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Message-ID: <20210811173254.shwupnunaaoadpjb@skbuf>
Date: Wed, 11 Aug 2021 20:32:54 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: DENG Qingfang <dqfext@...il.com>
Cc: Florian Fainelli <f.fainelli@...il.com>,
Vladimir Oltean <vladimir.oltean@....com>,
netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
Andrew Lunn <andrew@...n.ch>,
Vivien Didelot <vivien.didelot@...il.com>,
Kurt Kanzenbach <kurt@...utronix.de>,
Woojung Huh <woojung.huh@...rochip.com>,
UNGLinuxDriver@...rochip.com, Sean Wang <sean.wang@...iatek.com>,
Landen Chao <Landen.Chao@...iatek.com>,
Matthias Brugger <matthias.bgg@...il.com>,
Claudiu Manoil <claudiu.manoil@....com>,
Alexandre Belloni <alexandre.belloni@...tlin.com>,
George McCollister <george.mccollister@...il.com>
Subject: Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a
loop" antipattern from the core
On Wed, Aug 11, 2021 at 01:04:47AM +0800, DENG Qingfang wrote:
> On Tue, Aug 10, 2021 at 07:35:33PM +0300, Vladimir Oltean wrote:
> > If I were to guess where Qingfang was hinting at, is that the receive
> > path now needs to iterate over a list, whereas before it simply indexed
> > an array:
> >
> > static inline struct net_device *dsa_master_find_slave(struct net_device *dev,
> > int device, int port)
> > {
> > struct dsa_port *cpu_dp = dev->dsa_ptr;
> > struct dsa_switch_tree *dst = cpu_dp->dst;
> > struct dsa_port *dp;
> >
> > list_for_each_entry(dp, &dst->ports, list)
> > if (dp->ds->index == device && dp->index == port &&
> > dp->type == DSA_PORT_TYPE_USER)
> > return dp->slave;
> >
> > return NULL;
> > }
> >
> > I will try in the following days to make a prototype implementation of
> > converting back the linked list into an array and see if there is any
> > justifiable performance improvement.
> >
> > [ even if this would make the "multiple CPU ports in LAG" implementation
> > harder ]
>
> Yes, you got my point.
>
> There is RTL8390M series SoC, which has 52+ ports but a weak CPU (MIPS
> 34kc 700MHz). In that case the linear lookup time and the potential cache
> miss could make a difference.
Then I am not in a position to make relevant performance tests for that
scenario.
I have been testing with the following setup: an NXP LS1028A switch
(ocelot/felix driver) using IPv4 forwarding of 64 byte UDP datagrams
sent by a data generator. 2 ports at 1Gbps each, 100% port load. IP
forwarding takes place between 1 port and the other.
Generator port A sends from 192.168.100.1 to 192.168.200.1
Generator port B sends from 192.168.200.1 to 192.168.100.1
Flow control is enabled on all switch ports, the user ports and the CPU port
(I don't really have a setup that I can test in any meaningful way
without flow control).
