Re: power-efficient scheduling design

[Rafael J. Wysocki]

On Saturday, June 08, 2013 12:28:04 PM Catalin Marinas wrote:
> On Fri, Jun 07, 2013 at 07:08:47PM +0100, Preeti U Murthy wrote:
> > On 06/07/2013 08:21 PM, Catalin Marinas wrote:
> > > I think you are missing Ingo's point. It's not about the scheduler
> > > complying with decisions made by various governors in the kernel
> > > (which may or may not have enough information) but rather the
> > > scheduler being in a better position for making such decisions.
> > 
> > My mail pointed out that I disagree with this design ("the scheduler
> > being in a better position for making such decisions").
> > I think it should be a 2 way co-operation. I have elaborated below.

I agree with that.

> > > Take the cpuidle example, it uses the load average of the CPUs,
> > > however this load average is currently controlled by the scheduler
> > > (load balance). Rather than using a load average that degrades over
> > > time and gradually putting the CPU into deeper sleep states, the
> > > scheduler could predict more accurately that a run-queue won't have
> > > any work over the next x ms and ask for a deeper sleep state from the
> > > beginning.
> > 
> > How will the scheduler know that there will not be work in the near
> > future? How will the scheduler ask for a deeper sleep state?
> > 
> > My answer to the above two questions are, the scheduler cannot know how
> > much work will come up. All it knows is the current load of the
> > runqueues and the nature of the task (thanks to the PJT's metric). It
> > can then match the task load to the cpu capacity and schedule the tasks
> > on the appropriate cpus.
> 
> The scheduler can decide to load a single CPU or cluster and let the
> others idle. If the total CPU load can fit into a smaller number of CPUs
> it could as well tell cpuidle to go into deeper state from the
> beginning as it moved all the tasks elsewhere.

So why can't it do that today?  What's the problem?

> Regarding future work, neither cpuidle nor the scheduler know this but
> the scheduler would make a better prediction, for example by tracking
> task periodicity.

Well, basically, two pieces of information are needed to make target idle
state selections: (1) when the CPU (core or package) is going to be used
next time and (2) how much latency for going back to the non-idle state
can be tolerated.  While the scheduler knows (1) to some extent (arguably,
it generally cannot predict when hardware interrupts are going to occur),
I'm not really sure about (2).

> > As a consequence, it leaves certain cpus idle. The load of these cpus
> > degrade. It is via this load that the scheduler asks for a deeper sleep
> > state. Right here we have scheduler talking to the cpuidle governor.
> 
> So we agree that the scheduler _tells_ the cpuidle governor when to go
> idle (but not how deep).

It does indicate to cpuidle how deep it can go, however, by providing it with
the information about when the CPU is going to be used next time (from the
scheduler's perspective).

> IOW, the scheduler drives the cpuidle decisions. Two problems: (1) the
> cpuidle does not get enough information from the scheduler (arguably this
> could be fixed)

OK, so what information is missing in your opinion?

> and (2) the scheduler does not have any information about the idle states
> (power gating etc.) to make any informed decision on which/when CPUs should
> go idle.

That's correct, which is a drawback.  However, on some systems it may never
have that information (because hardware coordinates idle states in a way that
is opaque to the OS - e.g. by autopromoting deeper states when idle for
sufficiently long time) and on some systems that information may change over
time (i.e. the availablility of specific idle states may depend on factors
that aren't constant).

If you attempted to take all of the possible complications related to hardware
designs in that area in the scheduler, you'd end up with completely
unmaintainable piece of code.

> As you said, it is a non-optimal one-way communication but the solution
> is not feedback loop from cpuidle into scheduler. It's like the
> scheduler managed by chance to get the CPU into a deeper sleep state and
> now you'd like the scheduler to get feedback form cpuidle and not
> disturb that CPU anymore. That's the closed loop I disagree with. Could
> the scheduler not make this informed decision before - it has this total
> load, let's get this CPU into deeper sleep state?

No, it couldn't in general, for the above reasons.

> > I don't see what the problem is with the cpuidle governor waiting for
> > the load to degrade before putting that cpu to sleep. In my opinion,
> > putting a cpu to deeper sleep states should happen gradually.

