[PATCH net] net: avoid indirect memory pressure calls

[Florian Westphal <fw@strlen.de>]

There is a noticeable tcp performance regression (loopback or cross-netns),
seen with iperf3 -Z (sendfile mode) when generic retpolines are needed.

With SK_RECLAIM_THRESHOLD checks gone number of calls to enter/leave
memory pressure happen much more often. For TCP indirect calls are
used.

We can't remove the if-set-return short-circuit check in
tcp_enter_memory_pressure because there are callers other than
sk_enter_memory_pressure.  Doing a check in the sk wrapper too
reduces the indirect calls enough to recover some performance.

Before,
0.00-60.00  sec   322 GBytes  46.1 Gbits/sec                  receiver

After:
0.00-60.04  sec   359 GBytes  51.4 Gbits/sec                  receiver

"iperf3 -c $peer -t 60 -Z -f g", connected via veth in another netns.

Fixes: 4890b686f408 ("net: keep sk->sk_forward_alloc as small as possible")
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 net/core/sock.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 341c565dbc26..45d247112aa5 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2809,22 +2809,26 @@ EXPORT_SYMBOL(sock_cmsg_send);
 
 static void sk_enter_memory_pressure(struct sock *sk)
 {
-	if (!sk->sk_prot->enter_memory_pressure)
+	unsigned long *memory_pressure = sk->sk_prot->memory_pressure;
+
+	if (!memory_pressure || READ_ONCE(*memory_pressure))
 		return;
 
-	sk->sk_prot->enter_memory_pressure(sk);
+	if (sk->sk_prot->enter_memory_pressure)
+		sk->sk_prot->enter_memory_pressure(sk);
 }
 
 static void sk_leave_memory_pressure(struct sock *sk)
 {
-	if (sk->sk_prot->leave_memory_pressure) {
-		sk->sk_prot->leave_memory_pressure(sk);
-	} else {
-		unsigned long *memory_pressure = sk->sk_prot->memory_pressure;
+	unsigned long *memory_pressure = sk->sk_prot->memory_pressure;
 
-		if (memory_pressure && READ_ONCE(*memory_pressure))
-			WRITE_ONCE(*memory_pressure, 0);
-	}
+	if (!memory_pressure || READ_ONCE(*memory_pressure) == 0)
+		return;
+
+	if (sk->sk_prot->leave_memory_pressure)
+		sk->sk_prot->leave_memory_pressure(sk);
+	else
+		WRITE_ONCE(*memory_pressure, 0);
 }
 
 DEFINE_STATIC_KEY_FALSE(net_high_order_alloc_disable_key);
-- 
2.39.2