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

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Message-ID: <aAQH6Q9tTmLvozyv@gpd3>
Date: Sat, 19 Apr 2025 22:30:33 +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 10:10:13PM +0200, Andrea Righi wrote:
> 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.

Hm... actually thinking more about this, a problem with the percpu variable
is that, if no rq is locked, we could move to a different CPU and end up
reading the wrong rq_locked via scx_locked_rq(). I don't think we want to
preempt_disable/enable all the callbacks just to fix this... Maybe storing
in current is a safer choice?

Thanks,
-Andrea