If we know in advance that the CPU can be put into idle state Cn, there is no
reason to put it into anything shallower than that.

On the other hand, if the CPU is in Cn already and there is a possibility to
put it into a deeper low-power state (which we didn't know about before), it
may make sense to promote it into that state (if that's safe) or even wake it
up and idle it again.

> > This means time will tell the governors what kinds of workloads are running
> > on the system. If the cpu is idle for long, it probably means that the system
> > is less loaded and it makes sense to put the cpus to deeper sleep
> > states. Of course there could be sporadic bursts or quieting down of
> > tasks, but these are corner cases.
> 
> It's nothing wrong with degrading given the information that cpuidle
> currently has. It's a heuristics that worked ok so far and may continue
> to do so. But see my comments above on why the scheduler could make more
> informed decisions.
> 
> We may not move all the power gating information to the scheduler but
> maybe find a way to abstract this by giving more hints via the CPU and
> cache topology. The cpuidle framework (it may not be much left of a
> governor) would then take hints about estimated idle time and invoke the
> low-level driver about the right C state.

Overall, it looks like it'd be better to split the governor "layer" between the
scheduler and the idle driver with a well defined interface between them.  That
interface needs to be general enough to be independent of the underlying
hardware.

We need to determine what kinds of information should be passed both ways and
how to represent it.

> > > Of course, you could export more scheduler information to cpuidle,
> > > various hooks (task wakeup etc.) but then we have another framework,
> > > cpufreq. It also decides the CPU parameters (frequency) based on the
> > > load controlled by the scheduler. Can cpufreq decide whether it's
> > > better to keep the CPU at higher frequency so that it gets to idle
> > > quicker and therefore deeper sleep states? I don't think it has enough
> > > information because there are at least three deciding factors
> > > (cpufreq, cpuidle and scheduler's load balancing) which are not
> > > unified.
> > 
> > Why not? When the cpu load is high, cpu frequency governor knows it has
> > to boost the frequency of that CPU. The task gets over quickly, the CPU
> > goes idle. Then the cpuidle governor kicks in to put the CPU to deeper
> > sleep state gradually.
> 
> The cpufreq governor boosts the frequency enough to cover the load,
> which means reducing the idle time. It does not know whether it is
> better to boost the frequency twice as high so that it gets to idle
> quicker. You can change the governor's policy but does it have any
> information from cpuidle?

Well, it may get that information directly from the hardware.  Actually,
intel_pstate does that, but intel_pstate is the governor and the scaling
driver combined.

> > Meanwhile the scheduler should ensure that the tasks are retained on
> > that CPU,whose frequency is boosted and should not load balance it, so
> > that they can get over quickly. This I think is what is missing. Again
> > this comes down to the scheduler taking feedback from the CPU frequency
> > governors which is not currently happening.
> 
> Same loop again. The cpu load goes high because (a) there is more work,
> possibly triggered by external events, and (b) the scheduler decided to
> balance the CPUs in a certain way. As for cpuidle above, the scheduler
> has direct influence on the cpufreq decisions. How would the scheduler
> know which CPU not to balance against? Are CPUs in a cluster
> synchronous? Is it better do let other CPU idle or more efficient to run
> this cluster at half-speed?
> 
> Let's say there is an increase in the load, does the scheduler wait
> until cpufreq figures this out or tries to take the other CPUs out of
> idle? Who's making this decision? That's currently a potentially
> unstable loop.

Yes, it is and I don't think we currently have good answers here.

The results of many measurements seem to indicate that it generally is better
to do the work as quickly as possible and then go idle again, but there are
costs associated with going back and forth from idle to non-idle etc.

The main problem with cpufreq that I personally have is that the governors
carry out their own sampling with pretty much arbitrary resolution that may
lead to suboptimal decisions.  It would be much better if the scheduler
indicated when to *consider* the changing of CPU performance parameters (that
may not be frequency alone and not even frequency at all in general), more or
less the same way it tells cpuidle about idle CPUs, but I'm not sure if it
should decide what performance points to run at.

