linux-kernel - Re: [PATCH v4 3/3] PM/runtime:Replace jiffies based accounting with ktime based accounting

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Message-ID: <CAJZ5v0jp-+rG2usiumWmORTvqCYDJJbFUB+ey3wLyQUrimdTLw@mail.gmail.com>
Date:   Fri, 21 Dec 2018 09:58:18 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        rodrigo.vivi@...el.com, David Airlie <airlied@...ux.ie>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 3/3] PM/runtime:Replace jiffies based accounting with
 ktime based accounting

On Thu, Dec 20, 2018 at 11:03 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> On Thu, 20 Dec 2018 at 15:17, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > From: Thara Gopinath <thara.gopinath@...aro.org>
> >
> > This patch replaces jiffies based accounting for runtime_active_time
> > and runtime_suspended_time with ktime base accounting. This makes the
> > runtime debug counters inline with genpd and other pm subsytems which
> > uses ktime based accounting.
> >
> > Signed-off-by: Thara Gopinath <thara.gopinath@...aro.org>
> > [move from ktime to raw nsec]
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  drivers/base/power/runtime.c | 17 +++++++++--------
> >  drivers/base/power/sysfs.c   | 11 ++++++++---
> >  include/linux/pm.h           |  6 +++---
> >  3 files changed, 20 insertions(+), 14 deletions(-)
> >
> > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > index e695544..f700524 100644
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -64,8 +64,8 @@ static int rpm_suspend(struct device *dev, int rpmflags);
> >   */
> >  void update_pm_runtime_accounting(struct device *dev)
> >  {
> > -       unsigned long now = jiffies;
> > -       unsigned long delta;
> > +       u64 now = ktime_to_ns(ktime_get());
> > +       u64 delta;
> >
> >         delta = now - dev->power.accounting_timestamp;
> >
> > @@ -75,9 +75,9 @@ void update_pm_runtime_accounting(struct device *dev)
> >                 return;
> >
> >         if (dev->power.runtime_status == RPM_SUSPENDED)
> > -               dev->power.suspended_jiffies += delta;
> > +               dev->power.suspended_time += delta;
> >         else
> > -               dev->power.active_jiffies += delta;
> > +               dev->power.active_time += delta;
> >  }
> >
> >  static void __update_runtime_status(struct device *dev, enum rpm_status status)
> > @@ -88,17 +88,18 @@ static void __update_runtime_status(struct device *dev, enum rpm_status status)
> >
> >  u64 pm_runtime_suspended_time(struct device *dev)
> >  {
> > -       unsigned long flags, time;
> > +       u64 time;
> > +       unsigned long flags;
> >
> >         spin_lock_irqsave(&dev->power.lock, flags);
> >
> >         update_pm_runtime_accounting(dev);
> >
> > -       time = dev->power.suspended_jiffies;
> > +       time = dev->power.suspended_time;
> >
> >         spin_unlock_irqrestore(&dev->power.lock, flags);
> >
> > -       return jiffies_to_nsecs(time);
> > +       return time;
> >  }
> >  EXPORT_SYMBOL_GPL(pm_runtime_suspended_time);
> >
> > @@ -1503,7 +1504,7 @@ void pm_runtime_init(struct device *dev)
> >         dev->power.request_pending = false;
> >         dev->power.request = RPM_REQ_NONE;
> >         dev->power.deferred_resume = false;
> > -       dev->power.accounting_timestamp = jiffies;
> > +       dev->power.accounting_timestamp = ktime_to_ns(ktime_get());
>
> This change worries me a bit, but it may not be a problem in practice.
>
> pm_runtime_init() is called when devices get initialized, via
> device_initialize(). If timekeeping has not been initialized prior a
> call to ktime_get() is done, it will fail and causing a NULL pointer
> deference or something along those lines.
>
> In other words, do we know that device_initialize() is always called
> after timekeeping has been initialized during boot?

We do.

timekeeping_init() is called early in start_kernel() which is way
before driver_init() (and that's when devices can start to be
initialized) called from rest_init() via kernel_init_freeable() and
do_basic_setup().