Re: [PATCH] cpufreq/schedutil: Only bind threads if needed

[Rafael J. Wysocki]

On Thu, Sep 12, 2024 at 3:53 PM Christian Loehle
<christian.loehle@xxxxxxx> wrote:
>
> Remove the unconditional binding of sugov kthreads to the affected CPUs
> if the cpufreq driver indicates that updates can happen from any CPU.
> This allows userspace to set affinities to either save power (waking up
> bigger CPUs on HMP can be expensive) or increasing performance (by
> letting the utilized CPUs run without preemption of the sugov kthread).
>
> Without this patch the behavior of sugov threads will basically be a
> boot-time dice roll on which CPU of the PD has to handle all the
> cpufreq updates. With the recent decreases of update filtering these
> two basic problems become more and more apparent:
> 1. The wake_cpu might be idle and we are waking it up from another
> CPU just for the cpufreq update. Apart from wasting power, the exit
> latency of it's idle state might be longer than the sugov threads
> running time, essentially delaying the cpufreq update unnecessarily.
> 2. We are preempting either the requesting or another busy CPU of the
> PD, while the update could be done from a CPU that we deem less
> important and pay the price of an IPI and two context-switches.
>
> The change is essentially not setting PF_NO_SETAFFINITY on
> dvfs_possible_from_any_cpu, no behavior change if userspace doesn't
> touch affinities.

I'd like to hear from Viresh on this.

> Signed-off-by: Christian Loehle <christian.loehle@xxxxxxx>
> ---
>  kernel/sched/cpufreq_schedutil.c | 6 +++++-
>  kernel/sched/syscalls.c          | 3 +++
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 43111a515a28..466fb79e0b81 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -683,7 +683,11 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
>         }
>
>         sg_policy->thread = thread;
> -       kthread_bind_mask(thread, policy->related_cpus);
> +       if (policy->dvfs_possible_from_any_cpu)
> +               set_cpus_allowed_ptr(thread, policy->related_cpus);
> +       else
> +               kthread_bind_mask(thread, policy->related_cpus);
> +
>         init_irq_work(&sg_policy->irq_work, sugov_irq_work);
>         mutex_init(&sg_policy->work_lock);
>
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index c62acf509b74..7d4a4edfcfb9 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1159,6 +1159,9 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
>         if (!task_has_dl_policy(p) || !dl_bandwidth_enabled())
>                 return 0;
>
> +       if (dl_entity_is_special(&p->dl))
> +               return 0;
> +

Care to explain this particular piece?

>         /*
>          * Since bandwidth control happens on root_domain basis,
>          * if admission test is enabled, we only admit -deadline
> --