Re: power-efficient scheduling design

[Morten Rasmussen]

On Fri, May 31, 2013 at 11:52:04AM +0100, Ingo Molnar wrote:
> 
> * Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> 
> > Hi,
> > 
> > A number of patch sets related to power-efficient scheduling have been 
> > posted over the last couple of months. Most of them do not have much 
> > data to back them up, so I decided to do some testing.
> 
> Thanks, numbers are always welcome!
> 
> > Measurement technique:
> > Time spent non-idle (not in idle state) for each cpu based on cpuidle
> > ftrace events. TC2 does not have per-core power-gating, so packing
> > inside the A7 cluster does not lead to any significant power savings.
> > Note that any product grade hardware (TC2 is a test-chip) will very
> > likely have per-core power-gating, so in those cases packing will have
> > an appreciable effect on power savings.
> > Measuring non-idle time rather than power should give a more clear idea
> > about the effect of the patch sets given that the idle back-end is
> > highly implementation specific.
> 
> Note that I still disagree with the whole design notion of having an "idle 
> back-end" (and a 'cpufreq back end') separate from scheduler power saving 
> policy, and none of the patch-sets offered so far solve this fundamental 
> design problem.
> 
> PeterZ and me tried to point out the design requirements previously, but 
> it still does not appear to be clear enough to people, so let me spell it 
> out again, in a hopefully clearer fashion.
> 
> The scheduler has valuable power saving information available:
> 
>  - when a CPU is busy: about how long the current task expects to run
> 
>  - when a CPU is idle: how long the current CPU expects _not_ to run
> 
>  - topology: it knows how the CPUs and caches interrelate and already 
>    optimizes based on that
> 
>  - various high level and low level load averages and other metrics about 
>    the recent past that show how busy a particular CPU is, how busy the 
>    whole system is, and what the runtime properties of individual tasks is 
>    (how often it sleeps, etc.)
> 
> so the scheduler is in an _ideal_ position to do a judgement call about 
> the near future and estimate how deep an idle state a CPU core should 
> enter into and what frequency it should run at.
> 
> The scheduler is also at a high enough level to host a "I want maximum 
> performance, power does not matter to me" user policy override switch and 
> similar user policy details.
> 
> No ifs and whens about that.
> 
> Today the power saving landscape is fragmented and sad: we just randomly 
> interface scheduler task packing changes with some idle policy (and 
> cpufreq policy), which might or might not combine correctly.
> 
> Even when the numbers improve, it's an entirely random, essentially 
> unmaintainable property: because there's no clear split (possible) between 
> 'scheduler policy' and 'idle policy'. This is why we removed the old, 
> broken power saving scheduler code a year ago: to make room for something 
> _better_.
> 
> So if we want to add back scheduler power saving then what should happen 
> is genuinely better code:
> 
> To create a new low level idle driver mechanism the scheduler could use 
> and integrate proper power saving / idle policy into the scheduler.
> 
> In that power saving framework the already existing scheduler topology 
> information should be extended with deep idle parameters:
> 
>  - enumeration of idle states
> 
>  - how long it takes to enter+exit a particular idle state
> 
>  - [ perhaps information about how destructive to CPU caches that
>      particular idle state is. ]
> 
>  - new driver entry point that allows the scheduler to enter any of the 
>    enumerated idle states. Platform code will not change this state, all 
>    policy decisions and the idle state is decided at the power saving 
>    policy level.
> 
> All of this combines into a 'cost to enter and exit an idle state' 
> estimation plus a way to enter idle states. It should be presented to the 
> scheduler in a platform independent fashion, but without policy embedded: 
> a low level platform driver interface in essence.
> 
> Thomas Gleixner's recent work to generalize platform idle routines will 
> further help the implementation of this. (that code is upstream already)
> 
> _All_ policy, all metrics, all averaging should happen at the scheduler 
> power saving level, in a single place, and then the scheduler should 
> directly drive the new low level idle state driver mechanism.
> 
> 'scheduler power saving' and 'idle policy' are one and the same principle 
> and they should be handled in a single place to offer the best power 
> saving results.
> 
> Note that any RFC patch-set that offers an implementation for this could 
> be structured in a gradual fashion: only implementing it for a limited CPU 
> range initially. The new framework can then be extended to more and more 
> CPUs and architectures, incorporating more complicated power saving 
> features gradually. (The old, existing idle policy code would remain 
> untouched and available - it would simply not be used when the new policy 
> is activated.)
> 
> I.e. I'm not asking for a 'rewrite the world' kind of impossible task - 
> I'm providing an actionable path to get improved power saving upstream, 
> but it has to use a _sane design_.
> 
> This is a "line in the sand", a 'must have' design property for any 
> scheduler power saving patches to be acceptable - and I'm NAK-ing 
> incomplete approaches that don't solve the root design cause of our power 
> saving troubles...

Thanks for sharing your view.

I agree with idea of having a high level user switch to change
power/performance policy trade-offs for the system. Not only for
scheduling. I also share your view that the scheduler is in the ideal
place to drive the frequency scaling and idle policies.

However, I think that an integrated solution with one unified policy
implemented in the scheduler would take a significant rewrite of the
scheduler and the power management frameworks even if we start with just
a few SoCs.

To reach an integrated solution that does better than the current
approach there is a range of things that need to be considered:

- Define a power-efficient scheduling policy. Depending on the power
  gating support on the particular system packing tasks may improve
  power-efficiency while spreading the tasks may be better for others.

- Define how the user policy switch works. In previous discussions it
  was proposed to have a high level switch that allows specification of
  what the system should strive to achieve - power saving or performance.
  In those discussions, what power meant wasn't exactly defined.

- Find a generic way to represent the power topology which includes
  power domains, voltage domains and frequency domains. Also, more
  importantly how we can derive the optimal power/performance policy for
  the specific platform. There may be dependencies between idle and
  frequency states like it is the case for frequency boost mode like Arjan
  mentions in his reply.

- The fact that not all platforms expose all idle states to the OS and
  that closed firmware may do whatever it likes behind the scenes. There
  are various reasons to do this. Not all of them are bad.

- Define a scheduler driven frequency scaling policy that at least
  matches the 'performance' of the current cpufreq policies and has
  potential for further improvements.

- Match the power savings of the current cpuidle governors which are
  based on arcane heuristics developed over years to predict things like
  the occurrence of the next interrupt.

- Thermal aspects add more complexity to the power/performance policy.
  Depending on the platform, overheating may be handled by frequency
  capping or restricting the number of active cpus.

- Asymmetric/heterogeneous multi-processors need to be dealt with.

This is not a complete list. My point is that moving all policy to the
scheduler will significantly increase the complexity of the scheduler.
It is my impression that the general opinion is that the scheduler is
already too complicated. Correct me if I'm wrong.

While the proposed task packing patches are not complete solutions, they
address the first item on the above list and can be seen as a step
towards the goal.

Should I read your recommendation as you prefer a complete and
potentially huge patch set over incremental patch sets?

It would be good to have even a high level agreement on the path forward
where the expectation first and foremost is to take advantage of the
schedulers ideal position to drive the power management while
simplifying the power management code.

Thanks,
Morten

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