netdev - Re: [PATCH net-next v6 3/5] net/smc: Fallback when handshake workqueue congested

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Date:   Wed, 9 Feb 2022 17:11:50 +0100
From:   Karsten Graul <kgraul@...ux.ibm.com>
To:     "D. Wythe" <alibuda@...ux.alibaba.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net-next v6 3/5] net/smc: Fallback when handshake
 workqueue congested

On 09/02/2022 15:11, D. Wythe wrote:
> From: "D. Wythe" <alibuda@...ux.alibaba.com>
> 
> This patch intends to provide a mechanism to allow automatic fallback to

I would like to avoid the wording fallback all over here. The term SMC fallback
is used for SMC connections that are in our socket list, but use TCP because 
something went wrong during handshake.
What you changes result in are TCP-only connections which are not handled by
the SMC module at all. So the comments should use a different naming for that.
What the patch actually does is to disable the SMC experimental TCP header option,
so the client receives no SMC indication and does not proceed with SMC.
Is this correct?

Please also see my comments below.

> TCP according to the pressure of SMC handshake process. At present,
> frequent visits will cause the incoming connections to be backlogged in
> SMC handshake queue, raise the connections established time. Which is
> quite unacceptable for those applications who base on short lived
> connections.
> 
> There are two ways to implement this mechanism:
> 
> 1. Fallback when TCP established.
> 2. Fallback before TCP established.
> 
> In the first way, we need to wait and receive CLC messages that the
> client will potentially send, and then actively reply with a decline
> message, in a sense, which is also a sort of SMC handshake, affect the
> connections established time on its way.
> 
> In the second way, the only problem is that we need to inject SMC logic
> into TCP when it is about to reply the incoming SYN, since we already do
> that, it's seems not a problem anymore. And advantage is obvious, few
> additional processes are required to complete the fallback.
> 
> This patch use the second way.
> 
> Link: https://lore.kernel.org/all/1641301961-59331-1-git-send-email-alibuda@linux.alibaba.com/
> Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>
> ---
>  include/linux/tcp.h  |  1 +
>  net/ipv4/tcp_input.c |  3 ++-
>  net/smc/af_smc.c     | 18 ++++++++++++++++++
>  3 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 78b91bb..1c4ae5d 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -394,6 +394,7 @@ struct tcp_sock {
>  	bool	is_mptcp;
>  #endif
>  #if IS_ENABLED(CONFIG_SMC)
> +	bool	(*smc_in_limited)(const struct sock *sk);

Better variable name: smc_hs_congested

>  	bool	syn_smc;	/* SYN includes SMC */
>  #endif
>  
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index af94a6d..e817ec6 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -6703,7 +6703,8 @@ static void tcp_openreq_init(struct request_sock *req,
>  	ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
>  	ireq->ir_mark = inet_request_mark(sk, skb);
>  #if IS_ENABLED(CONFIG_SMC)
> -	ireq->smc_ok = rx_opt->smc_ok;
> +	ireq->smc_ok = rx_opt->smc_ok && !(tcp_sk(sk)->smc_in_limited &&
> +			tcp_sk(sk)->smc_in_limited(sk));

Use new name here and elsewhere ...

>  #endif
>  }
>  
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index ebfce3d..8175f60 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -101,6 +101,22 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk, struct sk_buff
>  	return NULL;
>  }
>  
> +static bool smc_is_in_limited(const struct sock *sk)

Better function name: smc_hs_congested()

> +{
> +	const struct smc_sock *smc;
> +
> +	smc = (const struct smc_sock *)
> +		((uintptr_t)sk->sk_user_data & ~SK_USER_DATA_NOCOPY);
> +
> +	if (!smc)
> +		return true;
> +
> +	if (workqueue_congested(WORK_CPU_UNBOUND, smc_hs_wq))
> +		return true;
> +
> +	return false;
> +}
> +
>  static struct smc_hashinfo smc_v4_hashinfo = {
>  	.lock = __RW_LOCK_UNLOCKED(smc_v4_hashinfo.lock),
>  };
> @@ -2309,6 +2325,8 @@ static int smc_listen(struct socket *sock, int backlog)
>  
>  	inet_csk(smc->clcsock->sk)->icsk_af_ops = &smc->af_ops;
>  
> +	tcp_sk(smc->clcsock->sk)->smc_in_limited = smc_is_in_limited;

Use new names here, too.

> +
>  	rc = kernel_listen(smc->clcsock, backlog);
>  	if (rc) {
>  		smc->clcsock->sk->sk_data_ready = smc->clcsk_data_ready;