linux-kernel - [PATCH sched_ext/for-6.19] sched_ext: Pass locked CPU parameter to scx

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Message-ID: <aRaG9X9xfS_QL2QF@slm.duckdns.org>
Date: Thu, 13 Nov 2025 15:33:41 -1000
From: Tejun Heo <tj@...nel.org>
To: Doug Anderson <dianders@...omium.org>
Cc: David Vernet <void@...ifault.com>,
	Andrea Righi <andrea.righi@...ux.dev>,
	Changwoo Min <changwoo@...lia.com>,
	Dan Schatzberg <schatzberg.dan@...il.com>,
	Emil Tsalapatis <etsal@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Righi <arighi@...dia.com>
Subject: [PATCH sched_ext/for-6.19] 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 <dianders@...omium.org>
Signed-off-by: Tejun Heo <tj@...nel.org>
---
 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. */