Re: [PATCH][RFC v2] ACPI: acpi_pad: Do not launch acpi_pad threads on idle cpus

[Rafael J. Wysocki]

On Mon, May 14, 2018 at 5:45 PM, Yu Chen <yu.c.chen@xxxxxxxxx> wrote:
> On Sun, May 13, 2018 at 11:30:52AM +0200, Rafael J. Wysocki wrote:
>> On Saturday, May 5, 2018 1:53:22 PM CEST Chen Yu wrote:
>> > According to current implementation of acpi_pad driver,
>> > it does not make sense to spawn any power saving threads
>> > on the cpus which are already idle - it might bring
>> > unnecessary overhead on these idle cpus and causes power
>> > waste. So verify the condition that if the number of 'busy'
>> > cpus exceeds the amount of the 'forced idle' cpus is met.
>> > This is applicable due to round-robin attribute of the
>> > power saving threads, otherwise ignore the setting/ACPI
>> > notification.
>>
>> OK, but CPUs are busy, because they are running tasks.  If acpi_pad
>> kthreads run on them, the tasks they are running will migrate to the
>> currently idle CPUs (unless they have specific CPU affinity) and the
>> throttling will not really be effective.
>>
> OK, I think this makes sense, I missed the load balance scenario.
>
>> I would think that acpi_pad should ensure that the requested number of
>> CPUs will not run anything other than throttling kthreads.  Isn't that
>> the case?
>>
> Do you mean, we should check if the number of 'idle'(rather than the 'busy' one
> in this patch) cpus is larger than the requested one?

What I really meant was that acpi_pad kthreads should be started on as
many CPUs as requested by the firmware to prevent anything else from
running on them.

> Then I think we should also add a patch to use the play_idle() as power_clamp to treat
> the throttling kthreads as idle threads thus to stop system tick.

Yes, we should.

> Such as the patch Jacob proposed:

Looks OK from a quick glance, but I'll have a deeper look at it tomorrow.

> Index: linux/drivers/acpi/acpi_pad.c
> ===================================================================
> --- linux.orig/drivers/acpi/acpi_pad.c
> +++ linux/drivers/acpi/acpi_pad.c
> @@ -144,7 +144,6 @@ static unsigned int round_robin_time = 1
>  static int power_saving_thread(void *data)
>  {
>         struct sched_param param = {.sched_priority = 1};
> -       int do_sleep;
>         unsigned int tsk_index = (unsigned long)data;
>         u64 last_jiffies = 0;
>
> @@ -160,33 +159,13 @@ static int power_saving_thread(void *dat
>                         round_robin_cpu(tsk_index);
>                 }
>
> -               do_sleep = 0;
> -
> -               expire_time = jiffies + HZ * (100 - idle_pct) / 100;
> -
> -               while (!need_resched()) {
> -                       if (tsc_detected_unstable && !tsc_marked_unstable) {
> -                               /* TSC could halt in idle, so notify users */
> -                               mark_tsc_unstable("TSC halts in idle");
> -                               tsc_marked_unstable = 1;
> -                       }
> -                       local_irq_disable();
> -                       tick_broadcast_enable();
> -                       tick_broadcast_enter();
> -                       stop_critical_timings();
> -
> -                       mwait_idle_with_hints(power_saving_mwait_eax, 1);
> -
> -                       start_critical_timings();
> -                       tick_broadcast_exit();
> -                       local_irq_enable();
> -
> -                       if (time_before(expire_time, jiffies)) {
> -                               do_sleep = 1;
> -                               break;
> -                       }
> +               if (tsc_detected_unstable && !tsc_marked_unstable) {
> +                       /* TSC could halt in idle, so notify users */
> +                       mark_tsc_unstable("TSC halts in idle");
> +                       tsc_marked_unstable = 1;
>                 }
>
> +               play_idle(jiffies_to_msecs(HZ * (100 - idle_pct) / 100));
>                 /*
>                  * current sched_rt has threshold for rt task running time.
>                  * When a rt task uses 95% CPU time, the rt thread will be
> @@ -196,8 +175,7 @@ static int power_saving_thread(void *dat
>                  * borrow CPU time from this CPU and cause RT task use > 95%
>                  * CPU time. To make 'avoid starvation' work, takes a nap here.
>                  */
> -               if (unlikely(do_sleep))
> -                       schedule_timeout_killable(HZ * idle_pct / 100);
> +               schedule_timeout_killable(HZ * idle_pct / 100);
>
>                 /* If an external event has set the need_resched flag, then
>                  * we need to deal with it, or this loop will continue to
>> Thanks,
>> Rafael
>>
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