netdev - Re: [PATCH net-next v3 09/24] ovpn: implement basic TX path (UDP)

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Date: Mon, 13 May 2024 09:37:06 +0200
From: Antonio Quartulli <antonio@...nvpn.net>
To: Sabrina Dubroca <sd@...asysnail.net>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Sergey Ryazanov <ryazanov.s.a@...il.com>, Paolo Abeni <pabeni@...hat.com>,
 Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew@...n.ch>,
 Esben Haabendal <esben@...nix.com>
Subject: Re: [PATCH net-next v3 09/24] ovpn: implement basic TX path (UDP)

On 12/05/2024 23:35, Sabrina Dubroca wrote:
> 2024-05-06, 03:16:22 +0200, Antonio Quartulli wrote:
>> +/* send skb to connected peer, if any */
>> +static void ovpn_queue_skb(struct ovpn_struct *ovpn, struct sk_buff *skb,
>> +			   struct ovpn_peer *peer)
>> +{
>> +	int ret;
>> +
>> +	if (likely(!peer))
>> +		/* retrieve peer serving the destination IP of this packet */
>> +		peer = ovpn_peer_get_by_dst(ovpn, skb);
>> +	if (unlikely(!peer)) {
>> +		net_dbg_ratelimited("%s: no peer to send data to\n",
>> +				    ovpn->dev->name);
>> +		goto drop;
>> +	}
>> +
>> +	ret = ptr_ring_produce_bh(&peer->tx_ring, skb);
>> +	if (unlikely(ret < 0)) {
>> +		net_err_ratelimited("%s: cannot queue packet to TX ring\n",
>> +				    peer->ovpn->dev->name);
>> +		goto drop;
>> +	}
>> +
>> +	if (!queue_work(ovpn->crypto_wq, &peer->encrypt_work))
>> +		ovpn_peer_put(peer);
> 
> I wanted to come back to this after going through the crypto patch,
> because this felt like a strange construct when I first looked at this
> patch.
> 
> Why are you using a workqueue here? Based on the kdoc for crypto_wq
> ("used to schedule crypto work that may sleep during TX/RX"), it's to
> deal with async crypto.
> 
> If so, why not use the more standard way of dealing with async crypto
> in contexts that cannot sleep, ie letting the crypto core call the
> "done" callback asynchronously? You need to do all the proper refcount
> handling, but IMO it's cleaner and simpler than this workqueue and
> ptr_ring. You can see an example of that in macsec (macsec_encrypt_*
> in drivers/net/macsec.c).

Aha! You don't know how happy I was when I found the doc describing how 
to convert the async code into sync-looking :-) With the detail that I 
had to move to a different context, as the code may want to sleep (hence 
the introduction of the workqueue).

It looks like I am little fan of WQs, while you are telling me to avoid 
them if possible.

I presume that using WQs comes with a non-negligible cost, therefore if 
we can just get things done without having to use them, then I should 
just don't.

I think I could go back to no-workqueue encrypt/decrypt.
Do you think this may have any impact on any future multi-core 
optimization? Back then I also thought that going through workers may 
make improvements in this area easier. But I could just be wrong.

Regards,


-- 
Antonio Quartulli
OpenVPN Inc.