netdev - Re: [PATCH net-next 2/2] tcp: add exponential backoff in __tcp_send

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Date:   Wed, 30 Sep 2020 11:00:55 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 2/2] tcp: add exponential backoff in __tcp_send_ack()

On Wed, Sep 30, 2020 at 8:55 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.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 <edumazet@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.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
>