sched_ext: Pass locked CPU parameter to scx_hardlockup() and add docs

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 <[e-mail]>
Reviewed-by: Douglas Anderson <[e-mail]>
Acked-by: Andrea Righi <[e-mail]>
Reviewed-by: Emil Tsalapatis <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

include/linux/sched/ext.h

···
 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 */
···
 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 */

kernel/sched/ext.c

···
 	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;
···
  * 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)
 {
···
  * 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;
 }
 

kernel/watchdog.c

···
 		 * 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. */