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

Alan Cox
2010-05-27T23:44:22Z

> 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 ?