linux-kernel - Re: [PATCH 1/1] sched/fair: allow disabling newidle_balance with sched_relax_domain

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Date: Thu, 28 Mar 2024 15:43:01 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Vitalii Bursov <vitaly@...sov.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched/fair: allow disabling newidle_balance with sched_relax_domain_level

On Thu, 28 Mar 2024 at 01:31, Vitalii Bursov <vitaly@...sov.com> wrote:
>
> Change relax_domain_level checks so that it would be possible
> to exclude all domains from newidle balancing.
>
> This matches the behavior described in the documentation:
>   -1   no request. use system default or follow request of others.
>    0   no search.
>    1   search siblings (hyperthreads in a core).
>
> "2" enables levels 0 and 1, level_max excludes the last (level_max)
> level, and level_max+1 includes all levels.

I was about to say that max+1 is useless because it's the same as -1
but it's not exactly the same because it can supersede the system wide
default_relax_domain_level. I wonder if one should be able to enable
more levels than what the system has set by default.

>
> Signed-off-by: Vitalii Bursov <vitaly@...sov.com>
> ---
>  kernel/cgroup/cpuset.c  | 2 +-
>  kernel/sched/debug.c    | 1 +
>  kernel/sched/topology.c | 2 +-
>  3 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 4237c874871..da24187c4e0 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2948,7 +2948,7 @@ bool current_cpuset_is_being_rebound(void)
>  static int update_relax_domain_level(struct cpuset *cs, s64 val)
>  {
>  #ifdef CONFIG_SMP
> -       if (val < -1 || val >= sched_domain_level_max)
> +       if (val < -1 || val > sched_domain_level_max + 1)
>                 return -EINVAL;
>  #endif
>
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 8d5d98a5834..8454cd4e5e1 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -423,6 +423,7 @@ static void register_sd(struct sched_domain *sd, struct dentry *parent)
>
>  #undef SDM
>
> +       debugfs_create_u32("level", 0444, parent, (u32 *)&sd->level);

IMO, this should be a separate patch as it's not part of the fix

>         debugfs_create_file("flags", 0444, parent, &sd->flags, &sd_flags_fops);
>         debugfs_create_file("groups_flags", 0444, parent, &sd->groups->flags, &sd_flags_fops);
>  }
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 99ea5986038..3127c9b30af 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1468,7 +1468,7 @@ static void set_domain_attribute(struct sched_domain *sd,
>         } else
>                 request = attr->relax_domain_level;
>
> -       if (sd->level > request) {
> +       if (sd->level >= request) {

good catch and worth :
Fixes: 9ae7ab20b483 ("sched/topology: Don't set SD_BALANCE_WAKE on
cpuset domain relax")


>                 /* Turn off idle balance on this domain: */
>                 sd->flags &= ~(SD_BALANCE_WAKE|SD_BALANCE_NEWIDLE);
>         }
> --
> 2.20.1
>