Subject: Re: [bpf-next PATCH v3 2/6] bpf, sockmap: On receive programs try to fast track SK_PASS ingress

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 12 Oct 2020 11:03:19 +0200

Hey John,

Exiting to see this work :-)

On Fri, Oct 09, 2020 at 08:36 PM CEST, John Fastabend wrote:
> When we receive an skb and the ingress skb verdict program returns
> SK_PASS we currently set the ingress flag and put it on the workqueue
> so it can be turned into a sk_msg and put on the sk_msg ingress queue.
> Then finally telling userspace with data_ready hook.
>
> Here we observe that if the workqueue is empty then we can try to
> convert into a sk_msg type and call data_ready directly without
> bouncing through a workqueue. Its a common pattern to have a recv
> verdict program for visibility that always returns SK_PASS. In this
> case unless there is an ENOMEM error or we overrun the socket we
> can avoid the workqueue completely only using it when we fall back
> to error cases caused by memory pressure.
>
> By doing this we eliminate another case where data may be dropped
> if errors occur on memory limits in workqueue.
>
> Fixes: 51199405f9672 ("bpf: skb_verdict, support SK_PASS on RX BPF path")
> Signed-off-by: John Fastabend <[email-masked]>
> ---
>  net/core/skmsg.c |   17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 040ae1d75b65..4b160d97b7f9 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -773,6 +773,7 @@ static void sk_psock_verdict_apply(struct sk_psock *psock,
>  {
>  	struct tcp_skb_cb *tcp;
>  	struct sock *sk_other;
> +	int err = -EIO;
>
>  	switch (verdict) {
>  	case __SK_PASS:
> @@ -784,8 +785,20 @@ static void sk_psock_verdict_apply(struct sk_psock *psock,
>
>  		tcp = TCP_SKB_CB(skb);
>  		tcp->bpf.flags |= BPF_F_INGRESS;
> -		skb_queue_tail(&psock->ingress_skb, skb);
> -		schedule_work(&psock->work);
> +
> +		/* If the queue is empty then we can submit directly
> +		 * into the msg queue. If its not empty we have to
> +		 * queue work otherwise we may get OOO data. Otherwise,
> +		 * if sk_psock_skb_ingress errors will be handled by
> +		 * retrying later from workqueue.
> +		 */
> +		if (skb_queue_empty(&psock->ingress_skb)) {
> +			err = sk_psock_skb_ingress(psock, skb);

When going through the workqueue (sk_psock_backlog), we will also check
if socket didn't get detached from the process, that is if
psock->sk->sk_socket != NULL, before queueing into msg queue.

Do we need a similar check here?

> +		}
> +		if (err < 0) {
> +			skb_queue_tail(&psock->ingress_skb, skb);
> +			schedule_work(&psock->work);
> +		}
>  		break;
>  	case __SK_REDIRECT:
>  		sk_psock_skb_redirect(skb);