void update_top_cache_domain(int cpu)
 #ifdef CONFIG_SCHED_SMT
 	per_cpu(smt_id, cpu) = cpumask_first(cpu_smt_mask(cpu));
 #endif
+	per_cpu(is_idle, cpu) = 1;
 	rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
 
 	sd = lowest_flag_domain(cpu, SD_NUMA);
-- 
2.18.2