Re: [PATCH 2/2] PM-runtime: allow userspace to monitor runtime_status changes

[Akinobu Mita <akinobu.mita@gmail.com>]

2019年9月3日(火) 6:47 Rafael J. Wysocki <rafael@kernel.org>:
>
> On Sun, Sep 1, 2019 at 4:09 PM Akinobu Mita <akinobu.mita@gmail.com> wrote:
> >
> > This enables the /sys/devices/.../power/runtime_status attribute to
> > allow the user space to get notifications via poll/select when the device
> > runtime PM status is changed.
> >
> > An example use case is to avoid unnecessary accesses for device statistics
> > (e.g. diskstats for block devices) while the device is in runtime suspend
> > by user space LED device actitity trigger.
> >
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> > ---
> >  Documentation/ABI/testing/sysfs-devices-power | 2 ++
> >  drivers/base/power/power.h                    | 1 +
> >  drivers/base/power/runtime.c                  | 1 +
> >  drivers/base/power/sysfs.c                    | 5 +++++
> >  4 files changed, 9 insertions(+)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
> > index 3e50536..47dc357 100644
> > --- a/Documentation/ABI/testing/sysfs-devices-power
> > +++ b/Documentation/ABI/testing/sysfs-devices-power
> > @@ -269,3 +269,5 @@ Description:
> >                 the current runtime PM status of the device, which may be
> >                 "suspended", "suspending", "resuming", "active", "error" (fatal
> >                 error), or "unsupported" (runtime PM is disabled).
> > +               This attribute allows the user space to get notifications via
> > +               poll/select when the device runtime PM status is changed.
> > diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
> > index ec33fbdb..8891bf4 100644
> > --- a/drivers/base/power/power.h
> > +++ b/drivers/base/power/power.h
> > @@ -74,6 +74,7 @@ extern int pm_qos_sysfs_add_flags(struct device *dev);
> >  extern void pm_qos_sysfs_remove_flags(struct device *dev);
> >  extern int pm_qos_sysfs_add_latency_tolerance(struct device *dev);
> >  extern void pm_qos_sysfs_remove_latency_tolerance(struct device *dev);
> > +extern void sysfs_notify_runtime_status(struct device *dev);
> >
> >  #else /* CONFIG_PM */
> >
> > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > index b753355..3a3e413 100644
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -94,6 +94,7 @@ static void __update_runtime_status(struct device *dev, enum rpm_status status)
> >  {
> >         update_pm_runtime_accounting(dev);
> >         dev->power.runtime_status = status;
> > +       sysfs_notify_runtime_status(dev);
>
> There are concerns about this.
>
> First off, it adds overhead for devices that change the PM-runtime
> status relatively often.  I'm not sure if that's sufficiently
> justified.
>
> Second, it is called for status changes from "active" to "suspending"
> and from "suspending" to "suspended" (and analogously for resume)
> which may not be particularly useful.  At least, user space may not
> have enough time to act on such notifications.
>
> Finally, it is racy, because at the time user space does something on
> a device PM-runtime status change, it very well may have changed the
> other way around already.

I withdraw this patch now.  I hope I'll retry with a real use case example
program.