RE: [net-next PATCH] net: act_skbedit: Fix tc action skbedit queue_mapping

-----Original Message-----
From: Alexander Duyck <redacted>
Sent: Monday, August 9, 2021 5:51 PM
To: Nambiar, Amritha <redacted>
Cc: Netdev <redacted>; Jakub Kicinski <kuba@kernel.org>;
Jamal Hadi Salim [off-list ref]; Jiri Pirko [off-list ref]; Cong
Wang [off-list ref]; Samudrala, Sridhar
[off-list ref]
Subject: Re: [net-next PATCH] net: act_skbedit: Fix tc action skbedit
queue_mapping

On Mon, Aug 9, 2021 at 4:36 PM Amritha Nambiar
[off-list ref] wrote:

quoted

For skbedit action queue_mapping to select the transmit queue,
queue_mapping takes the value N for tx-N (where N is the actual
queue number). However, current behavior is the following:
1. Queue selection is off by 1, tx queue N-1 is selected for
   action skbedit queue_mapping N. (If the general syntax for queue
   index is 1 based, i.e., action skbedit queue_mapping N would
   transmit to tx queue N-1, where N >=1, then the last queue cannot
   be used for transmit as this fails the upper bound check.)
2. Transmit to first queue of TCs other than TC0 selects the
   next queue.
3. It is not possible to transmit to the first queue (tx-0) as
   this fails the bounds check, in this case the fallback
   mechanism for hash calculation is used.

Fix the call to skb_set_queue_mapping(), the code retrieving the
transmit queue uses skb_get_rx_queue() which subtracts the queue
index by 1. This makes it so that "action skbedit queue_mapping N"
will transmit to tx-N (including the first and last queue).

Signed-off-by: Amritha Nambiar <redacted>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
---
 net/sched/act_skbedit.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/act_skbedit.c b/net/sched/act_skbedit.c
index e5f3fb8b00e3..a7bba15c74c4 100644
--- a/net/sched/act_skbedit.c
+++ b/net/sched/act_skbedit.c

@@ -59,7 +59,7 @@ static int tcf_skbedit_act(struct sk_buff *skb, const

struct tc_action *a,

quoted

        }
        if (params->flags & SKBEDIT_F_QUEUE_MAPPING &&
            skb->dev->real_num_tx_queues > params->queue_mapping)
-               skb_set_queue_mapping(skb, params->queue_mapping);
+               skb_set_queue_mapping(skb, params->queue_mapping + 1);
        if (params->flags & SKBEDIT_F_MARK) {
                skb->mark &= ~params->mask;
                skb->mark |= params->mark & params->mask;

I don't think this is correct. It is conflating the rx_queue_mapping
versus the Tx queue mapping. This is supposed to be setting the Tx
queue mapping which applies after we have dropped the Rx queue
mapping, not before. Specifically this is run at the qdisc enqueue
stage with a single locked qdisc, after netdev_pick_tx and skb_tx_hash
have already run. It is something that existed before mq and is meant
to work with the mutliq qdisc.

If you are wanting to add a seperate override to add support for
programming the Rx queue mapping you may want to submit that as a
different patch rather than trying to change the existing Tx queue
mapping feature. Either that or you would need to change this so that
it has a different behavior depending on where the hook is added since
the behavior would be different if this is called before skb_tx_hash.

Hi Alex,
Thanks for the review. The goal is to select a transmit queue using tc egress rule
and the action skbedit (that will go through netdev_pick_tx and skb_tx_hash).
I am not sure of the correct syntax for the queue-mapping value in the
action (tx-N or tx-N+1). As per the man page
(https://man7.org/linux/man-pages/man8/tc-skbedit.8.html), I interpreted
it as "action skbedit queue_mapping N" will transmit to tx-N. But, the
3 observations I listed don't quite seem to be following the tc rule.
Hence, tried to fix this in the action module. 

-Amritha