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

Andrea Righi
2025-04-19T20:30:41Z

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