tcp: allow drivers to tweak TSQ logic

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>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -267,6 +267,7 @@ 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
@@ -451,6 +452,7 @@ struct sock {
 	kmemcheck_bitfield_end(flags);
 
 	u16			sk_gso_max_segs;
+	u8			sk_pacing_shift;
 	unsigned long	        sk_lingertime;
 	struct proto		*sk_prot_creator;
 	rwlock_t		sk_callback_lock;

net/core/sock.c

@@ -2746,6 +2746,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 
 	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

net/ipv4/tcp_output.c

@@ -1720,7 +1720,7 @@ u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
 {
 	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,
@@ -2198,7 +2198,7 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
 {
 	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,
 		      sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes);
 	limit <<= factor;