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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Thu, 28 Mar 2024 12:07:48 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: peterz@infradead.org, vincent.guittot@linaro.org,
	dietmar.eggemann@arm.com, qyousef@layalina.io,
	linux-kernel@vger.kernel.org, vschneid@redhat.com
Subject: Re: [PATCH v3 0/2] sched: Minor changes for rd->overload access
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* Ingo Molnar <mingo@kernel.org> wrote:

> Plus I've applied a patch to rename ::overload to ::overloaded. It is 
> silly to use an ambiguous noun instead of a clear adjective when naming 
> such a flag ...

Plus SG_OVERLOAD should be SG_OVERLOADED as well - it now looks in line 
with SG_OVERUTILIZED:

 /* Scheduling group status flags */
 #define SG_OVERLOADED           0x1 /* More than one runnable task on a CPU. */
 #define SG_OVERUTILIZED         0x2 /* One or more CPUs are over-utilized. */

My followup question is: why are these a bitmask, why not separate 
flags?

AFAICS we only ever set them separately:

 thule:~/tip> git grep SG_OVER kernel/sched/
 kernel/sched/fair.c:            set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
 kernel/sched/fair.c:                    *sg_status |= SG_OVERLOADED;
 kernel/sched/fair.c:                    *sg_status |= SG_OVERUTILIZED;
 kernel/sched/fair.c:                            *sg_status |= SG_OVERLOADED;
 kernel/sched/fair.c:            set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
 kernel/sched/fair.c:                                       sg_status & SG_OVERUTILIZED);
 kernel/sched/fair.c:    } else if (sg_status & SG_OVERUTILIZED) {
 kernel/sched/fair.c:            set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
 kernel/sched/sched.h:#define SG_OVERLOADED              0x1 /* More than one runnable task on a CPU. */
 kernel/sched/sched.h:#define SG_OVERUTILIZED            0x2 /* One or more CPUs are over-utilized. */
 kernel/sched/sched.h:           set_rd_overloaded(rq->rd, SG_OVERLOADED);

In fact this results in suboptimal code:

                /* update overload indicator if we are at root domain */
                set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
                        
                /* Update over-utilization (tipping point, U >= 0) indicator */
                set_rd_overutilized_status(env->dst_rq->rd,
                                           sg_status & SG_OVERUTILIZED);

Note how the bits that got mixed together in sg_status now have to be 
masked out individually.

The sg_status bitmask appears to make no sense at all to me.

By turning these into individual bool flags we could also do away with 
all the extra SG_OVERLOADED/SG_OVERUTILIZED abstraction.

Ie. something like the patch below? Untested.

Thanks,

	Ingo

=================>
From: Ingo Molnar <mingo@kernel.org>
Date: Thu, 28 Mar 2024 12:00:14 +0100
Subject: [PATCH] sched/balancing: Simplify the sg_status bitmask and use separate ->overloaded and ->overutilized flags

SG_OVERLOADED and SG_OVERUTILIZED flags plus the sg_status bitmask are an
unnecessary complication that only make the code harder to read and slower.

We only ever set them separately:

 thule:~/tip> git grep SG_OVER kernel/sched/
 kernel/sched/fair.c:            set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
 kernel/sched/fair.c:                    *sg_status |= SG_OVERLOADED;
 kernel/sched/fair.c:                    *sg_status |= SG_OVERUTILIZED;
 kernel/sched/fair.c:                            *sg_status |= SG_OVERLOADED;
 kernel/sched/fair.c:            set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
 kernel/sched/fair.c:                                       sg_status & SG_OVERUTILIZED);
 kernel/sched/fair.c:    } else if (sg_status & SG_OVERUTILIZED) {
 kernel/sched/fair.c:            set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
 kernel/sched/sched.h:#define SG_OVERLOADED              0x1 /* More than one runnable task on a CPU. */
 kernel/sched/sched.h:#define SG_OVERUTILIZED            0x2 /* One or more CPUs are over-utilized. */
 kernel/sched/sched.h:           set_rd_overloaded(rq->rd, SG_OVERLOADED);

And use them separately, which results in suboptimal code:

                /* update overload indicator if we are at root domain */
                set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);

                /* Update over-utilization (tipping point, U >= 0) indicator */
                set_rd_overutilized_status(env->dst_rq->rd,

Introduce separate sg_overloaded and sg_overutilized flags in update_sd_lb_stats()
and its lower level functions, and change all of them to 'bool'.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c  | 33 ++++++++++++++++-----------------
 kernel/sched/sched.h | 17 ++++++-----------
 2 files changed, 22 insertions(+), 28 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f29efd5f19f6..ebc8d5f855de 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6688,19 +6688,18 @@ static inline bool cpu_overutilized(int cpu)
 /*
  * overutilized value make sense only if EAS is enabled
  */
-static inline int is_rd_overutilized(struct root_domain *rd)
+static inline bool is_rd_overutilized(struct root_domain *rd)
 {
 	return !sched_energy_enabled() || READ_ONCE(rd->overutilized);
 }
 
-static inline void set_rd_overutilized(struct root_domain *rd,
-					      unsigned int status)
+static inline void set_rd_overutilized(struct root_domain *rd, bool flag)
 {
 	if (!sched_energy_enabled())
 		return;
 
