linux-kernel - Re: [PATCH v3 3/3] sched_ext: Guarantee rq lock on scx_bpf_cpu

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Message-ID: <aJh6BWX7rYCNrzGu@gpd4>
Date: Sun, 10 Aug 2025 12:52:53 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: Christian Loehle <christian.loehle@....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
Subject: Re: [PATCH v3 3/3] sched_ext: Guarantee rq lock on scx_bpf_cpu_rq()

On Sat, Aug 09, 2025 at 09:03:46AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Aug 05, 2025 at 12:10:36PM +0100, Christian Loehle wrote:
> > @@ -7420,10 +7420,20 @@ __bpf_kfunc s32 scx_bpf_task_cpu(const struct task_struct *p)
> >   */
> >  __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
> >  {
> > +	struct rq *rq;
> > +
> >  	if (!kf_cpu_valid(cpu, NULL))
> >  		return NULL;
> >  
> > -	return cpu_rq(cpu);
> > +	preempt_disable();
> > +	rq = cpu_rq(cpu);
> > +	if (rq != scx_locked_rq()) {
> > +		scx_kf_error("Accessing not locked rq %d", cpu);
> > +		rq = NULL;
> > +	}
> > +	preempt_enable();
> 
> So, this will break the existing scheduler binaries immediately, which I
> don't think is a good way to go about it. Can you add a pr_warn_once() to
> print deprecation warning and add e.g. scx_bpf_locked_cpu_rq() instead?

Yeah, this is not nice, but they would be still broken though, in PATCH 1/3
we force schedulers to check for NULL and, if they don't, the verifier
won't be happy, so this already breaks existing binaries.

Even if a scheduler performs the NULL check, this change might still cause
incorrect behaviors, which can be worse than triggering an error.

How about we introduce scx_bpf_locked_cpu_rq() and we still trigger an
error in scx_bpf_cpu_rq(), mentioning about the new locked kfunc and
scx_bpf_task_acquire_remote_curr()?

Thanks,
-Andrea