Re: [linux-pm] suspend blockers & Android integration

["Arve Hjønnevåg" <arve@android.com>]

2010/6/6 Alan Stern <stern@rowland.harvard.edu>:
> On Sat, 5 Jun 2010, Alan Stern wrote:
>
>> > If you are referring to the approach that we don't use suspend but
>> > freeze a cgroup instead, this only solves the problem of bad apps. It
>> > does not help pause timers in trusted user space code and in the
>> > kernel, so it does not lower our average power consumption.
>>
>> You can solve this problem if you restructure your "trusted" apps in
>> the right way.  Require a trusted app to guarantee that whenever it
>> doesn't hold any suspend blockers, it will do nothing but wait (in a
>> poll() system call for example) for a wakeup event.  When the event
>> occurs, it must then activate a suspend blocker.
>>
>> Better yet, make it more fine-grained.  Instead of trusted apps, have
>> trusted threads.  Freeze the untrusted threads along with everything
>> else, and require the trusted threads to satisfy this guarantee.
>>
>> In this way, while the system is idle no user timers will get renewed.
>> Kernel timers are another matter, but we should be able to handle them.
>> There's nothing Android-specific about wanting to reduce kernel timer
>> wakeups while in a low-power mode.
>
> In fact it's possible to do this with only minimal changes to the
> userspace, providing you can specify all your possible hardware wakeup
> sources.  (On the Android this list probably isn't very large -- I
> imagine it includes the keypad, the radio link(s), the RTC, and maybe
> a few switches, buttons, or other things.)
>
> Here's how you can do it.  Extend the userspace suspend-blocker API, so
> that each suspend blocker can optionally have an associated wakeup
> source.
>
> The power-manager process should keep a list of "active" wakeup
> sources.  A source gets removed from the list when an associated
> suspend blocker is activated.
>

How do you do this safely? If you remove the active wakeup only when
activating the suspend blocker, you will never unblock suspend if
another wakeup event happens after user-space blocked suspend but
before user-space read the events.

Also, I'm not sure we can easily associate a wakeup event with a user
space suspend blocker. For instance when an alarm triggers it is
sometimes because of a user-space alarm and sometimes because an
in-kernel alarm.

> When the "active" list is empty and no suspend blockers are activated,
> the power manager freezes ALL other processes, trusted and untrusted
> alike.  It then does a big poll() on all the wakeup sources.  When the
> poll() returns, its output is used to repopulate the "active" list and
> processes are unfrozen.
>
> (You can also include some error detection: If a source remains on the
> "active" list for too long then something has gone wrong.)
>
> To do all this you don't even need to use cgroups.  The existing PM
> implementation allows a user process to freeze everything but itself;
> that's how swsusp and related programs work.
>
> This is still a big-hammer sort of approach, but it doesn't require any
> kernel changes.
>
> Alan Stern
>
>



-- 
Arve Hjønnevåg