Commit a37c2134 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

tcp: add exponential backoff in __tcp_send_ack() 



Whenever host is under very high memory pressure,
__tcp_send_ack() skb allocation fails, and we setup
a 200 ms (TCP_DELACK_MAX) timer before retrying.

On hosts with high number of TCP sockets, we can spend
considerable amount of cpu cycles in these attempts,
add high pressure on various spinlocks in mm-layer,
ultimately blocking threads attempting to free space
from making any progress.

This patch adds standard exponential backoff to avoid
adding fuel to the fire.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Acked-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent b6b6d653

include/net/inet_connection_sock.h

+2 −1
Original line number Diff line number Diff line
@@ -110,7 +110,7 @@ struct inet_connection_sock { 
		__u8		  pending;	 /* ACK is pending			   */

		__u8		  quick;	 /* Scheduled number of quick acks	   */

		__u8		  pingpong;	 /* The session is interactive		   */

		/* one byte hole. */

		__u8		  retry;	 /* Number of attempts			   */

		__u32		  ato;		 /* Predicted tick of soft clock	   */

		unsigned long	  timeout;	 /* Currently scheduled timeout		   */

		__u32		  lrcvtime;	 /* timestamp of last received data packet */
@@ -199,6 +199,7 @@ static inline void inet_csk_clear_xmit_timer(struct sock *sk, const int what) 
#endif

	} else if (what == ICSK_TIME_DACK) {

		icsk->icsk_ack.pending = 0;

		icsk->icsk_ack.retry = 0;

#ifdef INET_CSK_CLEAR_TIMERS

		sk_stop_timer(sk, &icsk->icsk_delack_timer);

#endif

net/ipv4/tcp_output.c

+8 −3
Original line number Diff line number Diff line
@@ -3941,10 +3941,15 @@ void __tcp_send_ack(struct sock *sk, u32 rcv_nxt) 
	buff = alloc_skb(MAX_TCP_HEADER,

			 sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));

	if (unlikely(!buff)) {

		struct inet_connection_sock *icsk = inet_csk(sk);

		unsigned long delay;



		delay = TCP_DELACK_MAX << icsk->icsk_ack.retry;

		if (delay < TCP_RTO_MAX)

			icsk->icsk_ack.retry++;

		inet_csk_schedule_ack(sk);

		inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;

		inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,

					  TCP_DELACK_MAX, TCP_RTO_MAX);

		icsk->icsk_ack.ato = TCP_ATO_MIN;

		inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK, delay, TCP_RTO_MAX);

		return;

	}

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