The script I run on the board to set things up for IP forwarding is:
ip link set eno2 down && echo ocelot-8021q > /sys/class/net/eno2/dsa/tagging
ip link set swp0 address a0:00:00:00:00:02
ip link set swp1 address a0:00:00:00:00:04
for eth in swp0 swp1; do
ip link set ${eth} up
done
ip addr add 192.168.100.2/24 dev swp0
ip addr add 192.168.200.2/24 dev swp1
echo 1 > /proc/sys/net/ipv4/ip_forward
arp -s 192.168.100.1 00:01:02:03:04:05 dev swp0
arp -s 192.168.200.1 00:01:02:03:04:06 dev swp1
ethtool --config-nfc eno2 flow-type ip4 dst-ip 192.168.200.1 action 0
ethtool --config-nfc eno2 flow-type ip4 dst-ip 192.168.100.1 action 1
ethtool -K eno2 gro on rx-gro-list on
ethtool -K swp0 gro on rx-gro-list on
ethtool -K swp1 gro on rx-gro-list on
The DSA patch I used on top of today's net-next was:
-----------------------------[ cut here ]-----------------------------
>From 7733f643dd61431a93da5a8f5118848cdc037562 Mon Sep 17 00:00:00 2001
From: Vladimir Oltean <vladimir.oltean@....com>
Date: Wed, 11 Aug 2021 00:27:07 +0300
Subject: [PATCH] net: dsa: setup a linear port cache for faster receive path
Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
include/net/dsa.h | 5 +++++
net/dsa/dsa2.c | 46 ++++++++++++++++++++++++++++++++++++++++++----
net/dsa/dsa_priv.h | 9 ++++-----
3 files changed, 51 insertions(+), 9 deletions(-)
diff --git a/include/net/dsa.h b/include/net/dsa.h
index 3203b200cc38..2a9ea4f57910 100644
--- a/include/net/dsa.h
+++ b/include/net/dsa.h
@@ -153,9 +153,14 @@ struct dsa_switch_tree {
struct net_device **lags;
unsigned int lags_len;
+ struct dsa_port **port_cache;
+
/* Track the largest switch index within a tree */
unsigned int last_switch;
+ /* Track the largest port count in a switch within a tree */
+ unsigned int max_num_ports;
+
/* Track the bridges with forwarding offload enabled */
unsigned long fwd_offloading_bridges;
};
diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index 0b7497dd60c3..3d2b92dbd603 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -941,6 +941,39 @@ static void dsa_tree_teardown_lags(struct dsa_switch_tree *dst)
kfree(dst->lags);
}
+static int dsa_tree_setup_port_cache(struct dsa_switch_tree *dst)
+{
+ struct dsa_port *dp;
+
+ list_for_each_entry(dp, &dst->ports, list) {
+ if (dst->last_switch < dp->ds->index)
+ dst->last_switch = dp->ds->index;
+ if (dst->max_num_ports < dp->ds->num_ports)
+ dst->max_num_ports = dp->ds->num_ports;
+ }
+
+ dst->port_cache = kcalloc((dst->last_switch + 1) * dst->max_num_ports,
+ sizeof(struct dsa_port *), GFP_KERNEL);
+ if (!dst->port_cache)
+ return -ENOMEM;
+
+ list_for_each_entry(dp, &dst->ports, list)
+ dst->port_cache[dp->ds->index * dst->max_num_ports + dp->index] = dp;
+
+ return 0;
+}
+
+static void dsa_tree_teardown_port_cache(struct dsa_switch_tree *dst)
+{
+ int i;
+
+ for (i = 0; i < dst->max_num_ports * dst->last_switch; i++)
+ dst->port_cache[i] = NULL;
+
+ kfree(dst->port_cache);
+ dst->port_cache = NULL;
+}
+
static int dsa_tree_setup(struct dsa_switch_tree *dst)
