[PATCH net-next 2/2] tcp: add exponential backoff in __tcp_send_ack()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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: Eric Dumazet <eduma...@google.com>
---
 include/net/inet_connection_sock.h |  3 ++-
 net/ipv4/tcp_output.c              | 11 ++++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/net/inet_connection_sock.h 
b/include/net/inet_connection_sock.h
index 
79875f976190750819948425e63dd0309c699050..7338b3865a2a3d278dc27c0167bba1b966bbda9f
 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -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
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 
6bd4e383030ea20441332a30e98fbda8cd90f84a..bf48cd73e96787a14b6f9af8beddb1067a7cb8dc
 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -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;
        }
 
-- 
2.28.0.806.g8561365e88-goog