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

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Message-ID: <aJof5QVDR7x-TePI@slm.duckdns.org>
Date: Mon, 11 Aug 2025 06:52:53 -1000
From: Tejun Heo <tj@...nel.org>
To: Christian Loehle <christian.loehle@....com>
Cc: Andrea Righi <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
Subject: Re: [PATCH v3 3/3] sched_ext: Guarantee rq lock on scx_bpf_cpu_rq()

Hello,

On Sun, Aug 10, 2025 at 11:18:52PM +0100, Christian Loehle wrote:
...
> > 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.

I'll revert that change. We shouldn't be introducing breaking changes
without grace period.

> > 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()?
> 
> If we still trigger an error in scx_bpf_cpu_rq() what's the difference
> between scx_bpf_cpu_rq() and scx_bpf_cpu_rq_locked() then?
> Just the error message?

Let's add a warning to scx_bpf_cpu_rq() pointing to scx_bpf_cpu_rq_locked(),
update the schedulers, and drop scx_bpf_cpu_rq() in a couple releases.

Thanks.

-- 
tejun