-	WRITE_ONCE(rd->overutilized, status);
-	trace_sched_overutilized_tp(rd, !!status);
+	WRITE_ONCE(rd->overutilized, flag);
+	trace_sched_overutilized_tp(rd, flag);
 }
 
 static inline void check_update_overutilized_status(struct rq *rq)
@@ -6711,7 +6710,7 @@ static inline void check_update_overutilized_status(struct rq *rq)
 	 */
 
 	if (!is_rd_overutilized(rq->rd) && cpu_overutilized(rq->cpu))
-		set_rd_overutilized(rq->rd, SG_OVERUTILIZED);
+		set_rd_overutilized(rq->rd, 1);
 }
 #else
 static inline void check_update_overutilized_status(struct rq *rq) { }
@@ -9940,7 +9939,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 				      struct sd_lb_stats *sds,
 				      struct sched_group *group,
 				      struct sg_lb_stats *sgs,
-				      int *sg_status)
+				      bool *sg_overloaded,
+				      bool *sg_overutilized)
 {
 	int i, nr_running, local_group;
 
@@ -9961,10 +9961,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 		sgs->sum_nr_running += nr_running;
 
 		if (nr_running > 1)
-			*sg_status |= SG_OVERLOADED;
+			*sg_overloaded = 1;
 
 		if (cpu_overutilized(i))
-			*sg_status |= SG_OVERUTILIZED;
+			*sg_overutilized = 1;
 
 #ifdef CONFIG_NUMA_BALANCING
 		sgs->nr_numa_running += rq->nr_numa_running;
@@ -9986,7 +9986,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 			/* Check for a misfit task on the cpu */
 			if (sgs->group_misfit_task_load < rq->misfit_task_load) {
 				sgs->group_misfit_task_load = rq->misfit_task_load;
-				*sg_status |= SG_OVERLOADED;
+				*sg_overloaded = 1;
 			}
 		} else if (env->idle && sched_reduced_capacity(rq, env->sd)) {
 			/* Check for a task running on a CPU with reduced capacity */
@@ -10612,7 +10612,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 	struct sg_lb_stats *local = &sds->local_stat;
 	struct sg_lb_stats tmp_sgs;
 	unsigned long sum_util = 0;
-	int sg_status = 0;
+	bool sg_overloaded = 0, sg_overutilized = 0;
 
 	do {
 		struct sg_lb_stats *sgs = &tmp_sgs;
@@ -10628,7 +10628,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 				update_group_capacity(env->sd, env->dst_cpu);
 		}
 
-		update_sg_lb_stats(env, sds, sg, sgs, &sg_status);
+		update_sg_lb_stats(env, sds, sg, sgs, &sg_overloaded, &sg_overutilized);
 
 		if (!local_group && update_sd_pick_busiest(env, sds, sg, sgs)) {
 			sds->busiest = sg;
@@ -10657,13 +10657,12 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 
 	if (!env->sd->parent) {
 		/* update overload indicator if we are at root domain */
-		set_rd_overloaded(env->dst_rq->rd, sg_status & SG_OVERLOADED);
+		set_rd_overloaded(env->dst_rq->rd, sg_overloaded);
 
 		/* Update over-utilization (tipping point, U >= 0) indicator */
-		set_rd_overutilized(env->dst_rq->rd,
-					   sg_status & SG_OVERUTILIZED);
-	} else if (sg_status & SG_OVERUTILIZED) {
-		set_rd_overutilized(env->dst_rq->rd, SG_OVERUTILIZED);
+		set_rd_overutilized(env->dst_rq->rd, sg_overloaded);
+	} else if (sg_overutilized) {
+		set_rd_overutilized(env->dst_rq->rd, sg_overutilized);
 	}
 
 	update_idle_cpu_scan(env, sum_util);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 07c6669b8250..7c39dbf31f75 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -713,7 +713,7 @@ struct rt_rq {
 	} highest_prio;
 #endif
 #ifdef CONFIG_SMP
-	int			overloaded;
+	bool			overloaded;
 	struct plist_head	pushable_tasks;
 
 #endif /* CONFIG_SMP */
@@ -757,7 +757,7 @@ struct dl_rq {
 		u64		next;
 	} earliest_dl;
 
-	int			overloaded;
+	bool			overloaded;
 
 	/*
 	 * Tasks on this rq that can be pushed away. They are kept in
@@ -850,10 +850,6 @@ struct perf_domain {
 	struct rcu_head rcu;
 };
 
-/* Scheduling group status flags */
-#define SG_OVERLOADED		0x1 /* More than one runnable task on a CPU. */
-#define SG_OVERUTILIZED		0x2 /* One or more CPUs are over-utilized. */
-
 /*
  * We add the notion of a root-domain which will be used to define per-domain
  * variables. Each exclusive cpuset essentially defines an island domain by
@@ -874,10 +870,10 @@ struct root_domain {
 	 * - More than one runnable task
 	 * - Running task is misfit
 	 */
-	int			overloaded;
+	bool			overloaded;
 
 	/* Indicate one or more cpus over-utilized (tipping point) */
-	int			overutilized;
+	bool			overutilized;
 
 	/*
 	 * The bit corresponding to a CPU gets set here if such CPU has more
@@ -2540,9 +2536,8 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
 	}
 
 #ifdef CONFIG_SMP
-	if (prev_nr < 2 && rq->nr_running >= 2) {
-		set_rd_overloaded(rq->rd, SG_OVERLOADED);
-	}
+	if (prev_nr < 2 && rq->nr_running >= 2)
+		set_rd_overloaded(rq->rd, 1);
 #endif
 
 	sched_update_tick_dependency(rq);