> > >> I would repeat here that today we interface cpuidle/cpufrequency
> > >> policies with scheduler but not the other way around. They do their bit
> > >> when a cpu is busy/idle. However scheduler does not see that somebody
> > >> else is taking instructions from it and comes back to give different
> > >> instructions!
> > > 
> > > The key here is that cpuidle/cpufreq make their primary decision based
> > > on something controlled by the scheduler: the CPU load (via run-queue
> > > balancing). You would then like the scheduler take such decision back
> > > into account. It just looks like a closed loop, possibly 'unstable' .
> > 
> > Why? Why would you call a scheduler->cpuidle->cpufrequency interaction a
> > closed loop and not the new_scheduler = scheduler+cpuidle+cpufrequency a
> > closed loop? Here too the scheduler should be made well aware of the
> > decisions it took in the past right?
> 
> It's more like:
> 
> scheduler -> cpuidle/cpufreq -> hardware operating point
>    ^                                      |
>    +--------------------------------------+
> 
> You can argue that you can make an adaptive loop that works fine but
> there are so many parameters that I don't see how it would work. The
> patches so far don't seem to address this. Small task packing, while
> useful, it's some heuristics just at the scheduler level.

I agree.

> With a combined decision maker, you aim to reduce this separate decision
> process and feedback loop. Probably impossible to eliminate the loop
> completely because of hardware latencies, PLLs, CPU frequency not always
> the main factor, but you can make the loop more tolerant to
> instabilities.

Well, in theory. :-)

Another question to ask is whether or not the structure of our software
reflects the underlying problem.  I mean, on the one hand there is the
scheduler that needs to optimally assign work items to computational units
(hyperthreads, CPU cores, packages) and on the other hand there's hardware
with different capabilities (idle states, performance points etc.).  Arguably,
the scheduler internals cannot cover all of the differences between all of the
existing types of hardware Linux can run on, so there needs to be a layer of
code providing an interface between the scheduler and the hardware.  But that
layer of code needs to be just *one*, so why do we have *two* different
frameworks (cpuidle and cpufreq) that talk to the same hardware and kind of to
the scheduler, but not to each other?

To me, the reason is history, and more precisely the fact that cpufreq had been
there first, then came cpuidle and only then poeple started to realize that
some scheduler tweaks may allow us to save energy without sacrificing too
much performance.  However, it looks like there's time to go back and see how
we can integrate all that.  And there's more, because we may need to take power
budgets and thermal management into account as well (i.e. we may not be allowed
to use full performance of the processors all the time because of some
additional limitations) and the CPUs may be members of power domains, so what
we can do with them may depend on the states of other devices.

> > > So I think we either (a) come up with 'clearer' separation of
> > > responsibilities between scheduler and cpufreq/cpuidle 
> > 
> > I agree with this. This is what I have been emphasizing, if we feel that
> > the cpufrequency/ cpuidle subsystems are suboptimal in terms of the
> > information that they use to make their decisions, let us improve them.
> > But this will not yield us any improvement if the scheduler does not
> > have enough information. And IMHO, the next fundamental information that
> > the scheduler needs should come from cpufreq and cpuidle.
> 
> What kind of information? Your suggestion that the scheduler should
> avoid loading a CPU because it went idle is wrong IMHO. It went idle
> because the scheduler decided this in first instance.
> 
> > Then we should move onto supplying scheduler information from the power
> > domain topology, thermal factors, user policies.
> 
> I agree with this but at this point you get the scheduler to make more
> informed decisions about task placement. It can then give more precise
> hints to cpufreq/cpuidle like the predicted load and those frameworks
> could become dumber in time, just complying with the requested
> performance level (trying to break the loop above).

Well, there's nothing like "predicted load".  At best, we may be able to make
more or less educated guesses about it, so in my opinion it is better to use
the information about what happened in the past for making decisions regarding
the current settings and re-adjust them over time as we get more information.

So how much decision making regarding the idle state to put the given CPU into
should be there in the scheduler?  I believe the only information coming out
of the scheduler regarding that should be "OK, this CPU is now idle and I'll
need it in X nanoseconds from now" plus possibly a hint about the wakeup
latency tolerance (but those hints may come from other places too).  That said
the decision *which* CPU should become idle at the moment very well may require
some information about what options are available from the layer below (for
example, "putting core X into idle for Y of time will save us Z energy" or
something like that).