{
bool complete;
@@ -956,10 +989,14 @@ static int dsa_tree_setup(struct dsa_switch_tree *dst)
if (!complete)
return 0;
- err = dsa_tree_setup_cpu_ports(dst);
+ err = dsa_tree_setup_port_cache(dst);
if (err)
return err;
+ err = dsa_tree_setup_cpu_ports(dst);
+ if (err)
+ goto teardown_port_cache;
+
err = dsa_tree_setup_switches(dst);
if (err)
goto teardown_cpu_ports;
@@ -984,6 +1021,8 @@ static int dsa_tree_setup(struct dsa_switch_tree *dst)
dsa_tree_teardown_switches(dst);
teardown_cpu_ports:
dsa_tree_teardown_cpu_ports(dst);
+teardown_port_cache:
+ dsa_tree_teardown_port_cache(dst);
return err;
}
@@ -1003,6 +1042,8 @@ static void dsa_tree_teardown(struct dsa_switch_tree *dst)
dsa_tree_teardown_cpu_ports(dst);
+ dsa_tree_teardown_port_cache(dst);
+
list_for_each_entry_safe(dl, next, &dst->rtable, list) {
list_del(&dl->list);
kfree(dl);
@@ -1301,9 +1342,6 @@ static int dsa_switch_parse_member_of(struct dsa_switch *ds,
return -EEXIST;
}
- if (ds->dst->last_switch < ds->index)
- ds->dst->last_switch = ds->index;
-
return 0;
}
diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
index 6310a15afe21..5c27f66fd62a 100644
--- a/net/dsa/dsa_priv.h
+++ b/net/dsa/dsa_priv.h
@@ -188,12 +188,11 @@ static inline struct net_device *dsa_master_find_slave(struct net_device *dev,
struct dsa_switch_tree *dst = cpu_dp->dst;
struct dsa_port *dp;
- list_for_each_entry(dp, &dst->ports, list)
- if (dp->ds->index == device && dp->index == port &&
- dp->type == DSA_PORT_TYPE_USER)
- return dp->slave;
+ dp = dst->port_cache[device * dst->max_num_ports + port];
+ if (!dp || dp->type != DSA_PORT_TYPE_USER)
+ return NULL;
- return NULL;
+ return dp->slave;
}
/* netlink.c */
-----------------------------[ cut here ]-----------------------------
The results I got were:
Before the patch:
684 Kpps = 459 Mbps
perf record -e cycles -C 0 sleep 10 && perf report
10.17% ksoftirqd/0 [kernel.kallsyms] [k] enetc_pci_remove
6.13% ksoftirqd/0 [kernel.kallsyms] [k] eth_type_trans
5.48% ksoftirqd/0 [kernel.kallsyms] [k] enetc_poll
4.99% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_alloc
4.56% ksoftirqd/0 [kernel.kallsyms] [k] dev_gro_receive
2.89% ksoftirqd/0 [kernel.kallsyms] [k] enetc_start_xmit
2.77% ksoftirqd/0 [kernel.kallsyms] [k] __skb_flow_dissect
2.75% ksoftirqd/0 [kernel.kallsyms] [k] __siphash_aligned
2.55% ksoftirqd/0 [kernel.kallsyms] [k] __netif_receive_skb_core
2.48% ksoftirqd/0 [kernel.kallsyms] [k] build_skb
2.47% ksoftirqd/0 [kernel.kallsyms] [k] take_page_off_buddy
2.01% ksoftirqd/0 [kernel.kallsyms] [k] inet_gro_receive
1.86% ksoftirqd/0 [kernel.kallsyms] [k] dsa_slave_xmit
1.76% ksoftirqd/0 [kernel.kallsyms] [k] __dev_queue_xmit
1.68% ksoftirqd/0 [kernel.kallsyms] [k] skb_zcopy_clear
1.62% ksoftirqd/0 [kernel.kallsyms] [k] enetc_build_skb
1.60% ksoftirqd/0 [kernel.kallsyms] [k] __build_skb_around
1.50% ksoftirqd/0 [kernel.kallsyms] [k] dev_hard_start_xmit
1.49% ksoftirqd/0 [kernel.kallsyms] [k] sch_direct_xmit
1.42% ksoftirqd/0 [kernel.kallsyms] [k] __skb_get_hash
