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

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Message-ID: <aAQDIIPOUAU-nB_F@gpd3>
Date: Sat, 19 Apr 2025 22:10:08 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
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

On Sat, Apr 19, 2025 at 07:34:16AM -1000, Tejun Heo wrote:
> 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.

Yeah, I was debating between using a percpu variable or storing it in
current. I went with current just to stay consistent with kf_tasks.

But you're right about not to increasing the size of the task, and as you
pointed out, we can’t switch if the rq is locked, so a percpu variable
should work. I’ll update that in v2.

> 
> > +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.

Right, these are local CPU access only, I'll drop the barrier.

Thanks,
-Andrea