Re: [PATCH v10 04/27] PM / Domains: Add genpd governor for CPUs

[Ulf Hansson]

On Wed, 19 Dec 2018 at 10:54, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> wrote:
>
> On 29/11/2018 18:46, Ulf Hansson wrote:
> > As it's now perfectly possible that a PM domain managed by genpd contains
> > devices belonging to CPUs, we should start to take into account the
> > residency values for the idle states during the state selection process.
> > The residency value specifies the minimum duration of time, the CPU or a
> > group of CPUs, needs to spend in an idle state to not waste energy entering
> > it.
> >
> > To deal with this, let's add a new genpd governor, pm_domain_cpu_gov, that
> > may be used for a PM domain that have CPU devices attached or if the CPUs
> > are attached through subdomains.
> >
> > The new governor computes the minimum expected idle duration time for the
> > online CPUs being attached to the PM domain and its subdomains. Then in the
> > state selection process, trying the deepest state first, it verifies that
> > the idle duration time satisfies the state's residency value.
> >
> > It should be noted that, when computing the minimum expected idle duration
> > time, we use the information from tick_nohz_get_next_wakeup(), to find the
> > next wakeup for the related CPUs. Future wise, this may deserve to be
> > improved, as there are more reasons to why a CPU may be woken up from idle.
> >
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > Cc: Lina Iyer <ilina@xxxxxxxxxxxxxx>
> > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Co-developed-by: Lina Iyer <lina.iyer@xxxxxxxxxx>
> > Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> > ---
> >
> > Changes in v10:
> >       - Fold in patch that extended the new genpd CPU governor to cope with
> >         QoS constraints, as to avoid confusion.
> >       - Simplified the code according to suggestions from Rafael.
> >
> > ---
> >  drivers/base/power/domain_governor.c | 61 +++++++++++++++++++++++++++-
> >  include/linux/pm_domain.h            |  3 ++
> >  2 files changed, 63 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/base/power/domain_governor.c b/drivers/base/power/domain_governor.c
> > index 99896fbf18e4..61a7c3c03c98 100644
> > --- a/drivers/base/power/domain_governor.c
> > +++ b/drivers/base/power/domain_governor.c
> > @@ -10,6 +10,9 @@
> >  #include <linux/pm_domain.h>
> >  #include <linux/pm_qos.h>
> >  #include <linux/hrtimer.h>
> > +#include <linux/cpumask.h>
> > +#include <linux/ktime.h>
> > +#include <linux/tick.h>
> >
> >  static int dev_update_qos_constraint(struct device *dev, void *data)
> >  {
> > @@ -211,8 +214,10 @@ static bool default_power_down_ok(struct dev_pm_domain *pd)
> >       struct generic_pm_domain *genpd = pd_to_genpd(pd);
> >       struct gpd_link *link;
> >
> > -     if (!genpd->max_off_time_changed)
> > +     if (!genpd->max_off_time_changed) {
> > +             genpd->state_idx = genpd->cached_power_down_state_idx;
> >               return genpd->cached_power_down_ok;
> > +     }
> >
> >       /*
> >        * We have to invalidate the cached results for the masters, so
> > @@ -237,6 +242,7 @@ static bool default_power_down_ok(struct dev_pm_domain *pd)
> >               genpd->state_idx--;
> >       }
> >
> > +     genpd->cached_power_down_state_idx = genpd->state_idx;
> >       return genpd->cached_power_down_ok;
> >  }
> >
> > @@ -245,6 +251,54 @@ static bool always_on_power_down_ok(struct dev_pm_domain *domain)
> >       return false;
> >  }
> >
> > +static bool cpu_power_down_ok(struct dev_pm_domain *pd)
> > +{
> > +     struct generic_pm_domain *genpd = pd_to_genpd(pd);
> > +     ktime_t domain_wakeup, cpu_wakeup;
> > +     s64 idle_duration_ns;
> > +     int cpu, i;
> > +
> > +     /* Validate dev PM QoS constraints. */
> > +     if (!default_power_down_ok(pd))
> > +             return false;
> > +
> > +     if (!(genpd->flags & GENPD_FLAG_CPU_DOMAIN))
> > +             return true;
>
> Is it possible to have this function called without the
> GENPD_FLAG_CPU_DOMAIN flag set in the genpd?

Theoretically yes, however in practice, probably not.

Do note, if the GENPD_FLAG_CPU_DOMAIN isn't set, then we haven't
allocated the cpumask for the genpd, so then we shouldn't use it.

>
> > +     /*
> > +      * Find the next wakeup for any of the online CPUs within the PM domain
> > +      * and its subdomains. Note, we only need the genpd->cpus, as it already
> > +      * contains a mask of all CPUs from subdomains.
> > +      */
> > +     domain_wakeup = ktime_set(KTIME_SEC_MAX, 0);
> > +     for_each_cpu_and(cpu, genpd->cpus, cpu_online_mask) {
> > +             cpu_wakeup = tick_nohz_get_next_wakeup(cpu);
> > +             if (ktime_before(cpu_wakeup, domain_wakeup))
> > +                     domain_wakeup = cpu_wakeup;
> > +     }
> > +

[...]

Kind regards
Uffe