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: Tue, 13 Oct 2020 21:43:03 +0200

On Mon, Oct 12, 2020 at 05:33 PM CEST, John Fastabend wrote:
> Jakub Sitnicki wrote:

[...]

>> 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.
>
> The sk_socket check is only for the egress path,
>
>   sk_psock_handle_skb -> skb_send_sock_locked -> kernel_sendmsg_locked

Oh, okay. I thought it was because we want to forwarding into the socket
as soon as there is no process to read from the queue.

> Then the do_tcp_sendpages() uses sk_socket and I don't see any checks for
> sk_socket being set. Although I think its worth looking through to see
> if the psock/sk state is always such that we have sk_socket there I
> don't recall off-hand where that is null'd.

It's in sock_orphan().

> But, to answer your question this is ingress only and here we don't
> use sk_socket for anything so I don't see any reason the check is
> needed. All that is done here is converting to skmsg and posting
> onto ingress queue.

Queued skb won't be read out, but I don't see a problem with it.

[...]