netdevsim: call napi_schedule from a timer context

The netdevsim driver was experiencing NOHZ tick-stop errors during packet
transmission due to pending softirq work when calling napi_schedule().
This issue was observed when running the netconsole selftest, which
triggered the following error message:

  NOHZ tick-stop error: local softirq work is pending, handler #8!!!

To fix this issue, introduce a timer that schedules napi_schedule()
from a timer context instead of calling it directly from the TX path.

Create an hrtimer for each queue and kick it from the TX path,
which then schedules napi_schedule() from the timer context.

Suggested-by: Jakub Kicinski <[email protected]>
Signed-off-by: Breno Leitao <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: leitao

drivers/net/netdevsim/netdev.c

@@ -87,7 +87,8 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (unlikely(nsim_forward_skb(peer_dev, skb, rq) == NET_RX_DROP))
 		goto out_drop_cnt;
 
-	napi_schedule(&rq->napi);
+	if (!hrtimer_active(&rq->napi_timer))
+		hrtimer_start(&rq->napi_timer, us_to_ktime(5), HRTIMER_MODE_REL);
 
 	rcu_read_unlock();
 	u64_stats_update_begin(&ns->syncp);
@@ -426,6 +427,22 @@ static int nsim_init_napi(struct netdevsim *ns)
 	return err;
 }
 
+static enum hrtimer_restart nsim_napi_schedule(struct hrtimer *timer)
+{
+	struct nsim_rq *rq;
+
+	rq = container_of(timer, struct nsim_rq, napi_timer);
+	napi_schedule(&rq->napi);
+
+	return HRTIMER_NORESTART;
+}
+
+static void nsim_rq_timer_init(struct nsim_rq *rq)
+{
+	hrtimer_init(&rq->napi_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	rq->napi_timer.function = nsim_napi_schedule;
+}
+
 static void nsim_enable_napi(struct netdevsim *ns)
 {
 	struct net_device *dev = ns->netdev;
@@ -615,11 +632,13 @@ static struct nsim_rq *nsim_queue_alloc(void)
 		return NULL;
 
 	skb_queue_head_init(&rq->skb_queue);
+	nsim_rq_timer_init(rq);
 	return rq;
 }
 
 static void nsim_queue_free(struct nsim_rq *rq)
 {
+	hrtimer_cancel(&rq->napi_timer);
 	skb_queue_purge_reason(&rq->skb_queue, SKB_DROP_REASON_QUEUE_PURGE);
 	kfree(rq);
 }

drivers/net/netdevsim/netdevsim.h

@@ -97,6 +97,7 @@ struct nsim_rq {
 	struct napi_struct napi;
 	struct sk_buff_head skb_queue;
 	struct page_pool *page_pool;
+	struct hrtimer napi_timer;
 };
 
 struct netdevsim {