Re: [PATCH 0/8] Suspend block api (version 8)

Rafael J. Wysocki
2010-05-28T00:39:14Z

On Friday 28 May 2010, Alan Cox wrote:
> > That's correct, but to me the Arve's goal is simply to maximize battery life
> > and he found experimentally that the longest battery life is achieved if
> > system suspend is used whenever the system doesn't need to be active (from its
> > user's perspective). This actually is different from "when the system is
> > idle", because the system isn't idle, for example, when updatedb is running.
> > However, from the user's perspective the updatedb process doesn't really need
> > to run at this particular time, it can very well do it's job in parallel with
> > the user typing or reading news. So, the system may very well be suspended
> > when updatedb is running.
>
> This is where the original questions around QoS came in
>
> > Since I think we've now rejected the feature, do we have a clear picture about
> > what the Android people should do _instead_ and yet keep the battery life they
> > want? Because I don't think telling "let them do what they want, who cares"
> > is right.
>
> Today "idle" means "no task running"
>
> If you are prepared to rephrase that as "no task that matters is running"
> what would need to answer ?
>
> - How do we define who matters: QoS ?

That's reasonable IMO.

> - Can you describe "idle" in terms of QoS without then breaking the
> reliable wakeup for an event (and do you need to ?)
>
> Could this for example look like
>
> Set QoS of 'user apps' to QS_NONE
> Button pushed
> Button driver sets QoS of app it wakes to QS_ABOVESUSPEND
>
> That would I think solve the reliable wakeup case although
> drivers raising a QoS parameter is a bit unusual in the kernel.
> That would at least however be specific to a few Android drivers
> and maybe a tiny amount of shared driver stuff so probably not
> unacceptable. (wake_up_pri(&queue, priority); isn't going to
> kill anyone is it - especially if it usually ignores the
> priority argument)
>
> I am curious Thomas how that would tie in with PI in the RT
> world, it's effectively inheriting priority from the users finger.
>
> - Would a model where the UI side behaviour looked like
>
> Timeout
> Screen Off
> Set QoS of 'user apps' to QS_NONE
>
> Event
> [Some chain of activity]
> Screen On
> Set QoS of 'user apps' to QS_ABOVESUSPEND
>
> do the job combined with the ability to see who is stopping us dropping
> to suspend so user space can take action. This could be a data table
> from the Android cpu manager provided to Android specific policy in
> whoever owns the display.
>
>
> If so how do we fix the UI policy code doing
>
> Screen Off
> Button Press
> task to QS_ABOVESUSPEND
> task to QS_NONE
>
> without touching the app userspace code
>
>
> Perhaps
>
> count2 = tasks to QS_NONE | QS_NOTCHANGED
> Screen off
> Button Press
> task to QS_ABOVESUSPEND
> count = tasks that are QS_NOTCHANGED to QS_NONE
>
> if (count != count2) {
> Stuff happened ... rethink
> }
>
> That is still a bit weird and wonderful but all the logic is in the right
> places. The special magic remains in the Android policy code and in the
> kernel specifics for Android.
>
> Thoughts ?

Hmm. How do we prevent the "non-relevant" tasks from being scheduled
once we've decided to go for power saving?

Rafael