linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Message-ID: <alpine.DEB.2.00.1008031611100.6545@asgard.lang.hm>
Date:	Tue, 3 Aug 2010 16:19:25 -0700 (PDT)
From:	david@...g.hm
To:	Arve Hjønnevåg <arve@...roid.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Ted Ts'o" <tytso@....edu>, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, mjg59@...f.ucam.org, pavel@....cz,
	florian@...kler.org, rjw@...k.pl, stern@...land.harvard.edu,
	swetland@...gle.com, peterz@...radead.org, tglx@...utronix.de,
	alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Tue, 3 Aug 2010, Arve Hj?nnev?g wrote:

> 2010/8/2  <david@...g.hm>:
>> On Mon, 2 Aug 2010, Arve Hj?nnev?g wrote:
>>
>>> 2010/8/2  <david@...g.hm>:
>>>>
>>>> On Mon, 2 Aug 2010, Arve Hj?nnev?g wrote:
>>>>
>>>>> On Mon, Aug 2, 2010 at 5:08 PM,  <david@...g.hm> 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
>>>>>> suspend.
>>>>>>
>>>>>
>>>>> 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?
>>>>
>>>
>>> You are describing low power idle modes, not suspend. Most timers stop
>>> in suspend, so a timer set 10 seconds from now when entering suspend
>>> will go off 10 seconds after resume so it should have no impact on how
>>> long you decide to stay in suspend.
>>
>> so what is the fundamental difference between deciding to go into low-power
>> idle modes to wake up back up on a given point in the future and deciding
>> that you are going to be idle for so long that you may as well suspend until
>> there is user input?
>>
>
> Low power idle modes are supposed to be transparent. Suspend stops the
> monotonic clock, ignores ready threads and switches over to a separate
> set of wakeup events/interrupts. We don't suspend until there is user
> input, we suspend until there is a wakeup event (user-input, incoming
> network data/phone-calls, alarms etc..).

s/user input/wakeup event/ and my question still stands.

low power modes are not transparent to the user in all cases (if the 
screen backlight dimms/shuts off a user reading something will notice, if 
the system switches to a lower clock speed it can impact user response 
time, etc) The system is making it's best guess as to how to best srve the 
user by sacraficing some capibilities to save power now so that the power 
can be available later.

as I see it, suspending until a wakeup event (button press, incoming call, 
alarm, etc) is just another datapoint along the same path.

If the system could not wake itself up to respond to user input, phone 
call, alarm, etc and needed the power button pressed to wake up (or shut 
down to the point where the battery could be removed and reinstalled a 
long time later), I would see things moving into a different category, but 
as long as the system has the ability to wake itself up later (and is 
still consuming power) I see the suspend as being in the same category as 
the other low-power modes (it's just more expensive to go in and out of)


why should the suspend be put into a different category from the other 
low-power states?

David Lang