Re: [PATCH v6 3/3] sched/fair: Combine EAS check with overutilized access

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 3/26/24 1:56 PM, Vincent Guittot wrote:
> On Tue, 26 Mar 2024 at 08:58, Ingo Molnar <mingo@kernel.org> wrote:
>>
>>
>> * Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
>>
>>>  /*
>>> - * Ensure that caller can do EAS. overutilized value
>>> - * make sense only if EAS is enabled
>>> + * overutilized value make sense only if EAS is enabled
>>>   */
>>> -static inline int is_rd_overutilized(struct root_domain *rd)
>>> +static inline int is_rd_not_overutilized(struct root_domain *rd)
>>>  {
>>> -     return READ_ONCE(rd->overutilized);
>>> +     return sched_energy_enabled() && !READ_ONCE(rd->overutilized);
>>>  }
>>
>> While adding the sched_energy_enabled() condition looks OK, the _not prefix
>> This is silly: putting logical operators into functions names is far less
>> readable than a !fn()...
>>
>>> -     if (!is_rd_overutilized(rq->rd) && cpu_overutilized(rq->cpu))
>>> +     if (is_rd_not_overutilized(rq->rd) && cpu_overutilized(rq->cpu))
>>
>> Especially since we already have cpu_overutilized(). It's far more coherent
>> to have the same basic attribute functions and put any negation into
>> *actual* logical operators.
> 
> I was concerned by the || in this case that could defeat the purpose
> of sched_energy_enabled() but it will return early anyway
> 

> return !sched_energy_enabled() || READ_ONCE(rd->overutilized);

I think this would work. 

> 
>>
>> Thanks,
>>
>>         Ingo


If EAS - false, then is_rd_overutilized -> would be true always and all users of it do !is_rd_overutilized(). so No operation.
If EAS - true, it reads rd->overutilized value. 

Does this look correct?  
-------------------------------------------------------------------------------------
From 3adc0d58f87d8a2e96196a0f47bcd0d2afd057ae Mon Sep 17 00:00:00 2001
From: Shrikanth Hegde <sshegde@linux.ibm.com>
Date: Wed, 6 Mar 2024 03:58:58 -0500
Subject: [PATCH v7 3/3] sched/fair: Combine EAS check with overutilized access

Access to overutilized is always used with sched_energy_enabled in
the pattern:

if (sched_energy_enabled && !overutilized)
       do something

So modify the helper function to return this pattern. This is more
readable code as it would say, do something when root domain is not
overutilized. This function always return true when EAS is disabled.

No change in functionality intended.

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/fair.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 24a7530a7d3f..e222e3ad4cfe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6686,12 +6686,11 @@ static inline bool cpu_overutilized(int cpu)
 }

 /*
- * Ensure that caller can do EAS. overutilized value
- * make sense only if EAS is enabled
+ * overutilized value make sense only if EAS is enabled
  */
 static inline int is_rd_overutilized(struct root_domain *rd)
 {
-	return READ_ONCE(rd->overutilized);
+	return !sched_energy_enabled() || READ_ONCE(rd->overutilized);
 }

 static inline void set_rd_overutilized_status(struct root_domain *rd,
@@ -6710,8 +6709,6 @@ static inline void check_update_overutilized_status(struct rq *rq)
 	 * overutilized field is used for load balancing decisions only
 	 * if energy aware scheduler is being used
 	 */
-	if (!sched_energy_enabled())
-		return;

 	if (!is_rd_overutilized(rq->rd) && cpu_overutilized(rq->cpu))
 		set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
@@ -7999,7 +7996,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)

 	rcu_read_lock();
 	pd = rcu_dereference(rd->pd);
-	if (!pd || is_rd_overutilized(rd))
+	if (!pd)
 		goto unlock;

 	/*
@@ -8202,7 +8199,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		    cpumask_test_cpu(cpu, p->cpus_ptr))
 			return cpu;

-		if (sched_energy_enabled()) {
+		if (!is_rd_overutilized(this_rq()->rd)) {
 			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
 			if (new_cpu >= 0)
 				return new_cpu;
@@ -10903,12 +10900,9 @@ static struct sched_group *sched_balance_find_src_group(struct lb_env *env)
 	if (busiest->group_type == group_misfit_task)
 		goto force_balance;

-	if (sched_energy_enabled()) {
-		struct root_domain *rd = env->dst_rq->rd;
-
-		if (rcu_dereference(rd->pd) && !is_rd_overutilized(rd))
-			goto out_balanced;
-	}
+	if (!is_rd_overutilized(env->dst_rq->rd) &&
+	    rcu_dereference(env->dst_rq->rd->pd))
+		goto out_balanced;

 	/* ASYM feature bypasses nice load balance check */
 	if (busiest->group_type == group_asym_packing)
--
2.39.3