linux-kernel - Re: [PATCH] sched/fair: Combine EAS check with overutilized access

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Date: Tue, 26 Mar 2024 20:32:17 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: peterz@...radead.org, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, qyousef@...alina.io,
	linux-kernel@...r.kernel.org, vschneid@...hat.com
Subject: Re: [PATCH] sched/fair: Combine EAS check with overutilized access


* Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:

> 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@...aro.org>
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.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);

On a second thought, I'm not sure removing the open-coded 
sched_energy_enabled() branches is a good idea: the current code makes it 
really, really clear when we are within EAS code paths.

Hiding it within is_rd_overutilized() makes it a lot less obvious IMO.

And this one:

> @@ -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;

Didn't have a root_domain::overutilized check before?

Thanks,

	Ingo