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

[Will Deacon <will@kernel.org>]

On Thu, May 20, 2021 at 02:38:55PM +0200, Daniel Bristot de Oliveira wrote:
> On 5/20/21 12:33 PM, Quentin Perret wrote:
> > On Thursday 20 May 2021 at 11:16:41 (+0100), Will Deacon wrote:
> >> Ok, thanks for the insight. In which case, I'll go with what we discussed:
> >> require admission control to be disabled for sched_setattr() but allow
> >> execve() to a 32-bit task from a 64-bit deadline task with a warning (this
> >> is probably similar to CPU hotplug?).
> > 
> > Still not sure that we can let execve go through ... It will break AC
> > all the same, so it should probably fail as well if AC is on IMO
> > 
> 
> If the cpumask of the 32-bit task is != of the 64-bit task that is executing it,
> the admission control needs to be re-executed, and it could fail. So I see this
> operation equivalent to sched_setaffinity(). This will likely be true for future
> schedulers that will allow arbitrary affinities (AC should run on affinity
> change, and could fail).
> 
> I would vote with Juri: "I'd go with fail hard if AC is on, let it
> pass if AC is off (supposedly the user knows what to do)," (also hope nobody
> complains until we add better support for affinity, and use this as a motivation
> to get back on this front).

I can have a go at implementing it, but I don't think it's a great solution
and here's why:

Failing an execve() is _very_ likely to be fatal to the application. It's
also very likely that the task calling execve() doesn't know whether the
program it's trying to execute is 32-bit or not. Consequently, if we go
with failing execve() then all that will happen is that people will disable
admission control altogether. That has a negative impact on "pure" 64-bit
applications and so I think we end up with the tail wagging the dog because
admission control will be disabled for everybody just because there is a
handful of 32-bit programs which may get executed. I understand that it
also means that RT throttling would be disabled.

Allowing the execve() to continue with a warning is very similar to the
case in which all the 64-bit CPUs are hot-unplugged at the point of
execve(), and this is much closer to the illusion that this patch series
intends to provide.

So, personally speaking, I would prefer the behaviour where we refuse to
admit 32-bit tasks vioa sched_set_attr() if the root domain contains
64-bit CPUs, but we _don't_ fail execve() of a 32-bit program from a
64-bit deadline task.

However, you're the deadline experts so ultimately I'll implement what
you prefer. I just wanted to explain why I think it's a poor interface.

Have I changed anybody's mind?

Will