Subject: Re: [PATCH net-next v2 2/3] netconsole: pr_err() when netpoll_setup fails

From: Breno Leitao <[email-masked]>
Date: Wed, 21 Aug 2024 01:41:55 -0700

On Tue, Aug 20, 2024 at 04:24:09PM -0700, Jakub Kicinski wrote:
> On Mon, 19 Aug 2024 03:36:12 -0700 Breno Leitao wrote:
> > netpoll_setup() can fail in several ways, some of which print an error
> > message, while others simply return without any message. For example,
> > __netpoll_setup() returns in a few places without printing anything.
> > 
> > To address this issue, modify the code to print an error message on
> > netconsole if the target is not enabled. This will help us identify and
> > troubleshoot netcnsole issues related to netpoll setup failures
> > more easily.
> 
> Only if memory allocation fails, it seems, and memory allocation
> failures with GFP_KERNEL will be quite noisy.

Or anything that fails in ->ndo_netpoll_setup() and doesn't print
anything else.

Do you think this is useless?

> BTW I looked thru 4 random implementations of ndo_netpoll_setup
> and they look almost identical :S Perhaps they can be refactored?

correct.  This should be refactored.

In fact, since you opened this topic, there are a few things that also
come to my mind

1) Possible reduce refill_skb() work in the critical path (UDP send
path), moving it to a workqueue?

When sending a message, netpoll tries fill the whole skb poll, and then try to
allocate a new skb before sending the packet. 

netconsole needs to write a message, which calls netpoll_send_udp()

	send_ext_msg_udp() {
		netpoll_send_udp() {
			refill_skbs() {
				while (skb_pool.qlen < MAX_SKBS) {
					skb = alloc_skb(MAX_SKB_SIZE, GFP_ATOMIC);
				}
			}
			skb = alloc_skb(len, GFP_ATOMIC);
				if (!skb)
					skb = skb_dequeue(&skb_pool);
			}
		}
	}
		
Would it be better if the hot path just get one of the skbs from the
pool, and refill it in a workqueue? If the skb_poll() is empty, then
alloc_skb(len, GFP_ATOMIC) !?

2) Report statistic back from netpoll_send_udp(). netpoll_send_skb()
return values are being discarded, so, it is hard to know if the packet
was transmitted or got something as NET_XMIT_DROP, NETDEV_TX_BUSY,
NETDEV_TX_OK.

It is unclear where this should be reported two. Maybe a configfs entry?