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

[Andrea Righi <arighi@nvidia.com>]

Hi Breno,

On Wed, Jul 16, 2025 at 03:47:38AM -0700, Breno Leitao wrote:
> 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:

Looks good to me, feel free to add my:

Acked-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> 
> 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();
>  }
>  
>  /*