[PATCH] tcp: Add memory pressure flag to sockstat

[Jamie Bainbridge]

When tuning a system it can be helpful to know whether the protocol is
in memory pressure state or not. This can be determined by corresponding
the number of pages in "net.ipv4.tcp_mem" with the current allocation,
but a global variable already tracks this as the source of truth.

Expose that variable in sockstat where other protocol memory usage is
already reported.

Add "pressure" which is 0 in normal state and 1 under pressure:

 # grep TCP /proc/net/sockstat
 TCP: inuse 5 orphan 0 tw 0 alloc 7 mem 1 pressure 0

 # grep TCP /proc/net/sockstat
 TCP: inuse 5 orphan 0 tw 0 alloc 7 mem 1 pressure 1

Tested by writing a large value to global variable tcp_memory_pressure
(it usually stores jiffies when memory pressure was entered) and not
just by code review or editing example output.

Signed-off-by: Jamie Bainbridge <jamie.bainbridge@xxxxxxxxx>
---
 net/ipv4/proc.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index eaf1d3113b62f7dc93fdc7b7c4041140ac63bf69..f4c5ced2de49d5c6d7f5d7ccdaa76c89dcf8c932 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -51,16 +51,17 @@
 static int sockstat_seq_show(struct seq_file *seq, void *v)
 {
 	struct net *net = seq->private;
-	int orphans, sockets;
+	int orphans, sockets, tcp_pressure;
 
 	orphans = tcp_orphan_count_sum();
 	sockets = proto_sockets_allocated_sum_positive(&tcp_prot);
+	tcp_pressure = READ_ONCE(tcp_memory_pressure) ? 1 : 0;
 
 	socket_seq_show(seq);
-	seq_printf(seq, "TCP: inuse %d orphan %d tw %d alloc %d mem %ld\n",
+	seq_printf(seq, "TCP: inuse %d orphan %d tw %d alloc %d mem %ld pressure %d\n",
 		   sock_prot_inuse_get(net, &tcp_prot), orphans,
 		   refcount_read(&net->ipv4.tcp_death_row.tw_refcount) - 1,
-		   sockets, proto_memory_allocated(&tcp_prot));
+		   sockets, proto_memory_allocated(&tcp_prot), tcp_pressure);
 	seq_printf(seq, "UDP: inuse %d mem %ld\n",
 		   sock_prot_inuse_get(net, &udp_prot),
 		   proto_memory_allocated(&udp_prot));
-- 
2.41.0