Re: [PATCH sched_ext/for-7.1] sched_ext: Reduce DSQ lock contention in consume_dispatch_q()

[Tejun Heo <tj@kernel.org>]

Hello, Andrea.

On Sun, Mar 15, 2026 at 12:52:31AM +0100, Andrea Righi wrote:
...
> Benchmarks that generate many enqueue/dispatch events (e.g., schbench)
> show around 2-3x higher throughput with most of the scx schedulers with
> this change applied.

Can you share more details about the benchmark setup and results?

> +	/*
> +	 * Use trylock to avoid spinning on a contended DSQ, if we fail to
> +	 * acquire the lock kick the CPU to retry on the next balance.
> +	 *
> +	 * In bypass mode simply spin to acquire the lock, since
> +	 * scx_kick_cpu() is suppressed.
> +	 */
> +	if (scx_bypassing(sch, cpu)) {
> +		raw_spin_lock(&dsq->lock);
> +	} else if (!raw_spin_trylock(&dsq->lock)) {
> +		scx_kick_cpu(sch, cpu, 0);
> +		return false;
> +	}

But I'm not sure this is what we wanna do. If we *really* want to do this,
maybe we can add a try_move variant; however, I'm pretty deeply skeptical
about the approach for a few reasons.

- If a shared DSQ becomes a bottleneck, the right thing to do would be
  introducing multiple DSQs and shard them.

- This likely is trading off fairness to gain bandwidth and this approach
  depending on machine / workload may lead to severe starvation. One can
  argue that controlled trade off between fairness and bandwidth is useful
  for some use cases. However, even if that is the case, I don't think
  trylock is the way to get there. If we think that low overhead high
  fan-out shared queue is desirable, it'd be better to introduce dedicated
  data structure which can do so in a controlled manner.

Thakns.

-- 
tejun