Re: [PATCH RFC v1 2/8] kernel/cpu_pm: Manage runtime PM in the idle path for CPUs

[Lina Iyer]

On Thu, Oct 11 2018 at 14:56 -0600, Rafael J. Wysocki wrote:
> On Wednesday, October 10, 2018 11:20:49 PM CEST Raju P.L.S.S.S.N wrote:
> > From: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxx>
> > Co-developed-by: Lina Iyer <lina.iyer@xxxxxxxxxx>
> > Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> > Signed-off-by: Raju P.L.S.S.S.N <rplsssn@xxxxxxxxxxxxxx>
> > (am from https://patchwork.kernel.org/patch/10478153/)
> > ---
> >  kernel/cpu_pm.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/kernel/cpu_pm.c b/kernel/cpu_pm.c
> > index 67b02e1..492d4a8 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)
> >  {
> >  	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);
>
> This may cause a power domain to go off, but if it goes off, then the idle
> governor has already selected idle states for all of the CPUs in that domain.
>
> Is there any way to ensure that turning the domain off (and later on) will
> no cause the target residency and exit latency expectations for those idle
> states to be exceeded?
Good point.

The cluster states should account for that additional latency. Just the
CPU's power down states need not care about that.

But, it would be nice if the PM domain governor could be cognizant of
the idle state chosen for each CPU, that way we dont configure the
domain to be powered off when the CPUs have just chosen to power down
(not chosen a cluster state). I think that is a whole different topic to
discuss.

-- Lina

> > +
> >  	return ret;
> >  }
> >  EXPORT_SYMBOL_GPL(cpu_pm_enter);
> > @@ -118,6 +124,11 @@ int cpu_pm_enter(void)
> >   */
> >  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);