[RFC 1/1] sched/rt: Avoid RTO push to stopped CPUs during shutdown

[wangqk1]

During system poweroff, machine_power_off() invokes smp_send_stop(). A
CPU receiving IPI_CPU_STOP parks without running the CPU hotplug teardown.
Its RT overload state can therefore retain a stale rto_mask bit.

A CPU still executing the RTO irq_work iterator can select that stale bit
and queue work to the stopped CPU.

Observed call trace:
  irq_work_queue_on()
  rto_push_irq_work_func()
  irq_work_single()
  __flush_smp_call_function_queue()
  generic_smp_call_function_single_interrupt()
  ipi_handler()

Stop RTO forwarding after terminal system states begin and skip rto_mask
candidates that are already offline.

This is a workaround, not a complete synchronization protocol. The
cpu_online() test is not atomic with irq_work_queue_on(), and panic or
crash-kexec paths do not set a terminal system_state before stopping CPUs.

Signed-off-by: wangqk1 <wangqk1@xxxxxxxxxxxx>
---
 kernel/sched/rt.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 85303add726d..716ac30c5afb 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2084,6 +2084,14 @@ static void push_rt_tasks(struct rq *rq)
 
 #ifdef HAVE_RT_PUSH_IPI
 
+static inline bool rto_push_allowed(void)
+{
+	enum system_states state = READ_ONCE(system_state);
+
+	return state != SYSTEM_HALT && state != SYSTEM_POWER_OFF &&
+	       state != SYSTEM_RESTART;
+}
+
 /*
  * When a high priority task schedules out from a CPU and a lower priority
  * task is scheduled in, a check is made to see if there's any RT tasks
@@ -2131,6 +2139,11 @@ static int rto_next_cpu(struct root_domain *rd)
 	int next;
 	int cpu;
 
+	if (unlikely(!rto_push_allowed())) {
+		rd->rto_cpu = -1;
+		return -1;
+	}
+
 	/*
 	 * When starting the IPI RT pushing, the rto_cpu is set to -1,
 	 * rt_next_cpu() will simply return the first CPU found in
@@ -2155,8 +2168,13 @@ static int rto_next_cpu(struct root_domain *rd)
 		if (cpu == this_cpu)
 			continue;
 
-		if (cpu < nr_cpu_ids)
+		if (cpu < nr_cpu_ids) {
+			/* smp_send_stop() does not clear stale rto_mask bits. */
+			if (!cpu_online(cpu))
+				continue;
+
 			return cpu;
+		}
 
 		rd->rto_cpu = -1;
 
-- 
2.43.0