linux-kernel - Re: [PATCH v5 3/3] sched_ext: deprecation warn for scx_bpf_cpu

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Message-ID: <aLhlPaa_lLFPnX2A@slm.duckdns.org>
Date: Wed, 3 Sep 2025 05:56:45 -1000
From: Tejun Heo <tj@...nel.org>
To: Christian Loehle <christian.loehle@....com>
Cc: arighi@...dia.com, void@...ifault.com, linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev, changwoo@...lia.com, hodgesd@...a.com,
	mingo@...hat.com, peterz@...radead.org, jake@...lion.co.uk
Subject: Re: [PATCH v5 3/3] sched_ext: deprecation warn for scx_bpf_cpu_rq()

On Mon, Sep 01, 2025 at 02:26:04PM +0100, Christian Loehle wrote:
> scx_bpf_cpu_rq() works on an unlocked rq which generally isn't safe.
> For the common use-cases scx_bpf_cpu_rq_locked() and
> scx_bpf_remote_curr() work, so add a deprecation warning
> to scx_bpf_cpu_rq() so it can eventually be removed.
> 
> Signed-off-by: Christian Loehle <christian.loehle@....com>
> ---
>  kernel/sched/ext.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index e242a2520f06..a524d404ad09 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -7427,6 +7427,9 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
>  	if (!kf_cpu_valid(cpu, NULL))
>  		return NULL;
>  
> +	pr_warn_once("%s() is deprecated; use scx_bpf_cpu_rq_locked() when holding rq lock "
> +		     "or scx_bpf_remote_curr() to read remote curr safely.\n", __func__);
> +

I wonder whether it'd make more sense to tie the once testing to the
scheduler instance (see warned_zero_slice).

Thanks.

-- 
tejun