linux-kernel - Re: [PATCH 1/2] sched_ext: Track currently locked rq

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Message-ID: <aAPemAFUsJaF_C2X@slm.duckdns.org>
Date: Sat, 19 Apr 2025 07:34:16 -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 1/2] sched_ext: Track currently locked rq

Hello, Andrea.

On Sat, Apr 19, 2025 at 02:24:30PM +0200, Andrea Righi wrote:
> @@ -149,6 +149,7 @@ struct sched_ext_entity {
>  	s32			selected_cpu;
>  	u32			kf_mask;	/* see scx_kf_mask above */
>  	struct task_struct	*kf_tasks[2];	/* see SCX_CALL_OP_TASK() */
> +	struct rq		*locked_rq;	/* currently locked rq */

Can this be a percpu variable? While rq is locked, current can't switch out
anyway and that way we don't have to increase the size of task. Note that
kf_tasks[] are different in that some ops may, at least theoretically,
sleep.

> +static inline void update_locked_rq(struct rq *rq)
> +{
> +	/*
> +	 * Check whether @rq is actually locked. This can help expose bugs
> +	 * or incorrect assumptions about the context in which a kfunc or
> +	 * callback is executed.
> +	 */
> +	if (rq)
> +		lockdep_assert_rq_held(rq);
> +	current->scx.locked_rq = rq;
> +	barrier();

As these conditions are program-order checks on the local CPU, I don't think
any barrier is necessary.

Thanks.

-- 
tejun