Re: [PATCH] PCI/AER: Use IRQF_NO_THREAD on aer_irq

[Lukas Wunner <lukas@wunner.de>]

On Thu, Sep 04, 2025 at 03:27:40PM -0500, Crystal Wood wrote:
> On Thu, 2025-09-04 at 14:48 +0200, Lukas Wunner wrote:
> > On Thu, Sep 04, 2025 at 09:30:24AM +0200, Sebastian Andrzej Siewior wrote:
> > > Another way would be to let the secondary handler run at a slightly lower
> > > priority than the primary handler. In this case making the primary
> > > non-threaded should not cause any harm.
> > 
> > Why isn't the secondary handler always assigned a lower priority
> > by default?  I think a lot of drivers are built on the assumption
> > that the primary handler is scheduled sooner than the secondary
> > handler.
> 
> That also works, and I agree it's more intuitive.

Could you test whether the patch below fixes the issue for you?

-- >8 --

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2b27238..ceed23d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1894,6 +1894,7 @@ static inline int task_nice(const struct task_struct *p)
 extern int sched_setscheduler(struct task_struct *, int, const struct sched_param *);
 extern int sched_setscheduler_nocheck(struct task_struct *, int, const struct sched_param *);
 extern void sched_set_fifo(struct task_struct *p);
+extern void sched_set_fifo_minus_one(struct task_struct *p);
 extern void sched_set_fifo_low(struct task_struct *p);
 extern void sched_set_normal(struct task_struct *p, int nice);
 extern int sched_setattr(struct task_struct *, const struct sched_attr *);
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index c948373..b09c18a 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -1239,7 +1239,10 @@ static int irq_thread(void *data)
 
 	irq_thread_set_ready(desc, action);
 
-	sched_set_fifo(current);
+	if (action->handler == irq_forced_secondary_handler)
+		sched_set_fifo_minus_one(current);
+	else
+		sched_set_fifo(current);
 
 	if (force_irqthreads() && test_bit(IRQTF_FORCED_THREAD,
 					   &action->thread_flags))
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 77ae87f..8234223 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -847,6 +847,15 @@ void sched_set_fifo(struct task_struct *p)
 EXPORT_SYMBOL_GPL(sched_set_fifo);
 
 /*
+ * Secondary IRQ handler has slightly lower priority than primary IRQ handler.
+ */
+void sched_set_fifo_minus_one(struct task_struct *p)
+{
+	struct sched_param sp = { .sched_priority = MAX_RT_PRIO / 2 - 1 };
+	WARN_ON_ONCE(sched_setscheduler_nocheck(p, SCHED_FIFO, &sp) != 0);
+}
+
+/*
  * For when you don't much care about FIFO, but want to be above SCHED_NORMAL.
  */
 void sched_set_fifo_low(struct task_struct *p)