Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE

[Daniel Bristot de Oliveira]

On 5/21/21 12:37 PM, Will Deacon wrote:
> Interesting, thanks. Thinking about this some more, it strikes me that with
> these silly asymmetric systems there could be an interesting additional
> problem with hotplug and deadline tasks. Imagine the following sequence of
> events:
> 
>   1. All online CPUs are 32-bit-capable
>   2. sched_setattr() admits a 32-bit deadline task
>   3. A 64-bit-only CPU is onlined

At the point 3, the global scheduler assumption is broken. For instance, in a
system with four CPUs and five ready 32-bit-capable tasks, when the fifth CPU as
added, the working conserving rule is violated because the five highest priority
thread are not running (only four are) :-(.

So, at this point, for us to keep to the current behavior, the addition should
be.. blocked? :-((

>   4. Some of the 32-bit-capable CPUs are offlined

Assuming that point 3 does not exist (i.e., all CPUs are 32-bit-capable). At
this point, we will have an increase in the pressure on the 32-bit-capable CPUs.

This can also create bad effects for 64-bit tasks, as the "contended" 32-bit
tasks will still be "queued" in a future time where they were supposed to be
done (leaving time for the 64-bit tasks).

> I wonder if we can get into a situation where we think we have enough
> bandwidth available, but in reality the 32-bit task is in trouble because
> it can't make use of the 64-bit-only CPU.

I would have to think more, but there might be a case where this contended
32-bit tasks could cause deadline misses for the 64-bit too.

> If so, then it seems to me that admission control is really just
> "best-effort" for 32-bit deadline tasks on these systems because it's based
> on a snapshot in time of the available resources.

The admission test as is now is "best-effort" in the sense that it allows a
workload higher than it could handle (it is necessary, but not sufficient AC).
But it should not be considered "best-effort" because of violations in the
working conserving property as a result of arbitrary affinities among tasks.
Overall, we have been trying to close any "exception left" to this later case.

I know, it is a complex situation, I am just trying to illustrate our concerns,
because, in the near future we might have a scheduler that handles arbitrary
affinity correctly. But that might require us to stick to an AC. The AC is
something precious for us.

(yeah, SP would not face this problem... now that I made progress on RV I can
get back to it).

-- Daniel