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@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v3:
- Move the timer initialization and cancel close to the queue
  allocation/free (Jakub)
- Link to v2: https://lore.kernel.org/r/20250217-netdevsim-v2-1-fc7fe177b98f@debian.org

Changes in v2:
- The approach implemented in v1 will not work, given that
  ndo_start_xmit() can be called with interrupt disable, and calling
  local_bh_enable() inside that function has nasty side effected.
  Jakub suggested creating a timer and calling napi_schedule() from that
  timer. 
- Link to v1: https://lore.kernel.org/r/20250212-netdevsim-v1-1-20ece94daae8@debian.org
---
 drivers/net/netdevsim/netdev.c    | 21 ++++++++++++++++++++-
 drivers/net/netdevsim/netdevsim.h |  1 +
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index 9b394ddc5206a7a5ca5440341551aac50c43e20c..a41dc79e9c2e082367af156b10b61f04be8c41fb 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/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);
 }
diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
index 96d54c08043d3a62b0731efd43bc6a313998bf01..e757f85ed8617bb13ed0bf0e367803e4ddbd8e95 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 {

---
base-commit: 0784d83df3bfc977c13252a0599be924f0afa68d
change-id: 20250212-netdevsim-258d2d628175

Best regards,
-- 
Breno Leitao <leitao@debian.org>

On Wed, Feb 19, 2025 at 08:41:20AM -0800, Breno Leitao wrote:
> 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@kernel.org>
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---

Looking at the tests, 3 of them are failing:

https://netdev.bots.linux.dev/flakes.html


2/3 passed when retried and just one of them (ip6gre-custom-multipath-hash-sh) failed
also on the retry.

Looking at the flakes, I see that ip6gre-custom-multipath-hash-sh was
flake during yesterday:

	https://netdev.bots.linux.dev/flakes.html?min-flip=0&tn-needle=ip6gre-custom-multipath-hash-sh

I've testd manually it, and the tests is passing:

	# vng -v --run . --user root --cpus 4 --
		make -C tools/testing/selftests TARGETS=net/forwarding TEST_PROGS=ip6gre_custom_multipath_hash.sh TEST_GEN_PROGS="" run_tests

	...

	ok 1 selftests: net/forwarding: ip6gre_custom_multipath_hash.sh


So, from a NIPA testing perspective, it seems the patch is good