linux-kernel - Re: [PATCH 2/2] sched_ext/idle: Make scx_bpf_select_cpu

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Message-ID: <aCItuWh-BLp3jC6h@slm.duckdns.org>
Date: Mon, 12 May 2025 07:19:53 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched_ext/idle: Make scx_bpf_select_cpu_and() usable
 from any context

Hello,

On Mon, May 12, 2025 at 05:14:56PM +0200, Andrea Righi wrote:
>  #ifdef CONFIG_SMP
> +	/*
> +	 * If called from an unlocked context, try to acquire
> +	 * cpus_read_lock() to avoid races with CPU hotplug.
> +	 */
> +	if (scx_kf_allowed_if_unlocked())
> +		if (!cpus_read_trylock())
> +			return -EBUSY;

Is this meaningful? The idle CPU selection is already racy against CPU
hotplugs and we depend on the scheduler core to fix it up afterwards. Even
if scx_bpf_select_cpu_and() is not racy, it will drop the cpus lock before
returning and becomes racy again right there. ie. This doesn't add any
meaningful protection.

Thanks.

-- 
tejun