kernel: backport a patch that allows drivers to tweak the TSQ logic

Currently local TCP performance on wifi devices can be limited because
the TSQ (TCP Small Queues) code is tuned for wired ethernet latencies.

With this patch drivers can increase the amount of local buffering to
allow TCP to trigger larger aggregation sizes

This commit is modified from the upstream version to allow #ifdef based
backport feature detection

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/generic/backport-4.9/025-tcp-allow-drivers-to-tweak-TSQ-logic.patch

@@ -0,0 +1,85 @@
+From: Eric Dumazet <edumazet@google.com>
+Date: Sat, 11 Nov 2017 15:54:12 -0800
+Subject: [PATCH] tcp: allow drivers to tweak TSQ logic
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+I had many reports that TSQ logic breaks wifi aggregation.
+
+Current logic is to allow up to 1 ms of bytes to be queued into qdisc
+and drivers queues.
+
+But Wifi aggregation needs a bigger budget to allow bigger rates to
+be discovered by various TCP Congestion Controls algorithms.
+
+This patch adds an extra socket field, allowing wifi drivers to select
+another log scale to derive TCP Small Queue credit from current pacing
+rate.
+
+Initial value is 10, meaning that this patch does not change current
+behavior.
+
+We expect wifi drivers to set this field to smaller values (tests have
+been done with values from 6 to 9)
+
+They would have to use following template :
+
+if (skb->sk && skb->sk->sk_pacing_shift != MY_PACING_SHIFT)
+     skb->sk->sk_pacing_shift = MY_PACING_SHIFT;
+
+Ref: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1670041
+Signed-off-by: Eric Dumazet <edumazet@google.com>
+Cc: Johannes Berg <johannes.berg@intel.com>
+Cc: Toke Høiland-Jørgensen <toke@toke.dk>
+Cc: Kir Kolyshkin <kir@openvz.org>
+---
+--- a/include/net/sock.h
++++ b/include/net/sock.h
+@@ -260,6 +260,7 @@ struct sock_common {
+   *	@sk_gso_type: GSO type (e.g. %SKB_GSO_TCPV4)
+   *	@sk_gso_max_size: Maximum GSO segment size to build
+   *	@sk_gso_max_segs: Maximum number of GSO segments
++  *	@sk_pacing_shift: scaling factor for TCP Small Queues
+   *	@sk_lingertime: %SO_LINGER l_linger setting
+   *	@sk_backlog: always used with the per-socket spinlock held
+   *	@sk_callback_lock: used with the callbacks in the end of this struct
+@@ -421,6 +422,8 @@ struct sock {
+ 	kmemcheck_bitfield_end(flags);
+ 
+ 	u16			sk_gso_max_segs;
++#define sk_pacing_shift sk_pacing_shift /* for backport checks */
++	u8			sk_pacing_shift;
+ 	unsigned long	        sk_lingertime;
+ 	struct proto		*sk_prot_creator;
+ 	rwlock_t		sk_callback_lock;
+--- a/net/core/sock.c
++++ b/net/core/sock.c
+@@ -2478,6 +2478,7 @@ void sock_init_data(struct socket *sock,
+ 
+ 	sk->sk_max_pacing_rate = ~0U;
+ 	sk->sk_pacing_rate = ~0U;
++	sk->sk_pacing_shift = 10;
+ 	sk->sk_incoming_cpu = -1;
+ 	/*
+ 	 * Before updating sk_refcnt, we must commit prior changes to memory
+--- a/net/ipv4/tcp_output.c
++++ b/net/ipv4/tcp_output.c
+@@ -1581,7 +1581,7 @@ u32 tcp_tso_autosize(const struct sock *
+ {
+ 	u32 bytes, segs;
+ 
+-	bytes = min(sk->sk_pacing_rate >> 10,
++	bytes = min(sk->sk_pacing_rate >> sk->sk_pacing_shift,
+ 		    sk->sk_gso_max_size - 1 - MAX_TCP_HEADER);
+ 
+ 	/* Goal is to send at least one packet per ms,
+@@ -2083,7 +2083,7 @@ static bool tcp_small_queue_check(struct
+ {
+ 	unsigned int limit;
+ 
+-	limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 10);
++	limit = max(2 * skb->truesize, sk->sk_pacing_rate >> sk->sk_pacing_shift);
+ 	limit = min_t(u32, limit, sysctl_tcp_limit_output_bytes);
+ 	limit <<= factor;
+