Re: [PATCH sched_ext/for-6.19] sched_ext: Pass locked CPU parameter to scx_hardlockup() and add docs

[Andrea Righi <arighi@nvidia.com>]

On Thu, Nov 13, 2025 at 03:33:41PM -1000, Tejun Heo wrote:
> With the buddy lockup detector, smp_processor_id() returns the detecting CPU,
> not the locked CPU, making scx_hardlockup()'s printouts confusing. Pass the
> locked CPU number from watchdog_hardlockup_check() as a parameter instead.
> 
> Also add kerneldoc comments to handle_lockup(), scx_hardlockup(), and
> scx_rcu_cpu_stall() documenting their return value semantics.
> 
> Suggested-by: Doug Anderson <dianders@chromium.org>
> Signed-off-by: Tejun Heo <tj@kernel.org>

Makes sense.

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

Thanks,
-Andrea

> ---
>  include/linux/sched/ext.h |    4 ++--
>  kernel/sched/ext.c        |   25 ++++++++++++++++++++++---
>  kernel/watchdog.c         |    2 +-
>  3 files changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
> index 70ee5c28a74d..bcb962d5ee7d 100644
> --- a/include/linux/sched/ext.h
> +++ b/include/linux/sched/ext.h
> @@ -230,7 +230,7 @@ struct sched_ext_entity {
>  void sched_ext_dead(struct task_struct *p);
>  void print_scx_info(const char *log_lvl, struct task_struct *p);
>  void scx_softlockup(u32 dur_s);
> -bool scx_hardlockup(void);
> +bool scx_hardlockup(int cpu);
>  bool scx_rcu_cpu_stall(void);
> 
>  #else	/* !CONFIG_SCHED_CLASS_EXT */
> @@ -238,7 +238,7 @@ bool scx_rcu_cpu_stall(void);
>  static inline void sched_ext_dead(struct task_struct *p) {}
>  static inline void print_scx_info(const char *log_lvl, struct task_struct *p) {}
>  static inline void scx_softlockup(u32 dur_s) {}
> -static inline bool scx_hardlockup(void) { return false; }
> +static inline bool scx_hardlockup(int cpu) { return false; }
>  static inline bool scx_rcu_cpu_stall(void) { return false; }
> 
>  #endif	/* CONFIG_SCHED_CLASS_EXT */
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 8a3b8f64a06b..918573f3f088 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3687,6 +3687,17 @@ bool scx_allow_ttwu_queue(const struct task_struct *p)
>  	return false;
>  }
> 
> +/**
> + * handle_lockup - sched_ext common lockup handler
> + * @fmt: format string
> + *
> + * Called on system stall or lockup condition and initiates abort of sched_ext
> + * if enabled, which may resolve the reported lockup.
> + *
> + * Returns %true if sched_ext is enabled and abort was initiated, which may
> + * resolve the lockup. %false if sched_ext is not enabled or abort was already
> + * initiated by someone else.
> + */
>  static __printf(1, 2) bool handle_lockup(const char *fmt, ...)
>  {
>  	struct scx_sched *sch;
> @@ -3718,6 +3729,10 @@ static __printf(1, 2) bool handle_lockup(const char *fmt, ...)
>   * that may not be caused by the current BPF scheduler, try kicking out the
>   * current scheduler in an attempt to recover the system to a good state before
>   * issuing panics.
> + *
> + * Returns %true if sched_ext is enabled and abort was initiated, which may
> + * resolve the reported RCU stall. %false if sched_ext is not enabled or someone
> + * else already initiated abort.
>   */
>  bool scx_rcu_cpu_stall(void)
>  {
> @@ -3750,14 +3765,18 @@ void scx_softlockup(u32 dur_s)
>   * numerous affinitized tasks in a single queue and directing all CPUs at it.
>   * 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 hardlockdup. %false if sched_ext is not enabled or
> + * someone else already initiated abort.
>   */
> -bool scx_hardlockup(void)
> +bool scx_hardlockup(int cpu)
>  {
> -	if (!handle_lockup("hard lockup - CPU %d", smp_processor_id()))
> +	if (!handle_lockup("hard lockup - CPU %d", cpu))
>  		return false;
> 
>  	printk_deferred(KERN_ERR "sched_ext: Hard lockup - CPU %d, disabling BPF scheduler\n",
> -			smp_processor_id());
> +			cpu);
>  	return true;
>  }
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 8dfac4a8f587..873020a2a581 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -203,7 +203,7 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
>  		 * only once when sched_ext is enabled and will immediately
>  		 * abort the BPF scheduler and print out a warning message.
>  		 */
> -		if (scx_hardlockup())
> +		if (scx_hardlockup(cpu))
>  			return;
> 
>  		/* Only print hardlockups once. */