And what about performance scaling?  Quite frankly, in my opinion that
requires some more investigation, because there still are some open questions
in that area.  To start with we can just continue using the current heuristics,
but perhaps with the scheduler calling the scaling "governor" when it sees fit
instead of that "governor" running kind of in parallel with it.

> > > or (b) come up
> > > with a unified load-balancing/cpufreq/cpuidle implementation as per
> > > Ingo's request. The latter is harder but, with a good design, has
> > > potentially a lot more benefits.
> > > 
> > > A possible implementation for (a) is to let the scheduler focus on
> > > performance load-balancing but control the balance ratio from a
> > > cpufreq governor (via things like arch_scale_freq_power() or something
> > > new). CPUfreq would not be concerned just with individual CPU
> > > load/frequency but also making a decision on how tasks are balanced
> > > between CPUs based on the overall load (e.g. four CPUs are enough for
> > > the current load, I can shut the other four off by telling the
> > > scheduler not to use them).
> > > 
> > > As for Ingo's preferred solution (b), a proposal forward could be to
> > > factor the load balancing out of kernel/sched/fair.c and provide an
> > > abstract interface (like load_class?) for easier extending or
> > > different policies (e.g. small task packing). 
> > 
> >  Let me elaborate on the patches that have been posted so far on the
> > power awareness of the scheduler. When we say *power aware scheduler*
> > what exactly do we want it to do?
> > 
> > In my opinion, we want it to *avoid touching idle cpus*, so as to keep
> > them in that state longer and *keep more power domains idle*, so as to
> > yield power savings with them turned off. The patches released so far
> > are striving to do the latter. Correct me if I am wrong at this.
> 
> Don't take me wrong, task packing to keep more power domains idle is
> probably in the right direction but it may not address all issues. You
> realised this is not enough since you are now asking for the scheduler
> to take feedback from cpuidle. As I pointed out above, you try to create
> a loop which may or may not work, especially given the wide variety of
> hardware parameters.
> 
> > Also
> > feel free to point out any other expectation from the power aware
> > scheduler if I am missing any.
> 
> If the patches so far are enough and solved all the problems, you are
> not missing any. Otherwise, please see my view above.
> 
> Please define clearly what the scheduler, cpufreq, cpuidle should be
> doing and what communication should happen between them.
> 
> > If I have got Ingo's point right, the issues with them are that they are
> > not taking a holistic approach to meet the said goal.
> 
> Probably because scheduler changes, cpufreq and cpuidle are all trying
> to address the same thing but independent of each other and possibly
> conflicting.
> 
> > Keeping more power
> > domains idle (by packing tasks) would sound much better if the scheduler
> > has taken all aspects of doing such a thing into account, like
> > 
> > 1. How idle are the cpus, on the domain that it is packing
> > 2. Can they go to turbo mode, because if they do,then we cant pack
> > tasks. We would need certain cpus in that domain idle.
> > 3. Are the domains in which we pack tasks power gated?
> > 4. Will there be significant performance drop by packing? Meaning do the
> > tasks share cpu resources? If they do there will be severe contention.
> 
> So by this you add a lot more information about the power configuration
> into the scheduler, getting it to make more informed decisions about
> task scheduling. You may eventually reach a point where cpuidle governor
> doesn't have much to do (which may be a good thing) and reach Ingo's
> goal.
> 
> That's why I suggested maybe starting to take the load balancing out of
> fair.c and make it easily extensible (my opinion, the scheduler guys may
> disagree). Then make it more aware of topology, power configuration so
> that it makes the right task placement decision. You then get it to
> tell cpufreq about the expected performance requirements (frequency
> decided by cpufreq) and cpuidle about how long it could be idle for (you
> detect a periodic task every 1ms, or you don't have any at all because
> they were migrated, the right C state being decided by the governor).

There is another angle to look at that as I said somewhere above.

What if we could integrate cpuidle with cpufreq so that there is one code
layer representing what the hardware can do to the scheduler?  What benefits
can we get from that, if any?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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