[RFC PATCH 3/4] sched/fair: Calculate the scan depth for idle balance based on system utilization

[Chen Yu]

When CPU is about to enter idle, it invokes newidle_balance() to pull
some tasks from other runqueues. Although there is per domain
max_newidle_lb_cost to throttle the newidle_balance(), it would be
good to further limit the scan based on overall system utilization.
The reason is that there is no limitation for newidle_balance() to
launch this balance simultaneously on multiple CPUs. Since each
newidle_balance() has to traverse all the CPUs to calculate the
statistics one by one, this total time cost on newidle_balance()
could be O(n^2). This is not good for performance or power saving.

For example, sqlite has spent quite some time on newidle balance()
on Intel Sapphire Rapids, which has 2 x 56C/112T = 224 CPUs:
6.69%    0.09%  sqlite3     [kernel.kallsyms]   [k] newidle_balance
5.39%    4.71%  sqlite3     [kernel.kallsyms]   [k] update_sd_lb_stats

Based on this observation, limit the scan depth of newidle_balance()
by considering the utilization of the LLC domain. Let the number of
scanned groups be a linear function of the utilization ratio:

nr_groups_to_scan = nr_groups * (1 - util_ratio)

Besides, save the total_load, total_capacity of the current
sched domain in each periodic load balance. This statistic
can be reused later by CPU_NEWLY_IDLE load balance if it quits
the scan earlier. Introduce a sched feature ILB_UTIL to
control this.

Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 include/linux/sched/topology.h |  4 ++++
 kernel/sched/fair.c            | 34 ++++++++++++++++++++++++++++++++++
 kernel/sched/features.h        |  1 +
 3 files changed, 39 insertions(+)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 1faececd5694..d7b2bac9bdf3 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -82,6 +82,10 @@ struct sched_domain_shared {
 	atomic_t	nr_busy_cpus;
 	int		has_idle_cores;
 	int		nr_idle_scan;
+	/* ilb scan depth and load balance statistic snapshot */
+	int		ilb_nr_scan;
+	unsigned long ilb_total_load;
+	unsigned long ilb_total_capacity;
 };
 
 struct sched_domain {
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b3a24aead848..f999e838114e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10122,6 +10122,39 @@ static void update_idle_cpu_scan(struct lb_env *env,
 		WRITE_ONCE(sd_share->nr_idle_scan, (int)y);
 }
 
+static void update_ilb_group_scan(struct lb_env *env,
+				  unsigned long sum_util,
+				  struct sched_domain_shared *sd_share,
+				  struct sd_lb_stats *sds)
+{
+	u64 tmp, nr_scan;
+
+	if (!sched_feat(ILB_UTIL) || env->idle == CPU_NEWLY_IDLE)
+		return;
+
+	if (!sd_share)
+		return;
+	/*
+	 * Limit the newidle balance scan depth based on overall system
+	 * utilization:
+	 * nr_groups_scan = nr_groups * (1 - util_ratio)
+	 * and util_ratio = sum_util / (sd_weight * SCHED_CAPACITY_SCALE)
+	 */
+	nr_scan = env->sd->nr_groups * sum_util;
+	tmp = env->sd->span_weight * SCHED_CAPACITY_SCALE;
+	do_div(nr_scan, tmp);
+	nr_scan = env->sd->nr_groups - nr_scan;
+	if ((int)nr_scan != sd_share->ilb_nr_scan)
+		WRITE_ONCE(sd_share->ilb_nr_scan, (int)nr_scan);
+
+	/* Also save the statistic snapshot of the periodic load balance */
+	if (sds->total_load != sd_share->ilb_total_load)
+		WRITE_ONCE(sd_share->ilb_total_load, sds->total_load);
+
+	if (sds->total_capacity != sd_share->ilb_total_capacity)
+		WRITE_ONCE(sd_share->ilb_total_capacity, sds->total_capacity);
+}
+
 /**
  * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
  * @env: The load balancing environment.
@@ -10200,6 +10233,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 	}
 
 	update_idle_cpu_scan(env, sum_util, sd_share);
+	update_ilb_group_scan(env, sum_util, sd_share, sds);
 }
 
 /**
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index ee7f23c76bd3..8f6e5b08408d 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -85,6 +85,7 @@ SCHED_FEAT(RT_PUSH_IPI, true)
 
 SCHED_FEAT(RT_RUNTIME_SHARE, false)
 SCHED_FEAT(LB_MIN, false)
+SCHED_FEAT(ILB_UTIL, true)
 SCHED_FEAT(ATTACH_AGE_LOAD, true)
 
 SCHED_FEAT(WA_IDLE, true)
-- 
2.25.1