Re: [PATCH sched_ext/for-7.1-fixes] sched_ext: Defer scx_hardlockup() out of NMI

[Andrea Righi]

On Fri, Apr 24, 2026 at 10:14:32AM -1000, Tejun Heo wrote:
> scx_hardlockup() runs from NMI and eventually calls scx_claim_exit(),
> which takes scx_sched_lock. scx_sched_lock isn't NMI-safe and grabbing
> it from NMI context can lead to deadlocks.
> 
> The hardlockup handler is best-effort recovery and the disable path it
> triggers runs off of irq_work anyway. Move the handle_lockup() call into
> an irq_work so it runs in IRQ context.
> 
> Fixes: ebeca1f930ea ("sched_ext: Introduce cgroup sub-sched support")
> Cc: stable@xxxxxxxxxxxxxxx # v7.1+

Apart from the Cc, as you mentioned. :)

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
>  kernel/sched/ext.c |   33 +++++++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 6 deletions(-)
> 
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -5142,6 +5142,25 @@ void scx_softlockup(u32 dur_s)
>  			smp_processor_id(), dur_s);
>  }
> 
> +/*
> + * scx_hardlockup() runs from NMI and eventually calls scx_claim_exit(),
> + * which takes scx_sched_lock. scx_sched_lock isn't NMI-safe and grabbing
> + * it from NMI context can lead to deadlocks. Defer via irq_work; the
> + * disable path runs off irq_work anyway.
> + */
> +static atomic_t scx_hardlockup_cpu = ATOMIC_INIT(-1);
> +
> +static void scx_hardlockup_irq_workfn(struct irq_work *work)
> +{
> +	int cpu = atomic_xchg(&scx_hardlockup_cpu, -1);
> +
> +	if (cpu >= 0 && handle_lockup("hard lockup - CPU %d", cpu))
> +		printk_deferred(KERN_ERR "sched_ext: Hard lockup - CPU %d, disabling BPF scheduler\n",
> +				cpu);
> +}
> +
> +static DEFINE_IRQ_WORK(scx_hardlockup_irq_work, scx_hardlockup_irq_workfn);
> +
>  /**
>   * scx_hardlockup - sched_ext hardlockup handler
>   *
> @@ -5150,17 +5169,19 @@ void scx_softlockup(u32 dur_s)
>   * Try kicking out the current scheduler in an attempt to recover the system to
>   * a good state before taking more drastic actions.
>   *
> - * Returns %true if sched_ext is enabled and abort was initiated, which may
> - * resolve the reported hardlockup. %false if sched_ext is not enabled or
> - * someone else already initiated abort.
> + * Queues an irq_work; the handle_lockup() call happens in IRQ context (see
> + * scx_hardlockup_irq_workfn).
> + *
> + * Returns %true if sched_ext is enabled and the work was queued, %false
> + * otherwise.
>   */
>  bool scx_hardlockup(int cpu)
>  {
> -	if (!handle_lockup("hard lockup - CPU %d", cpu))
> +	if (!rcu_access_pointer(scx_root))
>  		return false;
> 
> -	printk_deferred(KERN_ERR "sched_ext: Hard lockup - CPU %d, disabling BPF scheduler\n",
> -			cpu);
> +	atomic_cmpxchg(&scx_hardlockup_cpu, -1, cpu);
> +	irq_work_queue(&scx_hardlockup_irq_work);
>  	return true;
>  }
>