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Message-Id: <20181204155817.43704-1-edumazet@google.com>
Date: Tue, 4 Dec 2018 07:58:17 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>
Cc: netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Eric Dumazet <eric.dumazet@...il.com>,
Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: [PATCH net-next] tcp: reduce POLLOUT events caused by TCP_NOTSENT_LOWAT
TCP_NOTSENT_LOWAT socket option or sysctl was added in linux-3.12
as a step to enable bigger tcp sndbuf limits.
It works reasonably well, but the following happens :
Once the limit is reached, TCP stack generates
an [E]POLLOUT event for every incoming ACK packet.
This causes a high number of context switches.
This patch implements the strategy David Miller added
in sock_def_write_space() :
- If TCP socket has a notsent_lowat constraint of X bytes,
allow sendmsg() to fill up to X bytes, but send [E]POLLOUT
only if number of notsent bytes is below X/2
This considerably reduces TCP_NOTSENT_LOWAT overhead,
while allowing to keep the pipe full.
Tested:
100 ms RTT netem testbed between A and B, 100 concurrent TCP_STREAM
A:/# cat /proc/sys/net/ipv4/tcp_wmem
4096 262144 64000000
A:/# super_netperf 100 -H B -l 1000 -- -K bbr &
A:/# grep TCP /proc/net/sockstat
TCP: inuse 203 orphan 0 tw 19 alloc 414 mem 1364904 # This is about 54 MB of memory per flow :/
A:/# vmstat 5 5
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 0 256220672 13532 694976 0 0 10 0 28 14 0 1 99 0 0
2 0 0 256320016 13532 698480 0 0 512 0 715901 5927 0 10 90 0 0
0 0 0 256197232 13532 700992 0 0 735 13 771161 5849 0 11 89 0 0
1 0 0 256233824 13532 703320 0 0 512 23 719650 6635 0 11 89 0 0
2 0 0 256226880 13532 705780 0 0 642 4 775650 6009 0 12 88 0 0
A:/# echo 2097152 >/proc/sys/net/ipv4/tcp_notsent_lowat
A:/# grep TCP /proc/net/sockstat
TCP: inuse 203 orphan 0 tw 19 alloc 414 mem 86411 # 3.5 MB per flow
A:/# vmstat 5 5 # check that context switches have not inflated too much.
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
2 0 0 260386512 13592 662148 0 0 10 0 17 14 0 1 99 0 0
0 0 0 260519680 13592 604184 0 0 512 13 726843 12424 0 10 90 0 0
1 1 0 260435424 13592 598360 0 0 512 25 764645 12925 0 10 90 0 0
1 0 0 260855392 13592 578380 0 0 512 7 722943 13624 0 11 88 0 0
1 0 0 260445008 13592 601176 0 0 614 34 772288 14317 0 10 90 0 0
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
---
include/net/sock.h | 20 +++++++++++++++-----
include/net/tcp.h | 8 ++++++--
net/core/stream.c | 2 +-
3 files changed, 22 insertions(+), 8 deletions(-)
diff --git a/include/net/sock.h b/include/net/sock.h
index f665d74ae509b41d3ffe10faad4065b61f369341..df390a3e23fedca4c75e3a7e66ad9e35aac71746 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1110,7 +1110,7 @@ struct proto {
unsigned int inuse_idx;
#endif
- bool (*stream_memory_free)(const struct sock *sk);
+ bool (*stream_memory_free)(const struct sock *sk, int wake);
bool (*stream_memory_read)(const struct sock *sk);
/* Memory pressure */
void (*enter_memory_pressure)(struct sock *sk);
@@ -1192,19 +1192,29 @@ static inline void sk_refcnt_debug_release(const struct sock *sk)
#define sk_refcnt_debug_release(sk) do { } while (0)
#endif /* SOCK_REFCNT_DEBUG */
-static inline bool sk_stream_memory_free(const struct sock *sk)
+static inline bool __sk_stream_memory_free(const struct sock *sk, int wake)
{
if (sk->sk_wmem_queued >= sk->sk_sndbuf)
return false;
return sk->sk_prot->stream_memory_free ?
- sk->sk_prot->stream_memory_free(sk) : true;
+ sk->sk_prot->stream_memory_free(sk, wake) : true;
}
-static inline bool sk_stream_is_writeable(const struct sock *sk)
+static inline bool sk_stream_memory_free(const struct sock *sk)
+{
+ return __sk_stream_memory_free(sk, 0);
+}
+
+static inline bool __sk_stream_is_writeable(const struct sock *sk, int wake)
{
return sk_stream_wspace(sk) >= sk_stream_min_wspace(sk) &&
- sk_stream_memory_free(sk);
+ __sk_stream_memory_free(sk, wake);
+}
+
+static inline bool sk_stream_is_writeable(const struct sock *sk)
+{
+ return __sk_stream_is_writeable(sk, 0);
}
static inline int sk_under_cgroup_hierarchy(struct sock *sk,
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0681afc62354c6693e316ae66cb7e72a1cb8bd0a..e0a65c067662346f26a14754ac73de0600d797a8 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1870,12 +1870,16 @@ static inline u32 tcp_notsent_lowat(const struct tcp_sock *tp)
return tp->notsent_lowat ?: net->ipv4.sysctl_tcp_notsent_lowat;
}
-static inline bool tcp_stream_memory_free(const struct sock *sk)
+/* @wake is one when sk_stream_write_space() calls us.
+ * This sends EPOLLOUT only if notsent_bytes is half the limit.
+ * This mimics the strategy used in sock_def_write_space().
+ */
+static inline bool tcp_stream_memory_free(const struct sock *sk, int wake)
{
const struct tcp_sock *tp = tcp_sk(sk);
u32 notsent_bytes = tp->write_seq - tp->snd_nxt;
- return notsent_bytes < tcp_notsent_lowat(tp);
+ return (notsent_bytes << wake) < tcp_notsent_lowat(tp);
}
#ifdef CONFIG_PROC_FS
diff --git a/net/core/stream.c b/net/core/stream.c
index 7d329fb1f553a832e277e12989b8c49ede21ea0e..e94bb02a56295ec2db34ab423a8c7c890df0a696 100644
--- a/net/core/stream.c
+++ b/net/core/stream.c
@@ -32,7 +32,7 @@ void sk_stream_write_space(struct sock *sk)
struct socket *sock = sk->sk_socket;
struct socket_wq *wq;
- if (sk_stream_is_writeable(sk) && sock) {
+ if (__sk_stream_is_writeable(sk, 1) && sock) {
clear_bit(SOCK_NOSPACE, &sock->flags);
rcu_read_lock();
--
2.20.0.rc1.387.gf8505762e3-goog
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