Re: [PATCH 2/5] sched_ext: Allow only user DSQs for scx_bpf_consume(), scx_bpf_dsq_nr_queued() and bpf_iter_scx_dsq_new()

[David Vernet <void@manifault.com>]

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On Wed, Sep 25, 2024 at 11:04:15AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Wed, Sep 25, 2024 at 12:09:56PM -0500, David Vernet wrote:
> > On Tue, Sep 24, 2024 at 02:06:04PM -1000, Tejun Heo wrote:
> ...
> > This API impedance where you can dispatch but not consume feels unnatural, and
> > a bit leaky. I understand why we don't want to allow BPF to consume it -- we're
> > already doing it for the user as part of (and before) the dispatch loop. That's
> > also one-off logic that's separate from the normal interface for DSQs though,
> > and because of that, SCX_DSQ_GLOBAL imposes a cognitive overhead that IMO
> > arguably outweighs the convenience it provides.
> 
> I don't know. One can also argue that this makes all built-in DSQs behave
> more consistently as the local DSQs can only be dispatched to too. Either
> way, I don't think it makes meaningful differences.

That's a good point r.e. making it consistent with local. I don't think it
makes a big difference one way or the other, but this is something that folks
seem to get consistently confused about. To your point, maybe that won't be the
case now that you can't consume.

> > I'm still of the opinion that we should just hide SCX_DSQ_GLOBAL from the user
> > altogether. It makes sense why we'd need it as a backup DSQ for when we're e.g.
> > unloading the scheduler, but as a building block that's provided by the kernel
> > to the user, I'm not sure. In a realistic production scenario where you're
> > doing something like running a scheduler that's latency sensitive and cares
> > about deadlines, I'm not sure it would be viable or ever the optimal decision
> > to throw the task in a global DSQ and tolerate it being consumed before other
> > higher-priority tasks that were enqueued in normal DSQs. Or at the very least,
> > I could see users being surprised by the semantics, and having instead expected
> > it to function as just a built-in / pre-created DSQ that functions normally
> > otherwise.
> 
> Maybe we can further block it in the future but it doesn't seem material
> either way and I tend to prefer not putting extra restrictions unless
> necessary.

Fine with me, patch LG otherwise:

Acked-by: David Vernet <void@manifault.com>

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