Subject: Re: [PATCH] sctp: set ooo_okay properly for Transmit Packet Steering

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Wed, 30 Oct 2019 10:24:20 -0300

On Wed, Oct 30, 2019 at 12:07:17PM -0400, Wally Zhao wrote:
> Unlike tcp_transmit_skb,
> sctp_packet_transmit does not set ooo_okay explicitly,
> causing unwanted Tx queue switching when multiqueue is in use;

It is initialized to 0 by __alloc_skb() via:
        memset(skb, 0, offsetof(struct sk_buff, tail));
and never set to 1 by anyone for SCTP.

The patch description seems off. I don't see how the unwanted Tx queue
switching can happen. IOW, it's not fixing it OOO packets, but
improving it by allowing switching on the first packet. Am I missing
something?

> Tx queue switching may cause out-of-order packets.
> Change sctp_packet_transmit to allow Tx queue switching only for
> the first in flight packet, to avoid unwanted Tx queue switching.
> 
> Signed-off-by: Wally Zhao <[email-masked]>
> ---
>  net/sctp/output.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index dbda7e7..5ff75cc 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -626,6 +626,10 @@ int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
>  	/* neighbour should be confirmed on successful transmission or
>  	 * positive error
>  	 */
> +
> +	/* allow switch tx queue only for the first in flight pkt */
> +	head->ooo_okay = asoc->outqueue.outstanding_bytes == 0;

Considering we are talking about NIC queues here, we would have a
better result with tp->flight_size instead. As in, we can switch
queues if, for this transport, the queue is empty.

> +
>  	if (tp->af_specific->sctp_xmit(head, tp) >= 0 &&
>  	    tp->dst_pending_confirm)
>  		tp->dst_pending_confirm = 0;
> -- 
> 1.8.3.1
>