[linux-pm] Runtime PM discussion notes

[Paul Walmsley]

(cc'ing some lists)

Hi

a few thoughts here:

On Wed, 15 Jun 2011, Rafael J. Wysocki wrote:

> On Tuesday, June 14, 2011, Magnus Damm wrote:
>
> > As for freezing user space, yes, I agree. The other feature including
> > a different set of wakeup sources, not so sure why you would want to
> > have that. I can't think of any good use case for that, from my point
> > of view system-wide suspend is more like the limping cousin of the
> > full-blown kernel.
> 
> Well, the freezing of user space by itself doesn't buy you anything
> power management-wise, you pretty much need to do the other things too to
> really save energy this way. 

Freezing user space stops CPU-hogging processes from running.  When the 
runqueue is empty, the scheduler can go idle.  This in turn allows the 
CPU(s) to enter low power sleep states via CPUIdle.

As I understand it, this is really the basis for the modern Android 
use-case for wakelocks and opportunistic suspend.  They prevent 
non-power-privileged userspace applications from keeping the system from 
entering a low-power state.  (Secondarily, suspend also prevents kernel 
code from running, e.g., timers set by filesystems, the networking stack, 
etc.; but problems here should be fixed in the offending kernel code, 
rather than hacked around, since some of the users of those timers could 
be important)

> Anyway, the value of system suspend is to allow the user to tell the system
> "OK, now I'm not going to use you for a while and I don't want the USB mouse
> to wake you up", so that the system can go into a state in which it draws
> minimum energy.
> 
> Of course, you can argue that the system may detect when it's not used and
> put itself into that state, which is somewhat correct, but the system can
> _never_ know two things by itself: (1) that it's not going to be used _in_
> _advance_

But suspend users don't know this either, since they can't predict when 
the next external wakeup can happen.

Having some kind of indication that the user is done using a device for 
the time being is definitely useful.  But that's separate from system 
suspend.  System suspend is just one of many actions that the system might 
choose to take when the user gives that indication.

Consider something like a screensaver.  Usually, the longer latency the 
wakeup operation is, the longer the screensaver waits before deciding to, 
say, power the monitor off, or put the system into suspend (as we 
discussed that Mac OS X does at LinuxCon Boston).  Similarly, if there is 
some background task that really needs to run, an intelligent system will 
not allow a power economization mode to prevent that from happening.

> and (2) that it is supposed to ignore wakeup signals from certain
> sources. 

Isn't this configurable now without system suspend with the wakeup sources 
code?

> As I said, support for system suspend allows the user to do more with the
> system and you really can't implement that functionality differently.  You
> may choose not to implement it at all, but why should you?

The individual components of system suspend are definitely useful:

1. preventing userspace processes from keeping the system from entering 
   idle
2. preventing userspace processes from waking the system back up
3. preventing kernel code from keeping the system from entering idle
4. preventing kernel code from waking the system back up
5. requesting that drivers abort what they are currently doing

The main issues are that these:

1. are all bundled up into one operation

2. target the entire universe of entities that they manage, rather than 
selectively targeting non-power-privileged entities


- Paul