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Commit 6f458dfb authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

tcp: improve latencies of timer triggered events 



Modern TCP stack highly depends on tcp_write_timer() having a small
latency, but current implementation doesn't exactly meet the
expectations.

When a timer fires but finds the socket is owned by the user, it rearms
itself for an additional delay hoping next run will be more
successful.

tcp_write_timer() for example uses a 50ms delay for next try, and it
defeats many attempts to get predictable TCP behavior in term of
latencies.

Use the recently introduced tcp_release_cb(), so that the user owning
the socket will call various handlers right before socket release.

This will permit us to post a followup patch to address the
tcp_tso_should_defer() syndrome (some deferred packets have to wait
RTO timer to be transmitted, while cwnd should allow us to send them
sooner)

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: H.K. Jerry Chu <hkchu@google.com>
Cc: John Heffner <johnwheffner@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 9dc27415

include/linux/tcp.h

+3 −1
Original line number Diff line number Diff line
@@ -515,7 +515,9 @@ struct tcp_sock { 
enum tsq_flags {

	TSQ_THROTTLED,

	TSQ_QUEUED,

	TSQ_OWNED, /* tcp_tasklet_func() found socket was locked */

	TCP_TSQ_DEFERRED,	   /* tcp_tasklet_func() found socket was owned */

	TCP_WRITE_TIMER_DEFERRED,  /* tcp_write_timer() found socket was owned */

	TCP_DELACK_TIMER_DEFERRED, /* tcp_delack_timer() found socket was owned */

};



static inline struct tcp_sock *tcp_sk(const struct sock *sk)

include/net/tcp.h

+2 −0
Original line number Diff line number Diff line
@@ -350,6 +350,8 @@ extern int tcp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg, 
extern int tcp_sendpage(struct sock *sk, struct page *page, int offset,

			size_t size, int flags);

extern void tcp_release_cb(struct sock *sk);

extern void tcp_write_timer_handler(struct sock *sk);

extern void tcp_delack_timer_handler(struct sock *sk);

extern int tcp_ioctl(struct sock *sk, int cmd, unsigned long arg);

extern int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,

				 const struct tcphdr *th, unsigned int len);

net/ipv4/tcp_output.c

+29 −17
Original line number Diff line number Diff line
@@ -837,6 +837,13 @@ struct tsq_tasklet { 
};

static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);



static void tcp_tsq_handler(struct sock *sk)

{

	if ((1 << sk->sk_state) &

	    (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |

	     TCPF_CLOSE_WAIT  | TCPF_LAST_ACK))

		tcp_write_xmit(sk, tcp_current_mss(sk), 0, 0, GFP_ATOMIC);

}

/*

 * One tasklest per cpu tries to send more skbs.

 * We run in tasklet context but need to disable irqs when
@@ -864,16 +871,10 @@ static void tcp_tasklet_func(unsigned long data) 
		bh_lock_sock(sk);



		if (!sock_owned_by_user(sk)) {

			if ((1 << sk->sk_state) &

			    (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 |

			     TCPF_CLOSING | TCPF_CLOSE_WAIT | TCPF_LAST_ACK))

				tcp_write_xmit(sk,

					       tcp_current_mss(sk),

					       0, 0,

					       GFP_ATOMIC);

			tcp_tsq_handler(sk);

		} else {

			/* defer the work to tcp_release_cb() */

			set_bit(TSQ_OWNED, &tp->tsq_flags);

			set_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);

		}

		bh_unlock_sock(sk);
@@ -882,6 +883,9 @@ static void tcp_tasklet_func(unsigned long data) 
	}

}



#define TCP_DEFERRED_ALL ((1UL << TCP_TSQ_DEFERRED) |		\

			  (1UL << TCP_WRITE_TIMER_DEFERRED) |	\

			  (1UL << TCP_DELACK_TIMER_DEFERRED))

/**

 * tcp_release_cb - tcp release_sock() callback

 * @sk: socket
@@ -892,16 +896,24 @@ static void tcp_tasklet_func(unsigned long data) 
void tcp_release_cb(struct sock *sk)

{

	struct tcp_sock *tp = tcp_sk(sk);

	unsigned long flags, nflags;



	if (test_and_clear_bit(TSQ_OWNED, &tp->tsq_flags)) {

		if ((1 << sk->sk_state) &

		    (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 |

		     TCPF_CLOSING | TCPF_CLOSE_WAIT | TCPF_LAST_ACK))

			tcp_write_xmit(sk,

				       tcp_current_mss(sk),

				       0, 0,

				       GFP_ATOMIC);

