netdev - Re: [PATCH] net: reduce number of reference taken on sk

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Message-Id: <20090518.215823.98238538.davem@davemloft.net>
Date:	Mon, 18 May 2009 21:58:23 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	khc@...waw.pl, netdev@...r.kernel.org, satoru.satoh@...il.com
Subject: Re: [PATCH] net: reduce number of reference taken on sk_refcnt

From: Eric Dumazet <dada1@...mosbay.com>
Date: Sun, 10 May 2009 12:45:56 +0200

> Patch follows for RFC only (not Signed-of...), and based on net-next-2.6 

Thanks for the analysis.

> @@ -922,10 +922,13 @@ static inline int tcp_prequeue(struct sock *sk, struct sk_buff *skb)
>  	} else if (skb_queue_len(&tp->ucopy.prequeue) == 1) {
>  		wake_up_interruptible_poll(sk->sk_sleep,
>  					   POLLIN | POLLRDNORM | POLLRDBAND);
> -		if (!inet_csk_ack_scheduled(sk))
> +		if (!inet_csk_ack_scheduled(sk)) {
> +			unsigned int delay = (3 * tcp_rto_min(sk)) / 4;
> +
> +			delay = min(inet_csk(sk)->icsk_ack.ato, delay);
>  			inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
> -						  (3 * tcp_rto_min(sk)) / 4,
> -						  TCP_RTO_MAX);
> +						  delay, TCP_RTO_MAX);
> +		}
>  	}
>  	return 1;

I think this code is trying to aggressively stretch the ACK when
prequeueing.  In order to make sure there is enough time to get
the process on the CPU and send a response, and thus piggyback
the ACK.

If that turns out not to really matter, or matter less than your
problem, then we can make your change and I'm all for it.
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