inet: move icmp_global_{credit,stamp} to a separate cache line

icmp_global_credit was meant to be changed ~1000 times per second,
but if an admin sets net.ipv4.icmp_msgs_per_sec to a very high value,
icmp_global_credit changes can inflict false sharing to surrounding
fields that are read mostly.

Move icmp_global_credit and icmp_global_stamp to a separate
cacheline aligned group.

Fixes: b056b4cd9178 ("icmp: move icmp_global.credit and icmp_global.stamp to per netns storage")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260216142832.3834174-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/netns/ipv4.h

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 2dbd46f..8e971c7 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -88,6 +88,12 @@ struct netns_ipv4 {
 	int sysctl_tcp_rcvbuf_low_rtt;
 	__cacheline_group_end(netns_ipv4_read_rx);
 
+	/* ICMP rate limiter hot cache line. */
+	__cacheline_group_begin_aligned(icmp);
+	atomic_t	icmp_global_credit;
+	u32		icmp_global_stamp;
+	__cacheline_group_end_aligned(icmp);
+
 	struct inet_timewait_death_row tcp_death_row;
 	struct udp_table *udp_table;
 
@@ -141,8 +147,7 @@ struct netns_ipv4 {
 	int sysctl_icmp_ratemask;
 	int sysctl_icmp_msgs_per_sec;
 	int sysctl_icmp_msgs_burst;
-	atomic_t icmp_global_credit;
-	u32 icmp_global_stamp;
+
 	u32 ip_rt_min_pmtu;
 	int ip_rt_mtu_expires;
 	int ip_rt_min_advmss;