	}

	/* perform an atomic operation only if at least one flag is set */

	do {

		flags = tp->tsq_flags;

		if (!(flags & TCP_DEFERRED_ALL))

			return;

		nflags = flags & ~TCP_DEFERRED_ALL;

	} while (cmpxchg(&tp->tsq_flags, flags, nflags) != flags);



	if (flags & (1UL << TCP_TSQ_DEFERRED))

		tcp_tsq_handler(sk);



	if (flags & (1UL << TCP_WRITE_TIMER_DEFERRED))

		tcp_write_timer_handler(sk);



	if (flags & (1UL << TCP_DELACK_TIMER_DEFERRED))

		tcp_delack_timer_handler(sk);

}

EXPORT_SYMBOL(tcp_release_cb);

net/ipv4/tcp_timer.c

+37 −33
Original line number Diff line number Diff line
@@ -32,17 +32,6 @@ int sysctl_tcp_retries2 __read_mostly = TCP_RETR2; 
int sysctl_tcp_orphan_retries __read_mostly;

int sysctl_tcp_thin_linear_timeouts __read_mostly;



static void tcp_write_timer(unsigned long);

static void tcp_delack_timer(unsigned long);

static void tcp_keepalive_timer (unsigned long data);



void tcp_init_xmit_timers(struct sock *sk)

{

	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,

				  &tcp_keepalive_timer);

}

EXPORT_SYMBOL(tcp_init_xmit_timers);



static void tcp_write_err(struct sock *sk)

{

	sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
@@ -205,21 +194,11 @@ static int tcp_write_timeout(struct sock *sk) 
	return 0;

}



static void tcp_delack_timer(unsigned long data)

void tcp_delack_timer_handler(struct sock *sk)

{

	struct sock *sk = (struct sock *)data;

	struct tcp_sock *tp = tcp_sk(sk);

	struct inet_connection_sock *icsk = inet_csk(sk);



	bh_lock_sock(sk);

	if (sock_owned_by_user(sk)) {

		/* Try again later. */

		icsk->icsk_ack.blocked = 1;

		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);

		sk_reset_timer(sk, &icsk->icsk_delack_timer, jiffies + TCP_DELACK_MIN);

		goto out_unlock;

	}



	sk_mem_reclaim_partial(sk);



	if (sk->sk_state == TCP_CLOSE || !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
@@ -260,7 +239,21 @@ static void tcp_delack_timer(unsigned long data) 
out:

	if (sk_under_memory_pressure(sk))

		sk_mem_reclaim(sk);

out_unlock:

}



static void tcp_delack_timer(unsigned long data)

{

	struct sock *sk = (struct sock *)data;



	bh_lock_sock(sk);

	if (!sock_owned_by_user(sk)) {

		tcp_delack_timer_handler(sk);

	} else {

		inet_csk(sk)->icsk_ack.blocked = 1;

		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);

		/* deleguate our work to tcp_release_cb() */

		set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags);

	}

	bh_unlock_sock(sk);

	sock_put(sk);

}
@@ -450,19 +443,11 @@ void tcp_retransmit_timer(struct sock *sk) 
out:;

}



static void tcp_write_timer(unsigned long data)

void tcp_write_timer_handler(struct sock *sk)

{

	struct sock *sk = (struct sock *)data;

	struct inet_connection_sock *icsk = inet_csk(sk);

	int event;



	bh_lock_sock(sk);

	if (sock_owned_by_user(sk)) {

		/* Try again later */

		sk_reset_timer(sk, &icsk->icsk_retransmit_timer, jiffies + (HZ / 20));

		goto out_unlock;

	}



	if (sk->sk_state == TCP_CLOSE || !icsk->icsk_pending)

		goto out;
@@ -485,7 +470,19 @@ static void tcp_write_timer(unsigned long data) 


out:

	sk_mem_reclaim(sk);

out_unlock:

}



static void tcp_write_timer(unsigned long data)

{

	struct sock *sk = (struct sock *)data;



	bh_lock_sock(sk);

	if (!sock_owned_by_user(sk)) {

		tcp_write_timer_handler(sk);

	} else {

		/* deleguate our work to tcp_release_cb() */

		set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags);

	}

	bh_unlock_sock(sk);

	sock_put(sk);

}
@@ -602,3 +599,10 @@ static void tcp_keepalive_timer (unsigned long data) 
	bh_unlock_sock(sk);

	sock_put(sk);

}



void tcp_init_xmit_timers(struct sock *sk)

{

	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,

				  &tcp_keepalive_timer);

}

EXPORT_SYMBOL(tcp_init_xmit_timers);