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 Tue, Sep 24, 2024 at 02:06:04PM -1000, Tejun Heo wrote:

Hi Tejun,

> SCX_DSQ_GLOBAL is special in that it can't be used as a priority queue and
> is consumed implicitly, but all BPF DSQ related kfuncs could be used on it.
> SCX_DSQ_GLOBAL will be split per-node for scalability and those operations
> won't make sense anymore. Disallow SCX_DSQ_GLOBAL on scx_bpf_consume(),
> scx_bpf_dsq_nr_queued() and bpf_iter_scx_dsq_new(). This means that
> SCX_DSQ_GLOBAL can only be used as a dispatch target from BPF schedulers.

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'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.

Thanks,
David

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