Re: [PATCH] sched/ext: Suppress warning in __this_cpu_write() by disabling preemption

[Peter Zijlstra]

On Wed, Jul 16, 2025 at 08:54:47AM -0400, Steven Rostedt wrote:
> On Wed, 16 Jul 2025 05:46:15 -0700
> Breno Leitao <leitao@xxxxxxxxxx> wrote:
> 
> > __this_cpu_write() emits a warning if used with preemption enabled.
> > 
> > Function update_locked_rq() might be called with preemption enabled,
> > which causes the following warning:
> > 
> > 	BUG: using __this_cpu_write() in preemptible [00000000] code: scx_layered_6-9/68770
> > 
> > Disable preemption around the __this_cpu_write() call in
> > update_locked_rq() to suppress the warning, without affecting behavior.
> > 
> > If preemption triggers a  jump to another CPU during the callback it's
> > fine, since we would track the rq state on the other CPU with its own
> > local variable.
> > 
> > Suggested-by: Andrea Righi <arighi@xxxxxxxxxx>
> > Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> > Fixes: 18853ba782bef ("sched_ext: Track currently locked rq")
> > Acked-by: Andrea Righi <arighi@xxxxxxxxxx>
> > ---
> >  kernel/sched/ext.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> > index b498d867ba210..24fcbd7331f73 100644
> > --- a/kernel/sched/ext.c
> > +++ b/kernel/sched/ext.c
> > @@ -1258,7 +1258,14 @@ static inline void update_locked_rq(struct rq *rq)
> >  	 */
> >  	if (rq)
> >  		lockdep_assert_rq_held(rq);
> 
> <blink>
> 
> If an rq lock is expected to be held, there had better be no preemption
> enabled. How is this OK?

The rq=NULL case; but from the usage I've seen that also happens with
rq lock held.

Specifically I think the check ought to be:

	if (rq)
		lockdep_assert_rq_held(rq)
	else
		lockdep_assert_rq_held(__this_cpu_read(locked_rq));