Re: [PATCH] net: sched: check tc_skip_classify as far as possible

On Thu, Oct 28, 2021 at 10:28 PM Daniel Borkmann [off-list ref] wrote:

On 10/28/21 3:56 PM, xiangxia.m.yue@gmail.com wrote:

quoted

From: Tonghao Zhang <redacted>

We look up and then check tc_skip_classify flag in net
sched layer, even though skb don't want to be classified.
That case may consume a lot of cpu cycles.

Install the rules as below:
$ for id in $(seq 1 100); do
$     tc filter add ... egress prio $id ... action mirred egress redirect dev ifb0
$ done

Do you actually have such a case in practice or is this just hypothetical?

Hi Daniel, I did some research about this for k8s in production. There
are not so many tc prio(~5 different prio).
butg in this test, I use the 100 prio.

I reviewed the code, for the tx path, I think we check the
tc_skip_classify too later. In the rx path, we check it
in __netif_receive_skb_core.

Asking as this feels rather broken to begin with.

quoted

netperf:
$ taskset -c 1 netperf -t TCP_RR -H ip -- -r 32,32
$ taskset -c 1 netperf -t TCP_STREAM -H ip -- -m 32

Without this patch:
10662.33 tps
108.95 Mbit/s

With this patch:
12434.48 tps
145.89 Mbit/s

For TCP_RR, there are 16.6% improvement, TCP_STREAM 33.9%.

Cc: Willem de Bruijn <willemb@google.com>
Cc: Cong Wang <redacted>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Tonghao Zhang <redacted>
---
  net/core/dev.c      | 3 ++-
  net/sched/act_api.c | 3 ---
  2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index eb61a8821b3a..856ac1fb75b4 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -4155,7 +4155,8 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
  #ifdef CONFIG_NET_CLS_ACT
      skb->tc_at_ingress = 0;
  # ifdef CONFIG_NET_EGRESS
-     if (static_branch_unlikely(&egress_needed_key)) {
+     if (static_branch_unlikely(&egress_needed_key) &&
+         !skb_skip_tc_classify(skb)) {
              skb = sch_handle_egress(skb, &rc, dev);
              if (!skb)
                      goto out;

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index 7dd3a2dc5fa4..bd66f27178be 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c

@@ -722,9 +722,6 @@ int tcf_action_exec(struct sk_buff *skb, struct tc_action **actions,
      int i;
      int ret = TC_ACT_OK;

-     if (skb_skip_tc_classify(skb))
-             return TC_ACT_OK;
-

I think this might imply a change in behavior which could have the potential
to break setups in the wild.

we may not change this code, i will send v2, if not comment.

quoted

  restart_act_graph:
      for (i = 0; i < nr_actions; i++) {
              const struct tc_action *a = actions[i];

-- 
Best regards, Tonghao