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

[Juri Lelli <juri.lelli@redhat.com>]

On 20/05/21 19:01, Will Deacon wrote:
> 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.

Completely understand your perplexity. But how can the kernel still give
guarantees to "pure" 64-bit applications if there are 32-bit
applications around that essentially broke admission control when they
were restricted to a subset of cores?

> 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, for hotplug we currently have a check that would make hotplug
operations fail if removing a CPU would mean not enough bandwidth to run
the currently admitted set of DEADLINE tasks.

> 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.

OK, this is interesting and I guess a very valid alternative. That would
force users to create exclusive domains for 32-bit tasks, right?

> 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?

Partly! :)

Thanks a lot for the discussion so far.

Juri


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