linux-kernel - Re: [PATCH v8 09/26] kernel/cpu_pm: Manage runtime PM in the idle path for CPUs

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Date:   Wed, 18 Jul 2018 12:11:06 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>, linux-pm@...r.kernel.org,
        Kevin Hilman <khilman@...nel.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Lina Iyer <lina.iyer@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Juri Lelli <juri.lelli@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 09/26] kernel/cpu_pm: Manage runtime PM in the idle path for CPUs

On Wednesday, June 20, 2018 7:22:09 PM CEST Ulf Hansson wrote:
> To allow CPUs being power managed by PM domains, let's deploy support for
> runtime PM for the CPU's corresponding struct device.
> 
> More precisely, at the point when the CPU is about to enter an idle state,
> decrease the runtime PM usage count for its corresponding struct device,
> via calling pm_runtime_put_sync_suspend(). Then, at the point when the CPU
> resumes from idle, let's increase the runtime PM usage count, via calling
> pm_runtime_get_sync().
> 
> Cc: Lina Iyer <ilina@...eaurora.org>
> Co-developed-by: Lina Iyer <lina.iyer@...aro.org>
> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>

I finally got to this one, sorry for the huge delay.

Let me confirm that I understand the code flow correctly.

> ---
>  kernel/cpu_pm.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/cpu_pm.c b/kernel/cpu_pm.c
> index 67b02e138a47..492d4a83dca0 100644
> --- a/kernel/cpu_pm.c
> +++ b/kernel/cpu_pm.c
> @@ -16,9 +16,11 @@
>   */
>  
>  #include <linux/kernel.h>
> +#include <linux/cpu.h>
>  #include <linux/cpu_pm.h>
>  #include <linux/module.h>
>  #include <linux/notifier.h>
> +#include <linux/pm_runtime.h>
>  #include <linux/spinlock.h>
>  #include <linux/syscore_ops.h>
>  
> @@ -91,6 +93,7 @@ int cpu_pm_enter(void)

This is called from a cpuidle driver's ->enter callback for the target state
selected by the idle governor ->

>  {
>  	int nr_calls;
>  	int ret = 0;
> +	struct device *dev = get_cpu_device(smp_processor_id());
>  
>  	ret = cpu_pm_notify(CPU_PM_ENTER, -1, &nr_calls);
>  	if (ret)
> @@ -100,6 +103,9 @@ int cpu_pm_enter(void)
>  		 */
>  		cpu_pm_notify(CPU_PM_ENTER_FAILED, nr_calls - 1, NULL);
>  
> +	if (!ret && dev && dev->pm_domain)
> +		pm_runtime_put_sync_suspend(dev);

-> so this is going to invoke genpd_runtime_suspend() if the usage
counter of dev is 0.

That will cause cpu_power_down_ok() to be called (because this is
a CPU domain) and that will walk the domain cpumask and compute the
estimated idle duration as the minimum of tick_nohz_get_next_wakeup()
values over the CPUs in that cpumask.  [Note that the weight of the
cpumask must be seriously limited for that to actually work, as this
happens in the idle path.]  Next, it will return "true" if it can
find a domain state with residency within the estimated idle
duration.  [Note that this sort of overlaps with the idle governor's
job.]

Next, __genpd_runtime_suspend() will be invoked to run the device-specific
callback if any [Note that this has to be suitable for the idle path if
present.] and genpd_stop_dev() runs (which, again, may invoke a callback)
and genpd_power_off() runs under the domain lock (which must be a spinlock
then).

> +
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(cpu_pm_enter);
> @@ -118,6 +124,11 @@ EXPORT_SYMBOL_GPL(cpu_pm_enter);
>   */
>  int cpu_pm_exit(void)
>  {
> +	struct device *dev = get_cpu_device(smp_processor_id());
> +
> +	if (dev && dev->pm_domain)
> +		pm_runtime_get_sync(dev);
> +
>  	return cpu_pm_notify(CPU_PM_EXIT, -1, NULL);
>  }
>  EXPORT_SYMBOL_GPL(cpu_pm_exit);
> 

And this is called on wakeup when the cpuidle driver's ->enter callback
is about to return and it reverses the suspend flow (except that the
governor doesn't need to be called now).

Have I got that right?