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

[Tejun Heo <tj@kernel.org>]

Hello, Andrea.

Looks good overall. A few things:

On Thu, Sep 17, 2026 at 09:02:52AM +0200, Andrea Righi wrote:
> +		/*
> +		 * 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);

Can this fold into the loop above with for_each_cpu_or()? As is, a
lazy-only target keeps its cpus_to_kick_if_idle bit and gets locked a second
time by the idle loop.

> +__bpf_kfunc bool scx_bpf_task_set_slice_expiry(struct task_struct *p, bool lazy,
> +					       const struct bpf_prog_aux *aux)

The name reads a bit like it turns slice expiry on and off. Maybe something
like scx_bpf_task_set_lazy_resched(), with the ops flag and the task field
named to match?

> +	if (unlikely(flags & ~(SCX_KICK_IDLE | SCX_KICK_PREEMPT | SCX_KICK_WAIT |
> +			       SCX_KICK_PREEMPT_LAZY))) {
> +		scx_error(sch, "invalid kick flags 0x%llx", flags);
> +		return;
> +	}

Maybe add an enum with all the valid bits and test against that?

> +	if (unlikely((flags & SCX_KICK_IDLE) &&
> +		     (flags & (SCX_KICK_PREEMPT | SCX_KICK_PREEMPT_LAZY | SCX_KICK_WAIT)))) {
> +		scx_error(sch, "PREEMPT/WAIT cannot be used with SCX_KICK_IDLE");
> +		return;
> +	}

Does this need to move out of the IDLE branch? It's two tests on IDLE instead
of one, and as scx_error() aborts the scheduler anyway, the early return
doesn't buy much.

> +	 * ops.enable(), after which scx_bpf_task_set_slice_expiry() may override
> +	 * it per task.
> +	 * A task in user space still reschedules on the way back from the tick; a

Can you add a blank line between the paragraphs?

> + * compatibility wrapper is a no-op when the kfunc is unavailable. As with the
> + * setters above, discard the kfunc's authority result for consistency.
> + *
> + * Keep the wrapper until pre-v7.4 kernels fall out of the sched_ext scheduler
> + * support window.
> + */

Just noting the version it was added in is enough.

Thanks.

-- 
tejun