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

[Andrea Righi <arighi@nvidia.com>]

On Wed, Jul 16, 2025 at 03:06:52PM +0200, Peter Zijlstra wrote:
> 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@debian.org> 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@nvidia.com>
> > > Signed-off-by: Breno Leitao <leitao@debian.org>
> > > Fixes: 18853ba782bef ("sched_ext: Track currently locked rq")
> > > Acked-by: Andrea Righi <arighi@nvidia.com>
> > > ---
> > >  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));

Hm... but if the same CPU invokes two "unlocked" callbacks in a row,
locked_rq would be NULL during the second call and we would check rq_held
against NULL.

-Andrea