[PATCH 4/5] sched_ext: Abort directly from the hardlockup handler

[Tejun Heo <tj@kernel.org>]

scx_hardlockup() defers the abort to an irq_work because exit claiming used
to take scx_sched_lock and couldn't run from NMI. The deferral is now
unnecessary - claiming is NMI-safe and asserting ->aborting is exactly what
breaks the live-locks that hard-lock CPUs. Call handle_lockup() directly and
drop the irq_work. This also makes the self-detected case recoverable: the
perf watchdog fires on the hard-locked CPU itself, where a queued irq_work
never runs with IRQs off.

Also fix the return value: %true used to be returned whenever sched_ext was
loaded, suppressing the kernel's hardlockup report even when the abort was
refused. Return %true only when this call initiated the abort.

Fixes: bd2d76455b65 ("sched_ext: Defer scx_hardlockup() out of NMI")
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext/ext.c | 35 +++++++++--------------------------
 1 file changed, 9 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 4338dc6a3f3a..9f5729c2c5e2 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -5382,25 +5382,6 @@ void scx_softlockup(u32 dur_s)
 			cpu, 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(cpu, "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
  * @cpu: the target CPU
@@ -5410,19 +5391,21 @@ static DEFINE_IRQ_WORK(scx_hardlockup_irq_work, scx_hardlockup_irq_workfn);
  * Try kicking out the current scheduler in an attempt to recover the system to
  * a good state before taking more drastic actions.
  *
- * Queues an irq_work; the handle_lockup() call happens in IRQ context (see
- * scx_hardlockup_irq_workfn).
+ * Called from NMI. Aborting the scheduler sets ->aborting throughout the
+ * hierarchy before returning, which is what breaks the dispatch-path live-locks
+ * that can hard-lock CPUs.
  *
- * Returns %true if sched_ext is enabled and the work was queued, %false
- * otherwise.
+ * 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.
  */
 bool scx_hardlockup(int cpu)
 {
-	if (!rcu_access_pointer(scx_root))
+	if (!handle_lockup(cpu, "hard lockup - CPU %d", cpu))
 		return false;
 
-	atomic_cmpxchg(&scx_hardlockup_cpu, -1, cpu);
-	irq_work_queue(&scx_hardlockup_irq_work);
+	printk_deferred(KERN_ERR "sched_ext: Hard lockup - CPU %d, disabling BPF scheduler\n",
+			cpu);
 	return true;
 }
 
-- 
2.55.0