From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: smp: Wake ksoftirqd on PREEMPT_RT instead do_softirq().
Date: Mon, 27 Sep 2021 09:38:14 +0200

The softirq implementation on PREEMPT_RT does not provide do_softirq().
The other user of do_softirq() is replaced with a local_bh_disable()
+ enable() around the possible raise-softirq invocation. This can not be
done here because migration_cpu_stop() is invoked with disabled
preemption.

Wake the softirq thread on PREEMPT_RT if there are any pending softirqs.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20210927073814.x5h6osr4dgiu44sc@linutronix.de
---
v1…v2: Drop an empty line.

 kernel/smp.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)
---
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -690,10 +690,20 @@ void flush_smp_call_function_from_idle(v
 
 	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->idle, CFD_SEQ_NOCPU,
 		      smp_processor_id(), CFD_SEQ_IDLE);
+
 	local_irq_save(flags);
 	flush_smp_call_function_queue(true);
-	if (local_softirq_pending())
-		do_softirq();
+
+	if (local_softirq_pending()) {
+		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			do_softirq();
+		} else {
+			struct task_struct *ksoftirqd = this_cpu_ksoftirqd();
+
+			if (ksoftirqd && !task_is_running(ksoftirqd))
+				wake_up_process(ksoftirqd);
+		}
+	}
 
 	local_irq_restore(flags);
 }