1.29% ksoftirqd/0 [kernel.kallsyms] [k] __local_bh_enable_ip
1.26% ksoftirqd/0 [kernel.kallsyms] [k] udp_gro_receive
1.23% ksoftirqd/0 [kernel.kallsyms] [k] udp4_gro_receive
1.21% ksoftirqd/0 [kernel.kallsyms] [k] skb_segment_list
1.13% ksoftirqd/0 [kernel.kallsyms] [k] netdev_drivername
1.05% ksoftirqd/0 [kernel.kallsyms] [k] dev_shutdown
1.05% ksoftirqd/0 [kernel.kallsyms] [k] inet_gso_segment
1.01% ksoftirqd/0 [kernel.kallsyms] [k] dsa_switch_rcv
0.98% ksoftirqd/0 [kernel.kallsyms] [k] ocelot_rcv
0.91% ksoftirqd/0 [kernel.kallsyms] [k] napi_gro_receive
0.87% ksoftirqd/0 [kernel.kallsyms] [k] enetc_xmit
0.85% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_free_bulk
0.84% ksoftirqd/0 [kernel.kallsyms] [k] do_csum
0.84% ksoftirqd/0 [kernel.kallsyms] [k] memmove
0.80% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_rcv
0.79% ksoftirqd/0 [kernel.kallsyms] [k] __netif_receive_skb_list_core
0.78% ksoftirqd/0 [kernel.kallsyms] [k] netif_skb_features
0.76% ksoftirqd/0 [kernel.kallsyms] [k] netif_receive_skb_list_internal
0.76% ksoftirqd/0 [kernel.kallsyms] [k] skb_release_all
0.74% ksoftirqd/0 [kernel.kallsyms] [k] netdev_pick_tx
perf record -e cache-misses -C 0 sleep 10 && perf report
7.22% ksoftirqd/0 [kernel.kallsyms] [k] skb_zcopy_clear
6.46% ksoftirqd/0 [kernel.kallsyms] [k] inet_gro_receive
6.41% ksoftirqd/0 [kernel.kallsyms] [k] enetc_pci_remove
6.20% ksoftirqd/0 [kernel.kallsyms] [k] take_page_off_buddy
6.13% ksoftirqd/0 [kernel.kallsyms] [k] build_skb
5.06% ksoftirqd/0 [kernel.kallsyms] [k] inet_gso_segment
4.47% ksoftirqd/0 [kernel.kallsyms] [k] dev_gro_receive
4.28% ksoftirqd/0 [kernel.kallsyms] [k] memmove
3.77% ksoftirqd/0 [kernel.kallsyms] [k] enetc_poll
3.73% ksoftirqd/0 [kernel.kallsyms] [k] __copy_skb_header
3.46% ksoftirqd/0 [kernel.kallsyms] [k] eth_type_trans
3.06% ksoftirqd/0 [kernel.kallsyms] [k] skb_release_all
2.76% ksoftirqd/0 [kernel.kallsyms] [k] ip_send_check
2.36% ksoftirqd/0 [kernel.kallsyms] [k] __skb_get_hash
2.24% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_alloc
2.23% ksoftirqd/0 [kernel.kallsyms] [k] __netif_receive_skb_core
1.68% ksoftirqd/0 [kernel.kallsyms] [k] netdev_pick_tx
1.56% ksoftirqd/0 [kernel.kallsyms] [k] skb_segment_list
1.55% ksoftirqd/0 [kernel.kallsyms] [k] dsa_slave_xmit
1.54% ksoftirqd/0 [kernel.kallsyms] [k] skb_headers_offset_update
1.51% ksoftirqd/0 [kernel.kallsyms] [k] enetc_start_xmit
1.48% ksoftirqd/0 [kernel.kallsyms] [k] netdev_core_pick_tx
1.30% ksoftirqd/0 [kernel.kallsyms] [k] __dev_queue_xmit
1.14% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_rcv
1.05% ksoftirqd/0 [kernel.kallsyms] [k] __build_skb_around
1.05% ksoftirqd/0 [kernel.kallsyms] [k] skb_pull_rcsum
1.03% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_xmit
0.98% ksoftirqd/0 [kernel.kallsyms] [k] __skb_flow_dissect
0.89% ksoftirqd/0 [kernel.kallsyms] [k] ocelot_xmit
0.84% ksoftirqd/0 [kernel.kallsyms] [k] enetc_build_skb
0.73% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_free_bulk
