Re: [PATCHSET sched_ext/for-7.1] sched_ext: Implement SCX_ENQ_IMMED

[Tejun Heo <tj@kernel.org>]

Hello,

On Sat, Mar 07, 2026 at 11:36:46PM +0100, Andrea Righi wrote:
> This looks interesting, but I'm trying to understand the typical use case
> of this feature.
> 
> I agree that we need some kernel support to "atomically" determine when a
> CPU is available (it can't be done fully in BPF). Initially I thought the
> main target for ENQ_IMMED was to improve latency-sensitive workloads, but
> this actually hurts latency, due to the additional re-enqueue cost and in
> this case it might be better to be "less perfect" and not use ENQ_IMMED.

I don't see how it'd worsen latency. You atomically get the CPU or not. If
you don't, the only thing you can do is reenqueueing to find an alternate
cpu if available. If you don't do that, the task would end up waiting for
the CPU which is now busy doing something else to open up in the local DSQ.

Thanks.

-- 
tejun