Revert "sched/fair: Proportional newidle balance"

This reverts commit 33cf66d.

Signed-off-by: Eric Naim <dnaim@cachyos.org>

Author: 1Naim

include/linux/sched/topology.h

@@ -92,9 +92,6 @@ struct sched_domain {
 	unsigned int nr_balance_failed; /* initialise to 0 */
 
 	/* idle_balance() stats */
-	unsigned int newidle_call;
-	unsigned int newidle_success;
-	unsigned int newidle_ratio;
 	u64 max_newidle_lb_cost;
 	unsigned long last_decay_max_lb_cost;
 

kernel/sched/core.c

@@ -124,7 +124,6 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_exit_tp);
 EXPORT_TRACEPOINT_SYMBOL_GPL(sched_set_need_resched_tp);
 
 DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
-DEFINE_PER_CPU(struct rnd_state, sched_rnd_state);
 
 #ifdef CONFIG_SCHED_PROXY_EXEC
 DEFINE_STATIC_KEY_TRUE(__sched_proxy_exec);
@@ -8545,8 +8544,6 @@ void __init sched_init_smp(void)
 {
 	sched_init_numa(NUMA_NO_NODE);
 
-	prandom_init_once(&sched_rnd_state);
-
 	/*
 	 * There's no userspace yet to cause hotplug operations; hence all the
 	 * CPU masks are stable and all blatant races in the below code cannot

kernel/sched/fair.c

@@ -12279,27 +12279,11 @@ void update_max_interval(void)
 	max_load_balance_interval = HZ*num_online_cpus()/10;
 }
 
-static inline void update_newidle_stats(struct sched_domain *sd, unsigned int success)
-{
-	sd->newidle_call++;
-	sd->newidle_success += success;
-
-	if (sd->newidle_call >= 1024) {
-		sd->newidle_ratio = sd->newidle_success;
-		sd->newidle_call /= 2;
-		sd->newidle_success /= 2;
-	}
-}
-
-static inline bool
-update_newidle_cost(struct sched_domain *sd, u64 cost, unsigned int success)
+static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost)
 {
 	unsigned long next_decay = sd->last_decay_max_lb_cost + HZ;
 	unsigned long now = jiffies;
 
-	if (cost)
-		update_newidle_stats(sd, success);
-
 	if (cost > sd->max_newidle_lb_cost) {
 		/*
 		 * Track max cost of a domain to make sure to not delay the
@@ -12347,7 +12331,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 		 * Decay the newidle max times here because this is a regular
 		 * visit to all the domains.
 		 */
-		need_decay = update_newidle_cost(sd, 0, 0);
+		need_decay = update_newidle_cost(sd, 0);
 		max_cost += sd->max_newidle_lb_cost;
 
 		/*
@@ -12990,37 +12974,17 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 			break;
 
 		if (sd->flags & SD_BALANCE_NEWIDLE) {
-			unsigned int weight = 1;
-
-			if (sched_feat(NI_RANDOM)) {
-				/*
-				 * Throw a 1k sided dice; and only run
-				 * newidle_balance according to the success
-				 * rate.
-				 */
-				u32 d1k = sched_rng() % 1024;
-				weight = 1 + sd->newidle_ratio;
-				if (d1k > weight) {
-					update_newidle_stats(sd, 0);
-					continue;
-				}
-				weight = (1024 + weight/2) / weight;
-			}
 
 			pulled_task = sched_balance_rq(this_cpu, this_rq,
 						   sd, CPU_NEWLY_IDLE,
 						   &continue_balancing);
 
 			t1 = sched_clock_cpu(this_cpu);
 			domain_cost = t1 - t0;
+			update_newidle_cost(sd, domain_cost);
+
 			curr_cost += domain_cost;
 			t0 = t1;
-
-			/*
-			 * Track max cost of a domain to make sure to not delay the
-			 * next wakeup on the CPU.
-			 */
-			update_newidle_cost(sd, domain_cost, weight * !!pulled_task);
 		}
 
 		/*

kernel/sched/features.h

@@ -121,8 +121,3 @@ SCHED_FEAT(WA_BIAS, true)
 SCHED_FEAT(UTIL_EST, true)
 
 SCHED_FEAT(LATENCY_WARN, false)
-
-/*
- * Do newidle balancing proportional to its success rate using randomization.
- */
-SCHED_FEAT(NI_RANDOM, true)

kernel/sched/sched.h

@@ -5,7 +5,6 @@
 #ifndef _KERNEL_SCHED_SCHED_H
 #define _KERNEL_SCHED_SCHED_H
 
-#include <linux/prandom.h>
 #include <linux/sched/affinity.h>
 #include <linux/sched/autogroup.h>
 #include <linux/sched/cpufreq.h>
@@ -1375,12 +1374,6 @@ static inline bool is_migration_disabled(struct task_struct *p)
 }
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
-DECLARE_PER_CPU(struct rnd_state, sched_rnd_state);
-
-static inline u32 sched_rng(void)
-{
-	return prandom_u32_state(this_cpu_ptr(&sched_rnd_state));
-}
 
 static __always_inline struct rq *__this_rq(void)
 {

kernel/sched/topology.c

@@ -1674,12 +1674,6 @@ sd_init(struct sched_domain_topology_level *tl,
 
 		.last_balance		= jiffies,
 		.balance_interval	= sd_weight,
-
-		/* 50% success rate */
-		.newidle_call		= 512,
-		.newidle_success	= 256,
-		.newidle_ratio		= 512,
-
 		.max_newidle_lb_cost	= 0,
 		.last_decay_max_lb_cost	= jiffies,
 		.child			= child,