0.63% ksoftirqd/0 [kernel.kallsyms] [k] enetc_refill_rx_ring
0.55% ksoftirqd/0 [kernel.kallsyms] [k] netif_skb_features
0.46% ksoftirqd/0 [kernel.kallsyms] [k] dma_unmap_page_attrs
0.46% ksoftirqd/0 [kernel.kallsyms] [k] fib_table_lookup
0.36% ksoftirqd/0 [kernel.kallsyms] [k] fib_lookup_good_nhc
0.33% ksoftirqd/0 [kernel.kallsyms] [k] skb_release_data
0.33% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_tx_vid
0.32% ksoftirqd/0 [kernel.kallsyms] [k] enetc_flip_rx_buff
0.29% ksoftirqd/0 [kernel.kallsyms] [k] napi_consume_skb
After the patch:
650 Kpps = 426 Mbps
perf record -e cycles -C 0 sleep 10 && perf report
9.34% ksoftirqd/0 [kernel.kallsyms] [k] enetc_pci_remove
7.70% ksoftirqd/0 [kernel.kallsyms] [k] enetc_poll
5.49% ksoftirqd/0 [kernel.kallsyms] [k] eth_type_trans
4.62% ksoftirqd/0 [kernel.kallsyms] [k] take_page_off_buddy
4.55% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_alloc
4.36% ksoftirqd/0 [kernel.kallsyms] [k] dev_gro_receive
3.22% ksoftirqd/0 [kernel.kallsyms] [k] skb_zcopy_clear
2.59% ksoftirqd/0 [kernel.kallsyms] [k] __siphash_aligned
2.51% ksoftirqd/0 [kernel.kallsyms] [k] enetc_start_xmit
2.37% ksoftirqd/0 [kernel.kallsyms] [k] __skb_flow_dissect
2.20% ksoftirqd/0 [kernel.kallsyms] [k] __netif_receive_skb_core
1.84% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_free_bulk
1.69% ksoftirqd/0 [kernel.kallsyms] [k] inet_gro_receive
1.65% ksoftirqd/0 [kernel.kallsyms] [k] __dev_queue_xmit
1.63% ksoftirqd/0 [kernel.kallsyms] [k] dsa_slave_xmit
1.60% ksoftirqd/0 [kernel.kallsyms] [k] build_skb
1.45% ksoftirqd/0 [kernel.kallsyms] [k] enetc_build_skb
1.41% ksoftirqd/0 [kernel.kallsyms] [k] dev_hard_start_xmit
1.39% ksoftirqd/0 [kernel.kallsyms] [k] sch_direct_xmit
1.39% ksoftirqd/0 [kernel.kallsyms] [k] __build_skb_around
1.28% ksoftirqd/0 [kernel.kallsyms] [k] __skb_get_hash
1.16% ksoftirqd/0 [kernel.kallsyms] [k] __local_bh_enable_ip
1.14% ksoftirqd/0 [kernel.kallsyms] [k] udp4_gro_receive
1.12% ksoftirqd/0 [kernel.kallsyms] [k] skb_release_all
1.09% ksoftirqd/0 [kernel.kallsyms] [k] ocelot_rcv
1.08% ksoftirqd/0 [kernel.kallsyms] [k] netdev_drivername
1.08% ksoftirqd/0 [kernel.kallsyms] [k] dma_unmap_page_attrs
1.08% ksoftirqd/0 [kernel.kallsyms] [k] udp_gro_receive
1.05% ksoftirqd/0 [kernel.kallsyms] [k] dev_shutdown
1.04% ksoftirqd/0 [kernel.kallsyms] [k] napi_consume_skb
1.03% ksoftirqd/0 [kernel.kallsyms] [k] skb_segment_list
0.90% ksoftirqd/0 [kernel.kallsyms] [k] napi_gro_receive
0.86% ksoftirqd/0 [kernel.kallsyms] [k] inet_gso_segment
0.83% ksoftirqd/0 [kernel.kallsyms] [k] dsa_switch_rcv
0.77% ksoftirqd/0 [kernel.kallsyms] [k] memmove
0.75% ksoftirqd/0 [kernel.kallsyms] [k] do_csum
0.73% ksoftirqd/0 [kernel.kallsyms] [k] netif_skb_features
0.71% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_rcv
0.69% ksoftirqd/0 [kernel.kallsyms] [k] __netif_receive_skb_list_core
0.67% ksoftirqd/0 [kernel.kallsyms] [k] netif_receive_skb_list_internal
perf record -e cache-misses -C 0 sleep 10 && perf report
