netpoll: Optimize skb refilling on critical path

netpoll tries to refill the skb queue on every packet send, independently
if packets are being consumed from the pool or not. This was
particularly problematic while being called from printk(), where the
operation would be done while holding the console lock.

Introduce a more intelligent approach to skb queue management. Instead
of constantly attempting to refill the queue, the system now defers
refilling to a work queue and only triggers the workqueue when a buffer
is actually dequeued. This change significantly reduces operations with
the lock held.

Add a work_struct to the netpoll structure for asynchronous refilling,
updating find_skb() to schedule refill work only when necessary (skb is
dequeued).

These changes have demonstrated a 15% reduction in time spent during
netpoll_send_msg operations, especially when no SKBs are not consumed
from consumed from pool.

When SKBs are being dequeued, the improvement is even better, around
70%, mainly because refilling the SKB pool is now happening outside of
the critical patch (with console_owner lock held).

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250304-netpoll_refill_v2-v1-1-06e2916a4642@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netpoll.h

@@ -33,6 +33,7 @@ struct netpoll {
 	u16 local_port, remote_port;
 	u8 remote_mac[ETH_ALEN];
 	struct sk_buff_head skb_pool;
+	struct work_struct refill_wq;
 };
 
 struct netpoll_info {

net/core/netpoll.c

@@ -284,12 +284,13 @@ static struct sk_buff *find_skb(struct netpoll *np, int len, int reserve)
 	struct sk_buff *skb;
 
 	zap_completion_queue();
-	refill_skbs(np);
 repeat:
 
 	skb = alloc_skb(len, GFP_ATOMIC);
-	if (!skb)
+	if (!skb) {
 		skb = skb_dequeue(&np->skb_pool);
+		schedule_work(&np->refill_wq);
+	}
 
 	if (!skb) {
 		if (++count < 10) {
@@ -535,6 +536,7 @@ static void skb_pool_flush(struct netpoll *np)
 {
 	struct sk_buff_head *skb_pool;
 
+	cancel_work_sync(&np->refill_wq);
 	skb_pool = &np->skb_pool;
 	skb_queue_purge_reason(skb_pool, SKB_CONSUMED);
 }
@@ -621,6 +623,14 @@ int netpoll_parse_options(struct netpoll *np, char *opt)
 }
 EXPORT_SYMBOL(netpoll_parse_options);
 
+static void refill_skbs_work_handler(struct work_struct *work)
+{
+	struct netpoll *np =
+		container_of(work, struct netpoll, refill_wq);
+
+	refill_skbs(np);
+}
+
 int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 {
 	struct netpoll_info *npinfo;
@@ -666,6 +676,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 
 	/* fill up the skb queue */
 	refill_skbs(np);
+	INIT_WORK(&np->refill_wq, refill_skbs_work_handler);
 
 	/* last thing to do is link it to the net device structure */
 	rcu_assign_pointer(ndev->npinfo, npinfo);