[RFC PATCH 3/7] sched/fair: Save a snapshot of sched domain total_load and total_capacity

[Chen Yu]

Save the total_load, total_capacity of the current sched domain in each
periodic load balance. This statistic can be used later by CPU_NEWLY_IDLE
load balance if it quits the scan earlier. Introduce a sched feature
ILB_SNAPSHOT to control this. Code can check if sd_share->total_capacity
is non-zero to verify if the stat is valid.

In theory, if the system has reached a stable status, the total_capacity
and total_load should not change dramatically.

Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 include/linux/sched/topology.h |  2 ++
 kernel/sched/fair.c            | 25 +++++++++++++++++++++++++
 kernel/sched/features.h        |  2 ++
 3 files changed, 29 insertions(+)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index c07f2f00317a..d6a64a2c92aa 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -82,6 +82,8 @@ struct sched_domain_shared {
 	atomic_t	nr_busy_cpus;
 	int		has_idle_cores;
 	int		nr_idle_scan;
+	unsigned long	total_load;
+	unsigned long	total_capacity;
 };
 
 struct sched_domain {
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b3e25be58e2b..edcfee9965cd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10132,6 +10132,27 @@ static void update_idle_cpu_scan(struct lb_env *env,
 		WRITE_ONCE(sd_share->nr_idle_scan, (int)y);
 }
 
+static void ilb_save_stats(struct lb_env *env,
+			   struct sched_domain_shared *sd_share,
+			   struct sd_lb_stats *sds)
+{
+	if (!sched_feat(ILB_SNAPSHOT))
+		return;
+
+	if (!sd_share)
+		return;
+
+	/* newidle balance is too frequent */
+	if (env->idle == CPU_NEWLY_IDLE)
+		return;
+
+	if (sds->total_load != sd_share->total_load)
+		WRITE_ONCE(sd_share->total_load, sds->total_load);
+
+	if (sds->total_capacity != sd_share->total_capacity)
+		WRITE_ONCE(sd_share->total_capacity, sds->total_capacity);
+}
+
 /**
  * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
  * @env: The load balancing environment.
@@ -10140,6 +10161,7 @@ static void update_idle_cpu_scan(struct lb_env *env,
 
 static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sds)
 {
+	struct sched_domain_shared *sd_share = env->sd->shared;
 	struct sched_group *sg = env->sd->groups;
 	struct sg_lb_stats *local = &sds->local_stat;
 	struct sg_lb_stats tmp_sgs;
@@ -10209,6 +10231,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 	}
 
 	update_idle_cpu_scan(env, sum_util);
+
+	/* save a snapshot of stats during periodic load balance */
+	ilb_save_stats(env, sd_share, sds);
 }
 
 /**
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index ee7f23c76bd3..3cb71c8cddc0 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -101,3 +101,5 @@ SCHED_FEAT(LATENCY_WARN, false)
 
 SCHED_FEAT(ALT_PERIOD, true)
 SCHED_FEAT(BASE_SLICE, true)
+
+SCHED_FEAT(ILB_SNAPSHOT, true)
-- 
2.25.1