On Thu, Oct 31, 2019 at 3:03 AM Eric Dumazet [off-list ref] wrote:

On 10/30/19 9:07 AM, Wally Zhao wrote:

quoted

Unlike tcp_transmit_skb,
sctp_packet_transmit does not set ooo_okay explicitly,
causing unwanted Tx queue switching when multiqueue is in use;
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.

While the patch seems fine, the changelog is quite confusing.

When skb->ooo_olay is 0 (which is the default for freshly allocated skbs),
the core networking stack will stick to whatever TX queue was chosen
at the time the dst_entry was attached to the (connected) socket.

This means no reorder can happen at all by default.

By setting ooo_okay carefully (as you did in your patch), you allow
core networking stack to _switch_ to another TX queue based on
current CPU  (XPS selection)

So even without your fix, SCTP should not experience out-of-order packets.

Yes, you are right, as Marcelo also pointed out.
The changelog was given based on incorrect observation of a test
result, as I replied to Marcelo.
Since ooo_okay is default to 0, this is good enough; no need for any
patch from my side.
Thank you for your time on this.

quoted

Signed-off-by: Wally Zhao <redacted>
---
 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;
+
      if (tp->af_specific->sctp_xmit(head, tp) >= 0 &&
          tp->dst_pending_confirm)
              tp->dst_pending_confirm = 0;