[PATCH net-next v3 3/3] net: sched: make skip_sw actually skip software

From: Asbjørn Sloth Tønnesen
Date: Wed Mar 06 2024 - 11:59:49 EST

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TC filters come in 3 variants:
- no flag (try to process in hardware, but fallback to software))
- skip_hw (do not process filter by hardware)
- skip_sw (do not process filter by software)

However skip_sw is implemented so that the skip_sw
flag can first be checked, after it has been matched.

IMHO it's common when using skip_sw, to use it on all rules.

So if all filters in a block is skip_sw filters, then
we can bail early, we can thus avoid having to match
the filters, just to check for the skip_sw flag.

This patch adds a bypass, for when only TC skip_sw rules
are used. The bypass is guarded by a static key, to avoid
harming other workloads.

There are 3 ways that a packet from a skip_sw ruleset, can
end up in the kernel path. Although the send packets to a
non-existent chain way is only improved a few percents, then
I believe it's worth optimizing the trap and fall-though
use-cases.

+----------------------------+--------+--------+--------+
| Test description | Pre- | Post- | Rel. |
| | kpps | kpps | chg. |
+----------------------------+--------+--------+--------+
| basic forwarding + notrack | 3589.3 | 3587.9 | 1.00x |
| switch to eswitch mode | 3081.8 | 3094.7 | 1.00x |
| add ingress qdisc | 3042.9 | 3063.6 | 1.01x |
| tc forward in hw / skip_sw |37024.7 |37028.4 | 1.00x |
| tc forward in sw / skip_hw | 3245.0 | 3245.3 | 1.00x |
+----------------------------+--------+--------+--------+
| tests with only skip_sw rules below: |
+----------------------------+--------+--------+--------+
| 1 non-matching rule | 2694.7 | 3058.7 | 1.14x |
| 1 n-m rule, match trap | 2611.2 | 3323.1 | 1.27x |
| 1 n-m rule, goto non-chain | 2886.8 | 2945.9 | 1.02x |
| 5 non-matching rules | 1958.2 | 3061.3 | 1.56x |
| 5 n-m rules, match trap | 1911.9 | 3327.0 | 1.74x |
| 5 n-m rules, goto non-chain| 2883.1 | 2947.5 | 1.02x |
| 10 non-matching rules | 1466.3 | 3062.8 | 2.09x |
| 10 n-m rules, match trap | 1444.3 | 3317.9 | 2.30x |
| 10 n-m rules,goto non-chain| 2883.1 | 2939.5 | 1.02x |
| 25 non-matching rules | 838.5 | 3058.9 | 3.65x |
| 25 n-m rules, match trap | 824.5 | 3323.0 | 4.03x |
| 25 n-m rules,goto non-chain| 2875.8 | 2944.7 | 1.02x |
| 50 non-matching rules | 488.1 | 3054.7 | 6.26x |
| 50 n-m rules, match trap | 484.9 | 3318.5 | 6.84x |
| 50 n-m rules,goto non-chain| 2884.1 | 2939.7 | 1.02x |
+----------------------------+--------+--------+--------+

perf top (25 n-m skip_sw rules - pre patch):
20.39% [kernel] [k] __skb_flow_dissect
16.43% [kernel] [k] rhashtable_jhash2
10.58% [kernel] [k] fl_classify
10.23% [kernel] [k] fl_mask_lookup
4.79% [kernel] [k] memset_orig
2.58% [kernel] [k] tcf_classify
1.47% [kernel] [k] __x86_indirect_thunk_rax
1.42% [kernel] [k] __dev_queue_xmit
1.36% [kernel] [k] nft_do_chain
1.21% [kernel] [k] __rcu_read_lock

perf top (25 n-m skip_sw rules - post patch):
5.12% [kernel] [k] __dev_queue_xmit
4.77% [kernel] [k] nft_do_chain
3.65% [kernel] [k] dev_gro_receive
3.41% [kernel] [k] check_preemption_disabled
3.14% [kernel] [k] mlx5e_skb_from_cqe_mpwrq_nonlinear
2.88% [kernel] [k] __netif_receive_skb_core.constprop.0
2.49% [kernel] [k] mlx5e_xmit
2.15% [kernel] [k] ip_forward
1.95% [kernel] [k] mlx5e_tc_restore_tunnel
1.92% [kernel] [k] vlan_gro_receive

