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SubjectRe: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
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On Wed, May 26, 2010 at 6:16 AM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
>> Really, what are you getting at? Do you deny that there are programs,
>> that prevent a device from sleeping? (Just think of the bouncing
>> cows app)
>>
>> And if you have two kernels, one with which your device is dead after 1
>> hour and one with which your device is dead after 10 hours. Which would
>> you prefer? I mean really... this is ridiculous.
>
> The problem you have is that this is policy. If I have the device wired
> to a big screen and I want cows bouncing on it I'll be most upset if
> instead it suspends.

We never suspend when the screen is on. If the screen is off, I would
not be upset if it suspends.

> What you are essentially arguing for is for the
> kernel to disobey the userspace.

No I'm not. User-space asked the kernel to suspend when possible.
Suspend is an existing kernel feature. All opportunistic suspend adds
is the ability to use it safely when there are wakeup event you cannot
afford to ignore.

> It's as ridiculous (albeit usually less
> damaging) as a file system saying "Ooh thats a rude file name, the app
> can't have meant it, I'll put your document soemwhere else"
>
> The whole API feels wrong to me. It's breaking rule #1 of technology "You
> cannot solve a social problem with technology". In this case you have a
> social/economic problem which is crap code. You solve it with an
> economics solution - creative incentives not to produce crap code like
> boxes that keep popping up saying "App XYZ is using all your battery" and
> red-amber-green powermeter scores in app stores.
>
> That said if you want technical mitigation I think it makes more sense
> if you look at it from a different starting point. The starting point
> being this: We have idling logic in the kernel and improving this helps
> everyone. What is needed to improve the existing logic ?
>

Our actual stating point is this: Some systems can only enter their
lowest power state from suspend. So we added an api that allowed us to
use suspend without loosing wakeup events. Since suspending also
improves our battery life on platforms that enter the same power state
from idle and suspend and we already have a way to safely use suspend,
we would be crazy not to use it.

> - You don't know which processes should be ignored for the purpose of
>  suspend (except for kernel threads) and there is no way to set this
>
> - You don't know whether a move from a deep idle to a 'suspend' (which is
>  just a really deep idle in truth anyway) might break wakeups
>  requirements because a device has wake dependencies due to hardware
>  design (eg a port that has no electronics to kick the box out of
>  suspend into running). This is a problem we have already. [1]
>
> That maps onto two existing ideas
>
> Sandboxing/Resource Limits: handling apps that can't be trusted. So the
> phone runs the appstore code via something like

Sandboxing is problematic on Android since there are a lot of cross
process dependencies. When a call comes in I don't know where the name
and picture to display comes from. With suspend blockers we block
suspend when we get notified that we have an incoming call and we can
call into any process and get a response.

>
>                setpidle(getpid(), something);
>                exec()
>
> where 'something' is a value with meaning to both user space and to the
> existing idling logic in the kernel that basically says to what extent it
> is permitted to block idling/suspend. That also seems to tie into some of
> the realtime + idle problems. This I think deals with Kevin Hillman's
> thoughts on dealing with untrustworthy app code more cleanly and avoids
> the need for userspace hackery like the blocker API.
>
> And an entirely in kernel API where device drivers can indicate that in
> their current situation they require that the power level doesn't drop
> below some limit unless user requested. This is really important because
> the platform vendor of the phone/pda/tablet whatever effectively owns the
> kernel - so it's *their* problem, *their* control, *their* hardware and
> they can make it work as best for the device. Best of all it means its
> all free software stuff so if the vendor screws up you can still fix your
> phone.
>
> Implementation-wise it probably ties into setpidle, its simply that a task
> has a pair of idle values, a dynamic one and a base one, the dynamic one
> being the base one but updatable temporarily by drivers.
>


What about platforms that currently cannot enter low power states from
idle? Do you remove suspend support from the kernel?


> Alan
> --
>
> [1] Note I disagree with Kevin here on static/dynamic power management.
> There are IMHO two types of PM but they are 'user invoked' and
> 'automatic'. "Static" simply means it's not been made fast enough yet but
> its just a policy divide dependant on the users 'acceptable' resume time
> (which for hard RT may just as well rule out some more usual power states)
>
>
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--
Arve Hjønnevåg
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