Re: [PATCH net-next 3/3] net: sched: make skip_sw actually skip software
From: Asbjørn Sloth Tønnesen
Date: Fri Feb 16 2024 - 09:01:28 EST
Hi Vlad,
On 2/16/24 08:47, Vlad Buslov wrote:
On Thu 15 Feb 2024 at 16:04, Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx> wrote:
TC filters come in 3 variants:
- no flag (no opinion, process wherever possible)
- 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.
+----------------------------+--------+--------+--------+
| Test description | Pre | Post | Rel. |
| | kpps | kpps | chg. |
+----------------------------+--------+--------+--------+
| basic forwarding + notrack | 1264.9 | 1277.7 | 1.01x |
| switch to eswitch mode | 1067.1 | 1071.0 | 1.00x |
| add ingress qdisc | 1056.0 | 1059.1 | 1.00x |
+----------------------------+--------+--------+--------+
| 1 non-matching rule | 927.9 | 1057.1 | 1.14x |
| 10 non-matching rules | 495.8 | 1055.6 | 2.13x |
| 25 non-matching rules | 280.6 | 1053.5 | 3.75x |
| 50 non-matching rules | 162.0 | 1055.7 | 6.52x |
| 100 non-matching rules | 87.7 | 1019.0 | 11.62x |
+----------------------------+--------+--------+--------+
perf top (100 n-m skip_sw rules - pre patch):
25.57% [kernel] [k] __skb_flow_dissect
20.77% [kernel] [k] rhashtable_jhash2
14.26% [kernel] [k] fl_classify
13.28% [kernel] [k] fl_mask_lookup
6.38% [kernel] [k] memset_orig
3.22% [kernel] [k] tcf_classify
perf top (100 n-m skip_sw rules - post patch):
4.28% [kernel] [k] __dev_queue_xmit
3.80% [kernel] [k] check_preemption_disabled
3.68% [kernel] [k] nft_do_chain
3.08% [kernel] [k] __netif_receive_skb_core.constprop.0
2.59% [kernel] [k] mlx5e_xmit
2.48% [kernel] [k] mlx5e_skb_from_cqe_mpwrq_nonlinear
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 was in range 12.79 - 12.95 Mpps across all tests.
Signed-off-by: Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx>
---
include/net/pkt_cls.h | 5 +++++
net/core/dev.c | 3 +++
2 files changed, 8 insertions(+)
diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
index a4ee43f493bb..a065da4df7ff 100644
--- a/include/net/pkt_cls.h
+++ b/include/net/pkt_cls.h
@@ -74,6 +74,11 @@ static inline bool tcf_block_non_null_shared(struct tcf_block *block)
return block && block->index;
}
+static inline bool tcf_block_has_skip_sw_only(struct tcf_block *block)
+{
+ return block && atomic_read(&block->filtercnt) == atomic_read(&block->skipswcnt);
+}
Note that this introduces a read from heavily contended cache-line on
data path for all classifiers, including the ones that don't support
offloads. Wonder if this a concern for users running purely software tc.
Unfortunately, I don't have access to any multi-CPU machines, so I haven't been
able to test the impact of that.
Alternatively I guess I could also maintain a static key in the counter update logic.
+
static inline struct Qdisc *tcf_block_q(struct tcf_block *block)
{
WARN_ON(tcf_block_shared(block));
diff --git a/net/core/dev.c b/net/core/dev.c
index d8dd293a7a27..7cd014e5066e 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3910,6 +3910,9 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb,
if (!miniq)
return ret;
+ if (tcf_block_has_skip_sw_only(miniq->block))
+ return ret;
+
tc_skb_cb(skb)->mru = 0;
tc_skb_cb(skb)->post_ct = false;
tcf_set_drop_reason(skb, *drop_reason);
--
Best regards
Asbjørn Sloth Tønnesen
Network Engineer
Fiberby - AS42541