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

[Will Deacon <will@kernel.org>]

On Fri, May 21, 2021 at 03:00:42PM +0200, Daniel Bristot de Oliveira wrote:
> 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).

That's a really interesting point that I hadn't previously considered. It
means that the effects of 32-bit tasks with forced affinity are far
reaching when it comes to deadline 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.

I've implemented AC on execve() of a 32-bit program so we'll fail that system
call with -ENOEXEC if the root domain contains 64-bit-only CPUs. As expected,
the failure mode is awful because it seems as though the ELF binary is then
treated like a shell script by userspace and passed to /bin/sh:

$ sudo chrt -d -T 5000000 -P 16666666 0 ./hello32
./hello32: 1: Syntax error: word unexpected (expecting ")")

Anyway, I'll include this in v7.

Cheers,

Will