sched/balancing: Change 'enum cpu_idle_type' to have more natural definitions

The cpu_idle_type enum has the confusingly inverted property
that 'not idle' is 1, and 'idle' is '0'.

This resulted in a number of unnecessary complications in the code.

Reverse the order, remove the CPU_NOT_IDLE type, and convert
all code to a natural boolean form.

It's much more readable:

  -       enum cpu_idle_type idle = this_rq->idle_balance ?
  -                                               CPU_IDLE : CPU_NOT_IDLE;
  -
  +       enum cpu_idle_type idle = this_rq->idle_balance;

  --------------------------------

  -       if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
  +       if (!env->idle || !busiest->sum_nr_running)

  --------------------------------

And gets rid of the double negation in these usages:

  -               if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
  +               if (env->idle && env->src_rq->nr_running <= 1)

Furthermore, this makes code much more obvious where there's
differentiation between CPU_IDLE and CPU_NEWLY_IDLE.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: "Gautham R. Shenoy" <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20240308105901.1096078-4-mingo@kernel.org

include/linux/sched/idle.h

@@ -5,8 +5,8 @@
 #include <linux/sched.h>
 
 enum cpu_idle_type {
+	__CPU_NOT_IDLE = 0,
 	CPU_IDLE,
-	CPU_NOT_IDLE,
 	CPU_NEWLY_IDLE,
 	CPU_MAX_IDLE_TYPES
 };

kernel/sched/fair.c

@@ -9070,7 +9070,7 @@ static int detach_tasks(struct lb_env *env)
 		 * We don't want to steal all, otherwise we may be treated likewise,
 		 * which could at worst lead to a livelock crash.
 		 */
-		if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
+		if (env->idle && env->src_rq->nr_running <= 1)
 			break;
 
 		env->loop++;
@@ -9803,7 +9803,7 @@ static inline bool smt_vs_nonsmt_groups(struct sched_group *sg1,
 static inline bool smt_balance(struct lb_env *env, struct sg_lb_stats *sgs,
 			       struct sched_group *group)
 {
-	if (env->idle == CPU_NOT_IDLE)
+	if (!env->idle)
 		return false;
 
 	/*
@@ -9827,7 +9827,7 @@ static inline long sibling_imbalance(struct lb_env *env,
 	int ncores_busiest, ncores_local;
 	long imbalance;
 
-	if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
+	if (!env->idle || !busiest->sum_nr_running)
 		return 0;
 
 	ncores_busiest = sds->busiest->cores;
@@ -9927,8 +9927,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 				sgs->group_misfit_task_load = rq->misfit_task_load;
 				*sg_status |= SG_OVERLOAD;
 			}
-		} else if ((env->idle != CPU_NOT_IDLE) &&
-			   sched_reduced_capacity(rq, env->sd)) {
+		} else if (env->idle && sched_reduced_capacity(rq, env->sd)) {
 			/* Check for a task running on a CPU with reduced capacity */
 			if (sgs->group_misfit_task_load < load)
 				sgs->group_misfit_task_load = load;
@@ -9940,7 +9939,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 	sgs->group_weight = group->group_weight;
 
 	/* Check if dst CPU is idle and preferred to this group */
-	if (!local_group && env->idle != CPU_NOT_IDLE && sgs->sum_h_nr_running &&
+	if (!local_group && env->idle && sgs->sum_h_nr_running &&
 	    sched_group_asym(env, sgs, group))
 		sgs->group_asym_packing = 1;
 
@@ -10698,7 +10697,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 			 * waiting task in this overloaded busiest group. Let's
 			 * try to pull it.
 			 */
-			if (env->idle != CPU_NOT_IDLE && env->imbalance == 0) {
+			if (env->idle && env->imbalance == 0) {
 				env->migration_type = migrate_task;
 				env->imbalance = 1;
 			}
@@ -10913,7 +10912,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
 		goto force_balance;
 
 	if (busiest->group_type != group_overloaded) {
-		if (env->idle == CPU_NOT_IDLE) {
+		if (!env->idle) {
 			/*
 			 * If the busiest group is not overloaded (and as a
 			 * result the local one too) but this CPU is already
@@ -11121,7 +11120,7 @@ asym_active_balance(struct lb_env *env)
 	 * the lower priority @env::dst_cpu help it. Do not follow
 	 * CPU priority.
 	 */
-	return env->idle != CPU_NOT_IDLE && sched_use_asym_prio(env->sd, env->dst_cpu) &&
+	return env->idle && sched_use_asym_prio(env->sd, env->dst_cpu) &&
 	       (sched_asym_prefer(env->dst_cpu, env->src_cpu) ||
 		!sched_use_asym_prio(env->sd, env->src_cpu));
 }
@@ -11159,7 +11158,7 @@ static int need_active_balance(struct lb_env *env)
 	 * because of other sched_class or IRQs if more capacity stays
 	 * available on dst_cpu.
 	 */
-	if ((env->idle != CPU_NOT_IDLE) &&
+	if (env->idle &&
 	    (env->src_rq->cfs.h_nr_running == 1)) {
 		if ((check_cpu_capacity(env->src_rq, sd)) &&
 		    (capacity_of(env->src_cpu)*sd->imbalance_pct < capacity_of(env->dst_cpu)*100))
@@ -11735,8 +11734,8 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 				 * env->dst_cpu, so we can't know our idle
 				 * state even if we migrated tasks. Update it.
 				 */
-				idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;
-				busy = idle != CPU_IDLE && !sched_idle_cpu(cpu);
+				idle = idle_cpu(cpu);
+				busy = !idle && !sched_idle_cpu(cpu);
 			}
 			sd->last_balance = jiffies;
 			interval = get_sd_balance_interval(sd, busy);
@@ -12416,9 +12415,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
 {
 	struct rq *this_rq = this_rq();
-	enum cpu_idle_type idle = this_rq->idle_balance ?
-						CPU_IDLE : CPU_NOT_IDLE;
-
+	enum cpu_idle_type idle = this_rq->idle_balance;
 	/*
 	 * If this CPU has a pending nohz_balance_kick, then do the
 	 * balancing on behalf of the other idle CPUs whose ticks are