[PATCH 3/3] lib/irq_poll: Declare IRQ_POLL softirq vector as ksoftirqd-parking safe

Frederic Weisbecker
2022-02-08T16:16:56Z

The following warning may appear while setting a CPU down:

NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #20!!!

The IRQ_POLL_SOFTIRQ vector can be raised during the hotplug cpu_down()
path after ksoftirqd is parked and before the CPU actually dies. However
this is handled afterward at the CPUHP_IRQ_POLL_DEAD stage where the
queue gets migrated.

Hence this warning can be considered spurious and the vector can join
the "hotplug-safe" list.

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Paul Menzel <pmenzel@molgen.mpg.de>
---
include/linux/interrupt.h | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 9613326d2f8a..f40754caaefa 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -580,12 +580,15 @@ enum
};

/*
- * Ignoring the RCU vector after ksoftirqd is parked is fine
- * because:
- * 1) rcutree_migrate_callbacks() takes care of the queue.
+ * The following vectors can be safely ignored after ksoftirqd is parked:
+ *
+ * _ RCU:
+ * 1) rcutree_migrate_callbacks() migrates the queue.
* 2) rcu_report_dead() reports the final quiescent states.
+ *
+ * _ IRQ_POLL: irq_poll_cpu_dead() migrates the queue
*/
-#define SOFTIRQ_HOTPLUG_SAFE_MASK (BIT(RCU_SOFTIRQ))
+#define SOFTIRQ_HOTPLUG_SAFE_MASK (BIT(RCU_SOFTIRQ) | BIT(IRQ_POLL_SOFTIRQ))

/* map softirq index to softirq name. update 'softirq_to_name' in
* kernel/softirq.c when adding a new softirq.
--
2.25.1