netdev - Re: [RFC PATCH net-next 1/2] tcp: RTO Restart (RTOR)

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Date:	Mon, 07 Dec 2015 09:03:44 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Per Hurtig <per.hurtig@....se>
Cc:	davem@...emloft.net, edumazet@...gle.com, ncardwell@...gle.com,
	nanditad@...gle.com, tom@...bertland.com, ycheng@...gle.com,
	viro@...iv.linux.org.uk, fw@...len.de, mleitner@...hat.com,
	daniel@...earbox.net, willemb@...gle.com,
	ilpo.jarvinen@...sinki.fi, pasi.sarolahti@....fi,
	stephen@...workplumber.org, netdev@...r.kernel.org,
	anna.brunstrom@....se, apetlund@...ula.no, michawe@....uio.no,
	mohammad.rajiullah@....se
Subject: Re: [RFC PATCH net-next 1/2] tcp: RTO Restart (RTOR)

On Mon, 2015-12-07 at 10:00 +0100, Per Hurtig wrote:
\
>  
> +static u32 tcp_unsent_pkts(const struct sock *sk)
> +{
> +	struct sk_buff *skb = tcp_send_head(sk);
> +	u32 pkts = 0;
> +
> +	if (skb)
> +		tcp_for_write_queue_from(skb, sk)
> +			pkts += tcp_skb_pcount(skb);
> +
> +	return pkts;
> +}

write queue can be very big (consider GSO/TSO being off for example)

Parsing it just to implement later :

(tp->packets_out + tcp_unsent_pkts(sk) <
> +			    TCP_RTORESTART_THRESH)

is probably not very efficient.

I would rather implement a different helper, aborting the loop as soon
as the condition can not be met anymore.

> +
>  /* Restart timer after forward progress on connection.
>   * RFC2988 recommends to restart timer to now+rto.
>   */
> @@ -3027,6 +3040,17 @@ void tcp_rearm_rto(struct sock *sk)
>  			 */
>  			if (delta > 0)
>  				rto = delta;
> +		} else if (icsk->icsk_pending == ICSK_TIME_RETRANS &&
> +			   (sysctl_tcp_timer_restart == 1 ||
> +			    sysctl_tcp_timer_restart == 3) &&
> +			   (tp->packets_out + tcp_unsent_pkts(sk) <
> +			    TCP_RTORESTART_THRESH)) {
> +			struct sk_buff *skb = tcp_write_queue_head(sk);
> +			const u32 rto_time_stamp = tcp_skb_timestamp(skb);
> +			s32 delta = (s32)(tcp_time_stamp - rto_time_stamp);
> +
> +			if (delta > 0 && rto > delta)
> +				rto -= delta;
>  		}
>  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
>  					  TCP_RTO_MAX);


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