This patch is a result of our long-standing debug sessions, where it all
started as "networking is slow", and TCP network throughput suddenly
dropped from tens of Gbps to few Mbps, and we could not see anything in
the kernel log or netstat counters.

Currently, we have two memory pressure counters for TCP sockets [1],
which we manipulate only when the memory pressure is signalled through
the proto struct [2]. However, the memory pressure can also be signaled
through the cgroup memory subsystem, which we do not reflect in the
netstat counters. In the end, when the cgroup memory subsystem signals
that it is under pressure, we silently reduce the advertised TCP window
with tcp_adjust_rcv_ssthresh() to 4*advmss, which causes a significant
throughput reduction.

Keep in mind that when the cgroup memory subsystem signals the socket
memory pressure, it affects all sockets used in that cgroup.

This patch exposes a new file for each cgroup in sysfs which signals
the cgroup socket memory pressure. The file is accessible in
the following path.

  /sys/fs/cgroup/**/<cgroup name>/memory.net.socket_pressure

The output value is an integer matching the internal semantics of the
struct mem_cgroup for socket_pressure. It is a periodic re-arm clock,
representing the end of the said socket memory pressure, and once the
clock is re-armed it is set to jiffies + HZ.

Link: https://elixir.bootlin.com/linux/v6.15.4/source/include/uapi/linux/snmp.h#L231-L232 [1]
Link: https://elixir.bootlin.com/linux/v6.15.4/source/include/net/sock.h#L1300-L1301 [2]
Co-developed-by: Matyas Hurtik <redacted>
Signed-off-by: Matyas Hurtik <redacted>
Signed-off-by: Daniel Sedlak <redacted>
---
Changes:
v2 -> v3:
- Expose the socket memory pressure on the cgroups instead of netstat
- Split patch
- Link: https://lore.kernel.org/netdev/20250714143613.42184-1-daniel.sedlak@cdn77.com/ (local)

v1 -> v2:
- Add tracepoint
- Link: https://lore.kernel.org/netdev/20250707105205.222558-1-daniel.sedlak@cdn77.com/ (local)


 mm/memcontrol.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 902da8a9c643..8e8808fb2d7a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -4647,6 +4647,15 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
 	return nbytes;
 }
 
+static int memory_socket_pressure_show(struct seq_file *m, void *v)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
+
+	seq_printf(m, "%lu\n", READ_ONCE(memcg->socket_pressure));
+
+	return 0;
+}
+
 static struct cftype memory_files[] = {
 	{
 		.name = "current",

@@ -4718,6 +4727,11 @@ static struct cftype memory_files[] = {
 		.flags = CFTYPE_NS_DELEGATABLE,
 		.write = memory_reclaim,
 	},
+	{
+		.name = "net.socket_pressure",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.seq_show = memory_socket_pressure_show,
+	},
 	{ }	/* terminate */
 };
 

base-commit: e96ee511c906c59b7c4e6efd9d9b33917730e000
-- 
2.39.5