12.38% ksoftirqd/0 [kernel.kallsyms] [k] skb_zcopy_clear
9.34% ksoftirqd/0 [kernel.kallsyms] [k] take_page_off_buddy
8.62% ksoftirqd/0 [kernel.kallsyms] [k] enetc_pci_remove
5.61% ksoftirqd/0 [kernel.kallsyms] [k] memmove
5.44% ksoftirqd/0 [kernel.kallsyms] [k] inet_gro_receive
4.61% ksoftirqd/0 [kernel.kallsyms] [k] enetc_poll
4.20% ksoftirqd/0 [kernel.kallsyms] [k] inet_gso_segment
3.58% ksoftirqd/0 [kernel.kallsyms] [k] dev_gro_receive
3.19% ksoftirqd/0 [kernel.kallsyms] [k] build_skb
3.11% ksoftirqd/0 [kernel.kallsyms] [k] skb_release_all
2.80% ksoftirqd/0 [kernel.kallsyms] [k] __copy_skb_header
2.79% ksoftirqd/0 [kernel.kallsyms] [k] eth_type_trans
2.31% ksoftirqd/0 [kernel.kallsyms] [k] ip_send_check
1.95% ksoftirqd/0 [kernel.kallsyms] [k] __skb_get_hash
1.64% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_alloc
1.54% ksoftirqd/0 [kernel.kallsyms] [k] __netif_receive_skb_core
1.52% ksoftirqd/0 [kernel.kallsyms] [k] dsa_slave_xmit
1.51% ksoftirqd/0 [kernel.kallsyms] [k] kmem_cache_free_bulk
1.49% ksoftirqd/0 [kernel.kallsyms] [k] netdev_pick_tx
1.42% ksoftirqd/0 [kernel.kallsyms] [k] skb_headers_offset_update
1.34% ksoftirqd/0 [kernel.kallsyms] [k] skb_segment_list
1.20% ksoftirqd/0 [kernel.kallsyms] [k] enetc_build_skb
1.19% ksoftirqd/0 [kernel.kallsyms] [k] enetc_start_xmit
1.09% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_xmit
0.94% ksoftirqd/0 [kernel.kallsyms] [k] netdev_core_pick_tx
0.90% ksoftirqd/0 [kernel.kallsyms] [k] __dev_queue_xmit
0.87% ksoftirqd/0 [kernel.kallsyms] [k] ocelot_xmit
0.85% ksoftirqd/0 [kernel.kallsyms] [k] __skb_flow_dissect
0.68% ksoftirqd/0 [kernel.kallsyms] [k] dsa_8021q_rcv
0.63% ksoftirqd/0 [kernel.kallsyms] [k] skb_pull_rcsum
0.63% ksoftirqd/0 [kernel.kallsyms] [k] enetc_flip_rx_buff
0.61% ksoftirqd/0 [kernel.kallsyms] [k] napi_consume_skb
0.50% ksoftirqd/0 [kernel.kallsyms] [k] skb_release_data
0.48% ksoftirqd/0 [kernel.kallsyms] [k] dma_unmap_page_attrs
0.41% ksoftirqd/0 [kernel.kallsyms] [k] enetc_refill_rx_ring
0.37% ksoftirqd/0 [kernel.kallsyms] [k] __build_skb_around
0.33% ksoftirqd/0 [kernel.kallsyms] [k] fib_table_lookup
0.33% ksoftirqd/0 [kernel.kallsyms] [k] napi_skb_cache_put
0.28% ksoftirqd/0 [kernel.kallsyms] [k] gro_cells_receive
0.26% ksoftirqd/0 [kernel.kallsyms] [k] bpf_skb_load_helper_16
So the performance seems to be slightly worse with the patch, for
reasons that are not immediately apparent from perf, as far as I can
tell. If I just look at dsa_switch_rcv, I see there is a decrease in CPU
cycles from 1.01% to 0.83%, but that doesn't reflect in the throughput I
see for some reason.
(also please ignore some oddities in the perf reports like
"enetc_pci_remove", this comes from enetc_lock_mdio/enetc_unlock_mdio, I
have no idea why it gets printed like that)
So yeah, I cannot actually change my setup such that the list iteration
is more expensive than this (swp0 is the first element, swp1 is the second).
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