Test setup:
DUT: Intel Xeon D-1518 (2.20GHz) w/ Nvidia/Mellanox ConnectX-6 Dx 2x100G
Data rate measured on switch (Extreme X690), and DUT connected as
a router on a stick, with pktgen and pktsink as VLANs.
Pktgen-dpdk was in range 36.6-37.7 Mpps across all tests.
Full test data at https://files.fiberby.net/ast/2024/tc_skip_sw/v2_tests/

Signed-off-by: Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx>
---
include/net/pkt_cls.h | 9 +++++++++
include/net/sch_generic.h | 1 +
net/core/dev.c | 10 ++++++++++
net/sched/cls_api.c | 18 ++++++++++++++++++
4 files changed, 38 insertions(+)

diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
index a4ee43f493bb..41297bd38dff 100644
--- a/include/net/pkt_cls.h
+++ b/include/net/pkt_cls.h
@@ -74,6 +74,15 @@ static inline bool tcf_block_non_null_shared(struct tcf_block *block)
return block && block->index;
}

+#ifdef CONFIG_NET_CLS_ACT
+DECLARE_STATIC_KEY_FALSE(tcf_bypass_check_needed_key);
+
+static inline bool tcf_block_bypass_sw(struct tcf_block *block)
+{
+ return block && block->bypass_wanted;
+}
+#endif
+
static inline struct Qdisc *tcf_block_q(struct tcf_block *block)
{
WARN_ON(tcf_block_shared(block));
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 7af0621db226..60b0fdf2b1ad 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -477,6 +477,7 @@ struct tcf_block {
struct flow_block flow_block;
struct list_head owner_list;
bool keep_dst;
+ bool bypass_wanted;
atomic_t filtercnt; /* Number of filters */
atomic_t skipswcnt; /* Number of skip_sw filters */
atomic_t offloadcnt; /* Number of oddloaded filters */
diff --git a/net/core/dev.c b/net/core/dev.c
index fe054cbd41e9..b7c583f98e82 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2057,6 +2057,11 @@ void net_dec_egress_queue(void)
EXPORT_SYMBOL_GPL(net_dec_egress_queue);
#endif

+#ifdef CONFIG_NET_CLS_ACT
+DEFINE_STATIC_KEY_FALSE(tcf_bypass_check_needed_key);
+EXPORT_SYMBOL(tcf_bypass_check_needed_key);
+#endif
+
DEFINE_STATIC_KEY_FALSE(netstamp_needed_key);
EXPORT_SYMBOL(netstamp_needed_key);
#ifdef CONFIG_JUMP_LABEL
@@ -3911,6 +3916,11 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb,
if (!miniq)
return ret;

+ if (static_branch_unlikely(&tcf_bypass_check_needed_key)) {
+ if (tcf_block_bypass_sw(miniq->block))
+ return ret;
+ }
+
tc_skb_cb(skb)->mru = 0;
tc_skb_cb(skb)->post_ct = false;
tcf_set_drop_reason(skb, *drop_reason);
diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index 304a46ab0e0b..db0653993632 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -410,6 +410,23 @@ static void tcf_proto_get(struct tcf_proto *tp)
refcount_inc(&tp->refcnt);
}

+static void tcf_maintain_bypass(struct tcf_block *block)
+{
+ int filtercnt = atomic_read(&block->filtercnt);
+ int skipswcnt = atomic_read(&block->skipswcnt);
+ bool bypass_wanted = filtercnt > 0 && filtercnt == skipswcnt;
+
+ if (bypass_wanted != block->bypass_wanted) {
+#ifdef CONFIG_NET_CLS_ACT
+ if (bypass_wanted)
+ static_branch_inc(&tcf_bypass_check_needed_key);
+ else
+ static_branch_dec(&tcf_bypass_check_needed_key);
+#endif
+ block->bypass_wanted = bypass_wanted;
+ }
+}
+
static void tcf_block_filter_cnt_update(struct tcf_block *block, bool *counted, bool add)
{
lockdep_assert_not_held(&block->cb_lock);
@@ -424,6 +441,7 @@ static void tcf_block_filter_cnt_update(struct tcf_block *block, bool *counted,
*counted = false;
}
}
+ tcf_maintain_bypass(block);
up_write(&block->cb_lock);
}

--
2.43.0

Next message: Kieran Bingham: "Re: [PATCH 5/5] media: imx335: Limit analogue gain value"
Previous message: Asbjørn Sloth Tønnesen: "[PATCH net-next v3 2/3] net: sched: cls_api: add filter counter"
In reply to: Asbjørn Sloth Tønnesen: "[PATCH net-next v3 2/3] net: sched: cls_api: add filter counter"
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