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

Breno Leitao
2025-07-16T10:47:43Z

Hello Andrea,

On Tue, Jul 15, 2025 at 07:20:28PM +0200, Andrea Righi wrote:
> > On Tue, Apr 22, 2025 at 10:26:32AM +0200, Andrea Righi wrote:

> >
> > > + lockdep_assert_rq_held(rq);
> > > + __this_cpu_write(locked_rq, rq);
> >
> > This is hitting the following BUG() on some of my debug kernels:
> >
> > BUG: using __this_cpu_write() in preemptible [00000000] code: scx_layered_6-9/68770
> >
> > I have lockdep enabled, and I don't see the assert above. I am wondering
> > if rq is locked but preemption continues to be enabled (!?)
>
> Interesting. And it makes sense, because we may have callbacks called from
> a preemptible context (especially when rq == NULL).
>
> I think we can just put a preempt_disable() / preempt_enable() around
> __this_cpu_write(). If we 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. And if we were able to jump there, it means that preemption was
> disabled as well.

First of all thanks for the suggestion!

What about a patch like the following:

commit 9ed31e914181ec8f2d0b4484c42b00b6794661b9
Author: Breno Leitao <leitao@debian.org>
Date: Wed Jul 16 03:10:59 2025 -0700

sched/ext: Suppress warning in __this_cpu_write() by disabling preemption

__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")

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);
+ /*
+ * __this_cpu_write() emits a warning when used with preemption enabled.
+ * While there's no functional issue if the callback runs on another
+ * CPU, we disable preemption here solely to suppress that warning.
+ */
+ preempt_disable();
__this_cpu_write(locked_rq, rq);
+ preempt_enable();
}

/*