Re: [PATCH V3 2/2] sched: idle: IRQ based next prediction for idle period

From: Daniel Lezcano
Date: Tue Feb 23 2016 - 04:49:39 EST


Hi Rafael,

Added Ingo.

On 02/20/2016 12:43 AM, Rafael J. Wysocki wrote:
On Fri, Feb 19, 2016 at 4:01 PM, Daniel Lezcano
<daniel.lezcano@xxxxxxxxxx> wrote:
On 02/18/2016 07:57 PM, Rafael J. Wysocki wrote:

On Thu, Feb 18, 2016 at 11:25 AM, Daniel Lezcano
<daniel.lezcano@xxxxxxxxxx> wrote:

On 02/17/2016 11:21 PM, Rafael J. Wysocki wrote:

[ ... ]

Reviewed-by: Nicolas Pitre <nico@xxxxxxxxxx>



Well, I'm likely overlooking something, but how is this going to be
hooked up to the code in idle.c?



My somewhat educated guess is that sched_idle() in your patch is
intended to replace cpuidle_idle_call(), right?



Well, no. I was planning to first have it to use a different code path as
experimental code in order to focus improving the accuracy of the
prediction
and then merge or replace cpuidle_idle_call() with sched_idle().


In that case, what about making it a proper cpuidle governor that
people can test and play with in a usual way? Then it may potentially
benefit everybody and not just your experimental setup and you may get
coverage on systems you have no access to normally.

There is some boilerplate code to add for this purpose, but that's not
that bad IMO.


Hi Rafael,

sorry for the delay in the responses.

Actually, adding a new governor is precisely what I would like to avoid
because the objective is the scheduler acts as the governor.

But why, really?

Well, first of all I'm not sure what "the scheduler acts as the
governor" means. For the lack of a better explanation I'll refer to
the message at https://lkml.org/lkml/2016/1/12/530 that you pointed me
at.

For better explanation let's refer to Ingo's email:

http://lwn.net/Articles/552889/

Ingo said:

"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. [...]"

"[...] 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 patches I sent are supposed to provide the base brick to solve 'this fundamental design problem' by giving a couple of functions:

- when is the next event ?
- sleep X us with Y latency.

The component responsible of choosing X and Y will be the scheduler based on its knowledge of 'the near future' coming from 'when is the next event ?'

Moving the code I submitted into a governor has little sense with the goal described above because the sched_next_event will be called from the scheduler code directly. Today the integration is not obvious.

There is a branch in the cpuidle loop below but this is just for testing purpose at the moment and not part as a submitted patch.


Your code in there does something like:

if (sched_idle_enabled()) {
int latency = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
s64 duration = sched_idle_next_wakeup();
sched_idle(duration, latency);
} else {
cpuidle_idle_call();
}

which is quite questionable to be honest as it adds an extra branch to
the idle loop for no real benefit.

That is not the part of the submitted patchset. It is an example based on an embryonic code to show how to use the API.

How those API are integrated with the scheduler will be submitted in a separate patchset.

Now, what really is the difference between "governor" and "predictor"?
I don't quite see it except that the former is expected to provide a
specific interface.

The predictor is a standalone component giving the answer to the question 'when is the next event ?'.

The governor uses the next event information from the prediction to take a decision (governor == scheduler here).

If the code I am proposing is buried in the cpuidle framework inside a governor I don't see how the scheduler can become the ideal place to do a decision.

The way the idle loop works now (and I'm not sure if you can really
change it) is that when you get into it, you're idle no matter what
and you simply need to choose an idle state for the CPU to go into.
Some code needs to select that state, regardless of what name you want
to give to that code.

In the current setup, which I really don't think is unreasonable, this
is done by cpuidle_select() that simply invokes the governor's
->select() callback and that's it. That callback may very well be
part of the scheduler and registered from there if you want that, but
why do you want to change the whole mechanism? What's wrong with it
now?

There is nothing wrong. We lived with it for years and that satisfied everyone.

My point is I am sticking to how Ingo and Peter envision the integration work. It is clearly stated in Ingo's mail, he disagree on having the notion of 'idle backend'.

The couple of patches here are to be used by the scheduler directly.

Further, if you look at your sched_idle(), it looks almost like
cpuidle_idle_call() with a few really minor differences (apart from
the fact that it doesn't cover suspend-to-idle which it will have to
do eventually) that really look arbitrary and the "selection" if () in
it simply plays the role of the invocation of ->select(). So how is
it different really?

The question is not if there is duplicate code or not. Factoring code is easy but factoring code with a moving target is much more difficult.

The moving target is the cpuidle code itself. I have been consolidating the code several times and the effort got ruined several times by the changes.

It is much more easier to introduce the experimental code make it evolve wisely and then consolidate with the existing code.

And it does even more sense to branch it if the feature is by design the exact opposite of the current design (idle based statistics vs irq events) even if there is common code.

Here, it is the 'predictor' and the API to enter an idle state conforming the idle duration
and the latency constraint.

Isn't that just a simple rearrangement of the code? The latency still
comes from PM QoS and the duration is computed by your new code
instead of that being done by ->select() itself, but why actually
->select() cannot call sched_idle_next_wakeup() to get the duration
value it needs? Why do those values need to be passed to a
cpuidle_idle_call() replacement as arguments? Is there any particular
technical reason for doing that?

Yes. The select returns an idle state index. Even if, at some points, we may need to know what is the idle state fitting a set of timing constraints, the scheduler is interested in how long the cpu will be idle in order to take a decision.

One example when everything is integrated:

The topology gives the information CPU2 and CPU3 are cluster1.

1. CPU3 enters idle, the next wakeup event is stored for this cpu
2. CPU2 enters idle and looks when it should wake up
3. CPU2 looks when CPU3 wakes up
4. CPU2 takes the time intersection between CPU2/CPU3 in order to have the time the cluster is idle
5. CPU2 takes the decision if cluster power down is possible

With the current design it is not possible to do that.

And why that name, sched_idle_next_wakeup()? Does that function
really have anything to do with the scheduler now?

Concerning the testing, it is quite easy to switch from idle_sched to 'menu'
via on sched_debug or whatever option we want to add.


So I'm still unsure why you want to replace cpuidle_idle_call() with
sched_idle(). Is there anything wrong with it that it needs to be
replaced?


I don't want to replace cpuidle_idle_call() with sched_idle(). How we
integrate the API is something I would like to discuss with another patchset
focused in this integration only.

For reference: https://lkml.org/lkml/2016/1/12/530

Please answer my questions above. If you need to post a patchset for
this purpose, please do that.

...

I have to say that I was looking forward to the IRQ timings based
duration prediction, but the way you want to use it now is seriously
disappointing.

This comment is a bit patronizing but I can understand you were expecting more.

I will let Peter and Ingo give their opinions and I will follow the consensus.

Thanks for the review.

-- Daniel

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