udp: Introduce optional per-netns hash table.

The maximum hash table size is 64K due to the nature of the protocol. [0]
It's smaller than TCP, and fewer sockets can cause a performance drop.

On an EC2 c5.24xlarge instance (192 GiB memory), after running iperf3 in
different netns, creating 32Mi sockets without data transfer in the root
netns causes regression for the iperf3's connection.

  uhash_entries		sockets		length		Gbps
	    64K		      1		     1		5.69
			    1Mi		    16		5.27
			    2Mi		    32		4.90
			    4Mi		    64		4.09
			    8Mi		   128		2.96
			   16Mi		   256		2.06
			   32Mi		   512		1.12

The per-netns hash table breaks the lengthy lists into shorter ones.  It is
useful on a multi-tenant system with thousands of netns.  With smaller hash
tables, we can look up sockets faster, isolate noisy neighbours, and reduce
lock contention.

The max size of the per-netns table is 64K as well.  This is because the
possible hash range by udp_hashfn() always fits in 64K within the same
netns and we cannot make full use of the whole buckets larger than 64K.

  /* 0 < num < 64K  ->  X < hash < X + 64K */
  (num + net_hash_mix(net)) & mask;

Also, the min size is 128.  We use a bitmap to search for an available
port in udp_lib_get_port().  To keep the bitmap on the stack and not
fire the CONFIG_FRAME_WARN error at build time, we round up the table
size to 128.

The sysctl usage is the same with TCP:

  $ dmesg | cut -d ' ' -f 6- | grep "UDP hash"
  UDP hash table entries: 65536 (order: 9, 2097152 bytes, vmalloc)

  # sysctl net.ipv4.udp_hash_entries
  net.ipv4.udp_hash_entries = 65536  # can be changed by uhash_entries

  # sysctl net.ipv4.udp_child_hash_entries
  net.ipv4.udp_child_hash_entries = 0  # disabled by default

  # ip netns add test1
  # ip netns exec test1 sysctl net.ipv4.udp_hash_entries
  net.ipv4.udp_hash_entries = -65536  # share the global table

  # sysctl -w net.ipv4.udp_child_hash_entries=100
  net.ipv4.udp_child_hash_entries = 100

  # ip netns add test2
  # ip netns exec test2 sysctl net.ipv4.udp_hash_entries
  net.ipv4.udp_hash_entries = 128  # own a per-netns table with 2^n buckets

We could optimise the hash table lookup/iteration further by removing
the netns comparison for the per-netns one in the future.  Also, we
could optimise the sparse udp_hslot layout by putting it in udp_table.

[0]: https://lore.kernel.org/netdev/4ACC2815.7010101@gmail.com/

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 27 insertions, 0 deletions

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 815efc89ad73..727b25cc7ec4 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -1177,6 +1177,33 @@ udp_rmem_min - INTEGER
 udp_wmem_min - INTEGER
 	UDP does not have tx memory accounting and this tunable has no effect.
 
+udp_hash_entries - INTEGER
+	Show the number of hash buckets for UDP sockets in the current
+	networking namespace.
+
+	A negative value means the networking namespace does not own its
+	hash buckets and shares the initial networking namespace's one.
+
+udp_child_ehash_entries - INTEGER
+	Control the number of hash buckets for UDP sockets in the child
+	networking namespace, which must be set before clone() or unshare().
+
+	If the value is not 0, the kernel uses a value rounded up to 2^n
+	as the actual hash bucket size.  0 is a special value, meaning
+	the child networking namespace will share the initial networking
+	namespace's hash buckets.
+
+	Note that the child will use the global one in case the kernel
+	fails to allocate enough memory.  In addition, the global hash
+	buckets are spread over available NUMA nodes, but the allocation
+	of the child hash table depends on the current process's NUMA
+	policy, which could result in performance differences.
+
+	Possible values: 0, 2^n (n: 7 (128) - 16 (64K))
+
+	Default: 0
+
+
 RAW variables
 =============