Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Tue Aug 03 2010 - 00:46:35 EST

On Mon, 2 Aug 2010, Arve Hj?nnev?g wrote:

On Mon, Aug 2, 2010 at 5:08 PM, <david@xxxxxxx> wrote:
On Mon, 2 Aug 2010, Paul E. McKenney wrote:

you are close, but I think what I'm proposing is actually simpler (assuming
that the scheduler can be configured to generate the appropriate stats)

my thought was not to move applications between cgroups as they
aquire/release the suspend-block lock, bur rather to say that any
application that you would trust to get the suspend-block lock should be in
cgroup A while all other applications are in cgroup B

when you are deciding if the system shoudl go to sleep because it is idle,
ignore the activity of all applications in cgroup B

if cgroup A applications are busy, the system is not idle and should not

Triggering suspend from idle has been suggested before. However, idle
is not a signal that it is safe to suspend since timers stop in
suspend (or the code could temporarily be waiting on a non-wakeup
interrupt). If you add suspend blockers or wakelocks to prevent
suspend while events you care about are pending, then it does not make
a lot of sense to prevent suspend just because the cpu is not idle.

isn't this a matter of making the suspend decision look at what timers have been set to expire in the near future and/or tweaking how long the system needs to be idle before going to sleep?

to properly do a good job at suspending hyperagressivly you need to look at when you need to wake back up (after all, if you are only going to sleep for 1 second, it makes no sense to try and enter a sleep state that takes .5 seconds to get into and .6 seconds to get out of, you need to pick a lighter power saving mode)

if you really want to have the application be able to say 'I am ready for you to go to sleep NOW and I don't want any chance of waking back up until the system is ready for me to do so' it may be possible to have a special dev file that when a program attempts to read from it the program is considered to have been idle forever (expiring any 'delay since last activity' timers that are running for that process) and when the device wakes back up (or after a little bit of time if the device decides not to go back to sleep), allow the return from the blocking call to allow the app to continue running.

but I am not sure that something like that is needed. I think just checking for timers that are due to expire 'soon' and tweaking how long the system must be 'idle' before it decides to go to sleep should be good enough.

David Lang
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