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

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> 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 ?

- 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 ?