On Saturday 29 May 2010, Alan Stern wrote:
> On Sat, 29 May 2010, Florian Mickler wrote:
>
> > On Sat, 29 May 2010 12:42:37 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > > Now, all I'm interested in is providing interfaces from the kernel where
> > > needed, so that userspace can be optimally frugal with power usage, and
> > > can monitor/contain badly behaving tasks.
> > >
> >
> > I think this is a sensible approach.
>
> Here is an attempt to satisfy everyone as much as possible. But first
> an explicit disclaimer: When I say "suspend", I mean it as in
> "suspend-to-RAM"; i.e., a forced suspend and not a cpuidle mode.
>
> In place of in-kernel suspend blockers, there will be a new type of QoS
> constraint -- call it QOS_EVENTUALLY. It's a very weak constraint,
> compatible with all cpuidle modes in which runnable threads are allowed
> to run (which is all of them), but not compatible with suspend.
>
> The Android people want debugging and accountability. So in the most
> objectionable part of this proposal, we add a new way of registering
> QoS constraints: monitored constraints. The "monitored" implies that:
>
> The constraint has a name, which can be used for debugging
> and accounting;
>
> The kernel maintains statistics on the constraint's use and
> makes them available to userspace; and
>
> The PM core is notified whenever the number of active monitored
> constraints drops to 0.
>
> There is no /sys/power/policy file. In place of opportunistic suspend,
> we have "QoS-based suspend". This is initiated by userspace writing
> "qos" to /sys/power/state, and it is very much like suspend-to-RAM.
> However a QoS-based suspend fails immediately if there are any active
> normal QoS constraints incompatible with system suspend, in other
> words, any constraints requiring a throughput > 0 or an interrupt
> latency shorter than the time required for a suspend-to-RAM/resume
> cycle.
>
> If no such constraints are active, the QoS-based suspend blocks in an
> interruptible wait until the number of active QOS_EVENTUALLY
> constraints drops to 0. When that happens, it carries out a normal
> suspend-to-RAM -- except that it checks along the way to make sure that
> no new QoS constraints are activated while the suspend is in progress.
> If they are, the PM core backs out and fails the QoS-based suspend.
>
> Userspace suspend blockers don't exist at all, as far as the kernel is
> concerned. In their place, the Android runs a power-manager program
> that receives IPC requests from other processes when they need to
> prevent the system from suspending or allow it to suspend. The power
> manager's main loop looks like this:
>
> for (;;) {
> while (any IPC requests remain)
> handle them;
> if (any processes need to prevent suspend)
> sleep;
> else
> write "qos" to /sys/power/state;
> }
>
> The idea is that receipt of a new IPC request will cause a signal to be
> sent, interrupting the sleep or the "qos" write.
>
> There remains a question as to which kernel drivers should create
> monitored QOS_EVENTUALLY constraints. Perhaps userspace could be
> allowed to specify this (I don't know how). In any case, this is a
> relatively minor point.
>
> The advantages of this scheme are that this does everything the Android
> people need, and it does it in a way that's entirely compatible with
> pure QoS/cpuidle-based power management. It even starts along the path
> of making suspend-to-RAM just another kind of dynamic power state.
>
> If people such as Peter still want to complain that using
> suspend-to-RAM in Android phones isn't a good way to do power
> management, that's okay -- it's the designers' decision to program
> their phones the way they want. At least the kernel can give them the
> ability to do so in a way that doesn't compromise everybody else.
This sounds reasonable to me.
Rafael
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