Re: [PATCH 1/2] net: Assert proper context while calling napi_schedule()

[Breno Leitao]

Hello Jakub,

On Thu, Feb 13, 2025 at 07:14:26AM -0800, Jakub Kicinski wrote:
> ... How about we add an hrtimer to netdevsim,
> schedule it to fire 5usec in the future instead of scheduling NAPI
> immediately? We can call napi_schedule() from a timer safely.

I hacked a way to do so. Is this what you had in mind?

Author: Breno Leitao <leitao@xxxxxxxxxx>
Date:   Wed Feb 12 09:50:51 2025 -0800

    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 #08!!!
    
    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 <kuba@xxxxxxxxxx>
    Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index 42f247cbdceec..cd56904a39049 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -87,7 +87,7 @@ 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);
+	hrtimer_start(&rq->napi_timer, ns_to_ktime(5), HRTIMER_MODE_REL);
 
 	rcu_read_unlock();
 	u64_stats_update_begin(&ns->syncp);
@@ -426,6 +426,25 @@ 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);
+	/* TODO: Should HRTIMER_RESTART be returned if napi_schedule returns
+	 * false?
+	 */
+
+	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;
@@ -436,6 +455,7 @@ static void nsim_enable_napi(struct netdevsim *ns)
 
 		netif_queue_set_napi(dev, i, NETDEV_QUEUE_TYPE_RX, &rq->napi);
 		napi_enable(&rq->napi);
+		nsim_rq_timer_init(rq);
 	}
 }
 
@@ -461,6 +481,7 @@ static void nsim_del_napi(struct netdevsim *ns)
 	for (i = 0; i < dev->num_rx_queues; i++) {
 		struct nsim_rq *rq = ns->rq[i];
 
+		hrtimer_cancel(&rq->napi_timer);
 		napi_disable(&rq->napi);
 		__netif_napi_del(&rq->napi);
 	}
diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
index dcf073bc4802e..2b396c517ac1d 100644
--- a/drivers/net/netdevsim/netdevsim.h
+++ b/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 {