Merge branch 'net-introduce-net_aligned_data'

Eric Dumazet says:

====================
net: introduce net_aligned_data

____cacheline_aligned_in_smp on small fields like
tcp_memory_allocated and udp_memory_allocated is not good enough.

It makes sure to put these fields at the start of a cache line,
but does not prevent the linker from using the cache line for other
fields, with potential performance impact.

nm -v vmlinux|egrep -5 "tcp_memory_allocated|udp_memory_allocated"

...
ffffffff83e35cc0 B tcp_memory_allocated
ffffffff83e35cc8 b __key.0
ffffffff83e35cc8 b __tcp_tx_delay_enabled.1
ffffffff83e35ce0 b tcp_orphan_timer
...
ffffffff849dddc0 B udp_memory_allocated
ffffffff849dddc8 B udp_encap_needed_key
ffffffff849dddd8 B udpv6_encap_needed_key
ffffffff849dddf0 b inetsw_lock

One solution is to move these sensitive fields to a structure,
so that the compiler is forced to add empty holes between each member.

nm -v vmlinux|egrep -2 "tcp_memory_allocated|udp_memory_allocated|net_aligned_data"

ffffffff885af970 b mem_id_init
ffffffff885af980 b __key.0
ffffffff885af9c0 B net_aligned_data
ffffffff885afa80 B page_pool_mem_providers
ffffffff885afa90 b __key.2

v1: https://lore.kernel.org/netdev/20250627200551.348096-1-edumazet@google.com/
====================

Link: https://patch.msgid.link/20250630093540.3052835-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/include/net/aligned_data.h b/include/net/aligned_data.h
new file mode 100644
index 000000000000..e1a1c8aedc79
--- /dev/null
+++ b/include/net/aligned_data.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _NET_ALIGNED_DATA_H
+#define _NET_ALIGNED_DATA_H
+
+#include <linux/atomic.h>
+#include <linux/types.h>
+
+/* Structure holding cacheline aligned fields on SMP builds.
+ * Each field or group should have an ____cacheline_aligned_in_smp
+ * attribute to ensure no accidental false sharing can happen.
+ */
+struct net_aligned_data {
+	atomic64_t	net_cookie ____cacheline_aligned_in_smp;
+#if defined(CONFIG_INET)
+	atomic_long_t tcp_memory_allocated ____cacheline_aligned_in_smp;
+	atomic_long_t udp_memory_allocated ____cacheline_aligned_in_smp;
+#endif
+};
+
+extern struct net_aligned_data net_aligned_data;
+
+#endif /* _NET_ALIGNED_DATA_H */