[RFC PATCH 4/7] sched/fair: Calculate the scan depth for idle balance based on system utilization

[Chen Yu]

When the 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 groups to calculate the statistics one by one, this total
time cost on newidle_balance() could be O(n^2). n is the number
of groups. This issue is more severe if there are many groups
within 1 domain, for example, a system with a large number of
Cores in a LLC domain. This is not good for performance or
power saving.

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 sched domain. Let the number of
scanned groups be a linear function of the utilization ratio:

nr_groups_to_scan = nr_groups * (1 - util_ratio)

Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 include/linux/sched/topology.h |  1 +
 kernel/sched/fair.c            | 30 ++++++++++++++++++++++++++++++
 kernel/sched/features.h        |  1 +
 3 files changed, 32 insertions(+)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index d6a64a2c92aa..af2261308529 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -84,6 +84,7 @@ struct sched_domain_shared {
 	int		nr_idle_scan;
 	unsigned long	total_load;
 	unsigned long	total_capacity;
+	int		nr_sg_scan;
 };
 
 struct sched_domain {
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index edcfee9965cd..6925813db59b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10153,6 +10153,35 @@ static void ilb_save_stats(struct lb_env *env,
 		WRITE_ONCE(sd_share->total_capacity, sds->total_capacity);
 }
 
+static void update_ilb_group_scan(struct lb_env *env,
+				  unsigned long sum_util,
+				  struct sched_domain_shared *sd_share)
+{
+	u64 tmp, nr_scan;
+
+	if (!sched_feat(ILB_UTIL))
+		return;
+
+	if (!sd_share)
+		return;
+
+	if (env->idle == CPU_NEWLY_IDLE)
+		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->nr_sg_scan)
+		WRITE_ONCE(sd_share->nr_sg_scan, (int)nr_scan);
+}
+
 /**
  * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
  * @env: The load balancing environment.
@@ -10231,6 +10260,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 	}
 
 	update_idle_cpu_scan(env, sum_util);
+	update_ilb_group_scan(env, sum_util, sd_share);
 
 	/* 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 3cb71c8cddc0..30f6d1a2f235 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -103,3 +103,4 @@ SCHED_FEAT(ALT_PERIOD, true)
 SCHED_FEAT(BASE_SLICE, true)
 
 SCHED_FEAT(ILB_SNAPSHOT, true)
+SCHED_FEAT(ILB_UTIL, true)
-- 
2.25.1