tcp: tsq: restore minimal amount of queueing

[ Upstream commit 98e09386c0ef4dfd48af7ba60ff908f0d525cdee ]

After commit c9eeec26e32e ("tcp: TSQ can use a dynamic limit"), several
users reported throughput regressions, notably on mvneta and wifi
adapters.

802.11 AMPDU requires a fair amount of queueing to be effective.

This patch partially reverts the change done in tcp_write_xmit()
so that the minimal amount is sysctl_tcp_limit_output_bytes.

It also remove the use of this sysctl while building skb stored
in write queue, as TSO autosizing does the right thing anyway.

Users with well behaving NICS and correct qdisc (like sch_fq),
can then lower the default sysctl_tcp_limit_output_bytes value from
128KB to 8KB.

This new usage of sysctl_tcp_limit_output_bytes permits each driver
authors to check how their driver performs when/if the value is set
to a minimum of 4KB.

Normally, line rate for a single TCP flow should be possible,
but some drivers rely on timers to perform TX completion and
too long TX completion delays prevent reaching full throughput.

Fixes: c9eeec26e32e ("tcp: TSQ can use a dynamic limit")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Sujith Manoharan <sujith@msujith.org>
Reported-by: Arnaud Ebalard <arno@natisbad.org>
Tested-by: Sujith Manoharan <sujith@msujith.org>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index a46d78583ae1d9b1a8b49b96066f423f4b29d037..7d8dc93fe2eb25986832f26c4088c13570fd7a11 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -588,9 +588,6 @@ tcp_limit_output_bytes - INTEGER
        typical pfifo_fast qdiscs.
        tcp_limit_output_bytes limits the number of bytes on qdisc
        or device to reduce artificial RTT/cwnd and reduce bufferbloat.
-       Note: For GSO/TSO enabled flows, we try to have at least two
-       packets in flight. Reducing tcp_limit_output_bytes might also
-       reduce the size of individual GSO packet (64KB being the max)
        Default: 131072
 
 tcp_challenge_ack_limit - INTEGER

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 6e5617b9f9db6f3c2aeacc7923ea763660761d99..be5246e1d5b67c9534b817cd6d2d454e2a5468b6 100644 (file)
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -806,12 +806,6 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
                xmit_size_goal = min_t(u32, gso_size,
                                       sk->sk_gso_max_size - 1 - hlen);
 
-               /* TSQ : try to have at least two segments in flight
-                * (one in NIC TX ring, another in Qdisc)
-                */
-               xmit_size_goal = min_t(u32, xmit_size_goal,
-                                      sysctl_tcp_limit_output_bytes >> 1);
-
                xmit_size_goal = tcp_bound_to_half_wnd(tp, xmit_size_goal);
 
                /* We try hard to avoid divides here */

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d46f2143305c2632e7703f2677310dd177d67208..10e12a61bb5247cd98c0d15edfb61a87bc7bae36 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1875,8 +1875,12 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                 *  - better RTT estimation and ACK scheduling
                 *  - faster recovery
                 *  - high rates
+                * Alas, some drivers / subsystems require a fair amount
+                * of queued bytes to ensure line rate.
+                * One example is wifi aggregation (802.11 AMPDU)
                 */
-               limit = max(skb->truesize, sk->sk_pacing_rate >> 10);
+               limit = max_t(unsigned int, sysctl_tcp_limit_output_bytes,
+                             sk->sk_pacing_rate >> 10);
 
                if (atomic_read(&sk->sk_wmem_alloc) > limit) {
                        set_bit(TSQ_THROTTLED, &tp->tsq_flags);