[PATCH] softirq: allow raising NET_RX_SOFTIRQ from SMP call on RT

[wen.yang@linux.dev]

From: Wen Yang <wen.yang@linux.dev>

RPS distributes network processing load across CPUs by enqueuing
packets on a remote CPU's backlog and raising NET_RX_SOFTIRQ to
process them. This is done via an smp_call_function.

On PREEMPT_RT kernels, raising a softirq from an SMP call function
can increase preemption-off latencies. A WARN_ON_ONCE() in
do_softirq_post_smp_call_flush() enforces this.

A previous commit 6675ce20046d ("softirq: Allow raising SCHED_SOFTIRQ from SMP-call-function on RT kernel")
allowed SCHED_SOFTIRQ to be raised for idle load balancing purposes.
A similar issue exists for RPS. When RPS targets a remote CPU, it uses
an SMP call to raise NET_RX_SOFTIRQ, which triggers the warning.

This patch extends the exception to also allow NET_RX_SOFTIRQ, preventing
the warning when RPS is active on an RT kernel.

Signed-off-by: Wen Yang <wen.yang@linux.dev>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Ryo Takakura <ryotkkr98@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: linux-kernel@vger.kernel.org
---
 kernel/softirq.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 5f435c1e48d8..53cccdb29a8f 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -341,15 +341,15 @@ static inline void invoke_softirq(void)
 		wakeup_softirqd();
 }
 
-#define SCHED_SOFTIRQ_MASK	BIT(SCHED_SOFTIRQ)
+#define SCHED_SOFTIRQ_MASK	(BIT(SCHED_SOFTIRQ) | BIT(NET_RX_SOFTIRQ))
 
 /*
  * flush_smp_call_function_queue() can raise a soft interrupt in a function
  * call. On RT kernels this is undesired and the only known functionalities
  * are in the block layer which is disabled on RT, and in the scheduler for
  * idle load balancing. If soft interrupts get raised which haven't been
- * raised before the flush, warn if it is not a SCHED_SOFTIRQ so it can be
- * investigated.
+ * raised before the flush, warn if it is not SCHED_SOFTIRQ or NET_RX_SOFTIRQ
+ * so it can be investigated.
  */
 void do_softirq_post_smp_call_flush(unsigned int was_pending)
 {
-- 
2.25.1