linux-kernel - Re: [PATCH v2 3/3] sched/fair: Simplify some logic in update_sd_pick

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Date: Sun, 4 Feb 2024 12:48:11 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: David Vernet <void@...ifault.com>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, peterz@...radead.org, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, 
	kernel-team@...a.com
Subject: Re: [PATCH v2 3/3] sched/fair: Simplify some logic in update_sd_pick_busiest()

On Sun, 4 Feb 2024 at 05:46, David Vernet <void@...ifault.com> wrote:
>
> When comparing the current struct sched_group with the yet-busiest
> domain in update_sd_pick_busiest(), if the two groups have the same
> group type, we're currently doing a bit of unnecessary work for any
> group >= group_misfit_task. We're comparing the two groups, and then
> returning only if false (the group in question is not the busiest).
> Othewise, we break, do an extra unnecessary conditional check that's
> vacuously false for any group type > group_fully_busy, and then always
> return true.
>
> Let's just return directly in the switch statement instead. This doesn't
> change the size of vmlinux with llvm 17 (not surprising given that all
> of this is inlined in load_balance()), but it does shrink load_balance()
> by 88 bytes on x86. Given that it also improves readability, this seems
> worth doing.
>
> As a bonus, remove an unnecessary goto in update_sd_lb_stats().

The line above is not relevant to the content of the patch.

Other than that

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

>
> Reviewed-by: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: David Vernet <void@...ifault.com>
> ---
>  kernel/sched/fair.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 76d03106040d..fa049f866461 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10006,9 +10006,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>         switch (sgs->group_type) {
>         case group_overloaded:
>                 /* Select the overloaded group with highest avg_load. */
> -               if (sgs->avg_load <= busiest->avg_load)
> -                       return false;
> -               break;
> +               return sgs->avg_load > busiest->avg_load;
>
>         case group_imbalanced:
>                 /*
> @@ -10019,18 +10017,14 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>
>         case group_asym_packing:
>                 /* Prefer to move from lowest priority CPU's work */
> -               if (sched_asym_prefer(sg->asym_prefer_cpu, sds->busiest->asym_prefer_cpu))
> -                       return false;
> -               break;
> +               return sched_asym_prefer(sds->busiest->asym_prefer_cpu, sg->asym_prefer_cpu);
>
>         case group_misfit_task:
>                 /*
>                  * If we have more than one misfit sg go with the biggest
>                  * misfit.
>                  */
> -               if (sgs->group_misfit_task_load <= busiest->group_misfit_task_load)
> -                       return false;
> -               break;
> +               return sgs->group_misfit_task_load > busiest->group_misfit_task_load;
>
>         case group_smt_balance:
>                 /*
> --
> 2.43.0
>