Re: resume latency QoS support, unify suspend/resume into idle states

[Arve Hjønnevåg]

On Fri, May 28, 2010 at 2:21 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
>
> * Arve Hj?nnev?g <arve@xxxxxxxxxxx> wrote:
>
>> On Thu, May 27, 2010 at 3:23 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Thu, 27 May 2010 23:09:49 +0100
>> > Matthew Garrett <mjg59@xxxxxxxxxxxxx> wrote:
>> >
>> >> On Thu, May 27, 2010 at 11:08:06PM +0100, Alan Cox wrote:
>> >>
>> >> > This is I believe robust (and has been implemented on some non x86
>> >> > boxes). It depends on not forcing running tasks into suspend. That is the
>> >> > key.
>> >>
>> >> We've already established that ACPI systems require us to force running
>> >> tasks into suspend. How do we avoid the race in that situation?
>> >
>> > Android phones do not have ACPI. Embedded platforms do not have ACPI. MID
>> > x86 devices do not have ACPI.
>>
>> Android does not only run on phones. It is possible that no android devices
>> have ACPI, but I don't know that for a fact. What I do know is that people
>> want to run Android on x86 hardware and supporting suspend could be very
>> benficial.
>
> (If there's a sane framework then we'll fix x86 to fit into it and will deal
> with quirks.)
>
>> > I would imagine the existing laptops will handle power management limited
>> > by the functionality they have available. Just like any other piece of
>> > hardware.
>>
>> I think existing laptops (and desktops) can benefit from opportunistic
>> suspend support. If opportunistic suspend is used for auto-sleep after
>> inactivity instead of forced suspend, the user space suspend blocker api
>> will allow an application to delay this auto sleep until for instance a
>> download completes. This part could also be done with a user-space IPC call,
>> but having a standard kernel interface for it may make it more common. A
>> less common case, but more critical, is RTC alarms. I know my desktops can
>> wakeup at a specific time by programming an RTC alarm, but without suspend
>> blockers how do you ensure that the system does not suspend right after the
>> alarm triggered? I have a system that wakes up at specific times requested
>> by my DVR application, but I cannot use this system for anything else unless
>> I manually turn off the DVR application's auto-sleep feature. With suspend
>> blockers and something like the android alarm driver, I could use this
>> system for more than one application that have scheduled tasks and it would
>> be more usable for interactive applications.
>
> I really like the level of detail and care that went into suspend-blockers,
> and i think the Android solution is very mature in terms of functionality
> offered to users.
>
> In terms of bringing this depth of functionality and control to the upstream
> kernel, what do you think about Alan's QoS scheme, described in:
>
>   <20100528001514.28e593ef@xxxxxxxxxxxxxxxxxxx>
>
> ?
>
> It's in essence suspend-blockers on steroids. It consists of two main
> components:
>
>  - Unify the 'suspended' state into the regular chain of idle states, and
>   create a single, coherent and transparent way we handle system idleness.
>
>  - Give apps a QoS attribute that allows them to express how long they can
>   afford to wait for a wakeup. (A downloading app would set it to say 50msecs,
>   and thus the kernel would know it automatically which method of idleness is
>   still achievable. If all currently running apps have a max(QoS) attribute
>   of infinite, then the kernel can suspend for an unlimited amount of time.)
>
> AFAICS, and i have read through your suspend-blocker usecases, this should
> handle all the usecases you listed - and some more. (please yell if that's not
> so)
>
> Suspend-blockers are equivalent to: 'app sets idle QoS latency to 0 msecs'.
>
> (And on x86, for BIOS/CPU combos that allow it we can implement this scheme
> too.)
>
> Thoughts?

Tying the QoS attribute to apps does not work (all proposals I have
seen have race conditions), but replacing every suspend blocker with
unique QoS object will work, since is the same thing as what suspend
blockers provide. I think replacing suspend blockers with artificial
latency requirements is a bad idea though, since we use them to ensure
a specific level of functionality (tasks, timers and interrupts
operate normally). If we get a more generic constraint framework,
suspend blockers may possibly be absorbed by this, but I think the
current implementation is useful as is (it could even be useful to
someone working on a generic constraints framework).

-- 
Arve Hjønnevåg
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