Re: suspend blockers & Android integration

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, 4 Jun 2010, Ingo Molnar wrote:
> 
> What you say is absolutely true, hence this would be driven via sched_tick() + 
> TIF notifiers - i.e. only ever treat user-mode tasks as 'idle-able'. This can 
> be done with no overhead to the regular fastpaths.
> 
> The TIF notifier would be the one scheduling to idle - and would thus do it 
> only to user-mode tasks.

The thing is, unless there is some _really_ deep other reason to do 
something like this, I still think it's total overdesign to push any 
knowledge/choices like this into the scheduler. I'd rather keep things way 
more independent, less tied to each other and to deep kernel subsystems.

IOW, my personal opinion is that somethng like a suspend (blocker or not) 
decision simply shouldn't be important enough to be tied into the 
scheduler. Especially not if it could just be its own layer.

That said, as far as I know, the Android people have mostly been looking 
at the suspend angle from a single-core standpoint. And I'm not at all 
convinced that they should hijack the existing "/sys/power/state" thing 
which is what I think they do now.

And those two things go together. The /sys/power/state thing is a global 
suspend - which I don't think is appropriate for a opportunistic thing in 
the first place, especially for multi-core.

A well-designed opportunistic suspend should be a two-phase thing: an 
opportunistc CPU hotunplug (shutting down cores one by one as the system 
is idle), and not a "global" event in the first place. And only when 
you've reached single-core state should you then say "do I suspend the 
system too".

So I've tried to look a bit at the patches, and my admittedly rough 
comments so far is

 - I really do prefer the "off to the side" approach that the current 
   google opportunistic suspend patches have. As mentioned, I don't think 
   this should be deep in the scheduler. Not at all.

 - I do think there are possibly races and CPU idle issues there, but I 
   think they are mainly for the multi-core thing. And I think that's a 
   totally separate issue. Or it _should_ be.

 - once you're single-core (whether because you never had more cores to 
   begin with, or because the "opportunistic CPU offlining" has taken down 
   the other cores), I think the suspend-blocker is fine as a concept, and 
   certainly shouldn't need any deep scheduler hooks.

so I'd like to see the opportunistc suspend thing think about CPU 
offlining, and I'd like to see it disconnect from the existing 
/sys/power/state. And I'd really not like to involved deep internal kernel 
hooks into it.

But I'll also admit that maybe I'm not seeing some problems. I've frankly 
tried to avoid the whole discussion until Andrew pulled me in yesterday.

			Linus