[PATCH] tick/block: Add BLOCK_SOFTIRQ to the hotplug safe mask

[Wang Shuaiwei]

During CPU-hotplug stress testing combined with an I/O
workload, the following message is observed:

  "NOHZ tick-stop error: local softirq work is pending, handler #10!!!"

On CPU offline, ksoftirqd is parked at CPUHP_AP_SMPBOOT_THREADS while
the CPU still takes interrupts. A block completion raised after that
point stays pending, and if the dying CPU goes idle before teardown,
report_idle_softirq() sees it, keeps the tick alive and prints the
error above.

Such pending block completions on the outgoing CPU are drained later by
blk_softirq_cpu_dead() (CPUHP_BLOCK_SOFTIRQ_DEAD), so a pending
BLOCK_SOFTIRQ is harmless here. Add it to SOFTIRQ_HOTPLUG_SAFE_MASK
like the other teardown-covered vectors.

Signed-off-by: Wang Shuaiwei <wangshuaiwei1@xxxxxxxxxx>
---
 include/linux/interrupt.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 3bf969ad8fe0..52bb684090c0 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -573,9 +573,11 @@ enum
  * _ IRQ_POLL: irq_poll_cpu_dead() migrates the queue
  *
  * _ (HR)TIMER_SOFTIRQ: (hr)timers_dead_cpu() migrates the queue
+ *
+ * _ BLOCK_SOFTIRQ: blk_softirq_cpu_dead() completes the remaining requests
  */
-#define SOFTIRQ_HOTPLUG_SAFE_MASK (BIT(TIMER_SOFTIRQ) | BIT(IRQ_POLL_SOFTIRQ) |\
-				   BIT(HRTIMER_SOFTIRQ) | BIT(RCU_SOFTIRQ))
+#define SOFTIRQ_HOTPLUG_SAFE_MASK (BIT(TIMER_SOFTIRQ) | BIT(BLOCK_SOFTIRQ) |\
+				   BIT(IRQ_POLL_SOFTIRQ) | BIT(HRTIMER_SOFTIRQ) | BIT(RCU_SOFTIRQ))
 
 
 /* map softirq index to softirq name. update 'softirq_to_name' in
-- 
2.43.0