[RFC v2 1/2] sched/fair: filter out overloaded cpus in SIS

[Abel Wu]

It would bring benefit if the unoccupied cpus (sched-idle/idle cpus) can
start serving as soon as the non-idle tasks are available. Lots of effort
has already done, and task wakeup path is one of them.

When a task is woken up, the scheduler tends to put it on an unoccupied
cpu to make full use of cpu capacity. But due to scalability issues, the
search depth is bounded to a reasonable limit. IOW it's possible that a
task is woken up on a busy cpu while unoccupied cpus are still out there.

This patch focuses on improving the SIS searching efficiency by filtering
out the overloaded cpus, so as a result the more overloaded the system
is, the less cpus we will search.

Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
---
 include/linux/sched/topology.h | 12 ++++++++
 kernel/sched/core.c            |  1 +
 kernel/sched/fair.c            | 65 +++++++++++++++++++++++++++++++++++++++++-
 kernel/sched/sched.h           |  6 ++++
 kernel/sched/topology.c        |  4 ++-
 5 files changed, 86 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 56cffe42abbc..fb35a1983568 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -81,6 +81,18 @@ struct sched_domain_shared {
 	atomic_t	ref;
 	atomic_t	nr_busy_cpus;
 	int		has_idle_cores;
+
+	/*
+	 * The state of overloaded cpus is for different use against
+	 * the above elements and they are all hot, so start a new
+	 * cacheline to avoid false sharing.
+	 */
+	atomic_t	nr_overloaded	____cacheline_aligned;
+
+	/*
+	 * Must be last
+	 */
+	unsigned long	overloaded[];
 };
 
 struct sched_domain {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ef946123e9af..a372881f8eaf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9495,6 +9495,7 @@ void __init sched_init(void)
 		rq->wake_stamp = jiffies;
 		rq->wake_avg_idle = rq->avg_idle;
 		rq->max_idle_balance_cost = sysctl_sched_migration_cost;
+		rq->overloaded = 0;
 
 		INIT_LIST_HEAD(&rq->cfs_tasks);
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 16874e112fe6..fbeb05321615 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6284,6 +6284,15 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
 #endif /* CONFIG_SCHED_SMT */
 
 /*
+ * It would be very unlikely to find an unoccupied cpu when system is heavily
+ * overloaded. Even if we could, the cost might bury the benefit.
+ */
+static inline bool sched_domain_overloaded(struct sched_domain *sd, int nr_overloaded)
+{
+	return nr_overloaded > sd->span_weight - (sd->span_weight >> 4);
+}
+
+/*
  * Scan the LLC domain for idle CPUs; this is dynamically regulated by
  * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
  * average idle time for this rq (as found in rq->avg_idle).
@@ -6291,7 +6300,7 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
 static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool has_idle_core, int target)
 {
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
-	int i, cpu, idle_cpu = -1, nr = INT_MAX;
+	int i, cpu, idle_cpu = -1, nr = INT_MAX, nro;
 	struct rq *this_rq = this_rq();
 	int this = smp_processor_id();
 	struct sched_domain *this_sd;
@@ -6301,7 +6310,13 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 	if (!this_sd)
 		return -1;
 
+	nro = atomic_read(&sd->shared->nr_overloaded);
+	if (sched_domain_overloaded(sd, nro))
+		return -1;
+
 	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+	if (nro)
+		cpumask_andnot(cpus, cpus, sdo_mask(sd->shared));
 
 	if (sched_feat(SIS_PROP) && !has_idle_core) {
 		u64 avg_cost, avg_idle, span_avg;
@@ -7018,6 +7033,51 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 
 	return newidle_balance(rq, rf) != 0;
 }
+
+static inline bool cfs_rq_overloaded(struct rq *rq)
+{
+	return rq->cfs.h_nr_running - rq->cfs.idle_h_nr_running > 1;
+}
+
+/*
+ * Use locality-friendly rq->overloaded to cache the status of the rq
+ * to minimize the heavy cost on LLC shared data.
+ *
+ * Must be called with rq locked
+ */
+static void update_overload_status(struct rq *rq)
+{
+	struct sched_domain_shared *sds;
+	bool overloaded = cfs_rq_overloaded(rq);
+	int cpu = cpu_of(rq);
+
+	lockdep_assert_rq_held(rq);
+
+	if (rq->overloaded == overloaded)
+		return;
+
+	rcu_read_lock();
+	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+	if (unlikely(!sds))
+		goto unlock;
+
+	if (overloaded) {
+		cpumask_set_cpu(cpu, sdo_mask(sds));
+		atomic_inc(&sds->nr_overloaded);
+	} else {
+		cpumask_clear_cpu(cpu, sdo_mask(sds));
+		atomic_dec(&sds->nr_overloaded);
+	}
+
+	rq->overloaded = overloaded;
+unlock:
+	rcu_read_unlock();
+}
+
+#else
+
+static inline void update_overload_status(struct rq *rq) { }
+
 #endif /* CONFIG_SMP */
 
 static unsigned long wakeup_gran(struct sched_entity *se)
@@ -7365,6 +7425,8 @@ done: __maybe_unused;
 	if (new_tasks > 0)
 		goto again;
 
+	update_overload_status(rq);
+
 	/*
 	 * rq is about to be idle, check if we need to update the
 	 * lost_idle_time of clock_pelt
@@ -11183,6 +11245,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 	if (static_branch_unlikely(&sched_numa_balancing))
 		task_tick_numa(rq, curr);
 
+	update_overload_status(rq);
 	update_misfit_status(curr, rq);
 	update_overutilized_status(task_rq(curr));
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3da5718cd641..afa1bb68c3ec 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1012,6 +1012,7 @@ struct rq {
 
 	unsigned char		nohz_idle_balance;
 	unsigned char		idle_balance;
+	unsigned char		overloaded;
 
 	unsigned long		misfit_task_load;
 
@@ -1764,6 +1765,11 @@ static inline struct sched_domain *lowest_flag_domain(int cpu, int flag)
 	return sd;
 }
 
+static inline struct cpumask *sdo_mask(struct sched_domain_shared *sds)
+{
+	return to_cpumask(sds->overloaded);
+}
+
 DECLARE_PER_CPU(struct sched_domain __rcu *, sd_llc);
 DECLARE_PER_CPU(int, sd_llc_size);
 DECLARE_PER_CPU(int, sd_llc_id);
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 32841c6741d1..fea1294ebd16 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1621,6 +1621,8 @@ sd_init(struct sched_domain_topology_level *tl,
 		sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
 		atomic_inc(&sd->shared->ref);
 		atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
+		atomic_set(&sd->shared->nr_overloaded, 0);
+		cpumask_clear(sdo_mask(sd->shared));
 	}
 
 	sd->private = sdd;
@@ -2086,7 +2088,7 @@ static int __sdt_alloc(const struct cpumask *cpu_map)
 
 			*per_cpu_ptr(sdd->sd, j) = sd;
 
-			sds = kzalloc_node(sizeof(struct sched_domain_shared),
+			sds = kzalloc_node(sizeof(struct sched_domain_shared) + cpumask_size(),
 					GFP_KERNEL, cpu_to_node(j));
 			if (!sds)
 				return -ENOMEM;
-- 
2.11.0