Re: [PATCH 1/2] sched_ext: Add lazy preemption support

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] sched_ext: redundant idle kick processing for lazy preemptions
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commit 42fa214a424ea8999256e25137769c2bfce81537
Author: Andrea Righi <arighi@nvidia.com>
sched_ext: Add lazy preemption support

This commit adds lazy preemption support to sched_ext, allowing BPF
schedulers to request deferred rescheduling. It introduces new enqueue and
kick flags for lazy preemption, and updates the task slice expiry logic.

> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index f875c50f00bbe..819995667cc26 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
[ ... ]
> @@ -8574,6 +8626,14 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
>  			cpumask_clear_cpu(cpu, pcpu->cpus_to_kick_if_idle);
>  		}
>  
> +		/*
> +		 * kick_one_cpu() clears the lazy bit of every cpu it visited
> +		 * above; visit the remaining requests which contain lazy
> +		 * preemption, see scx_kick_cpu().
> +		 */
> +		for_each_cpu(cpu, pcpu->cpus_to_preempt_lazy)
> +			should_wait |= kick_one_cpu(cpu, pcpu, this_rq, ksyncs);
> +

[Severity: Low]
Does this loop in kick_cpus_irq_workfn() intentionally skip clearing
cpus_to_kick_if_idle for the CPUs it processes?

If a BPF scheduler issues both an idle kick and a lazy preemption kick on
the same target within a narrow window, but without a non-lazy kick, the
target CPU is serviced by this loop. 

Because this loop fails to clear cpus_to_kick_if_idle for the visited CPU,
the subsequent idle loop immediately below it will redundantly lock and
evaluate the same CPU again. 

Could this cause a minor performance inefficiency due to the redundant
evaluation, and should cpumask_clear_cpu(cpu, pcpu->cpus_to_kick_if_idle)
be added here as well?

>  		for_each_cpu(cpu, pcpu->cpus_to_kick_if_idle) {
>  			kick_one_cpu_if_idle(cpu, pcpu, this_rq);
>  			cpumask_clear_cpu(cpu, pcpu->cpus_to_kick_if_idle);
>  		}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260915194611.2674127-1-arighi@nvidia.com?part=1