netdev - Re: TCP not retransmitting

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Date:	Fri, 11 Dec 2009 13:57:34 +0100
From:	Frederic Leroy <fredo@...rox.org>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: TCP not retransmitting

Hi Ilpo,

On Thu, Dec 10, 2009 at 03:24:20PM +0200, Ilpo Järvinen wrote:
> Hi,
> 
> Now that the more important issues are out of the way, could try with the 
> following debug patch, if we could get some information on why the 
> retransmission are not happening when they're supposed to (the EAGAIN 
> problem we noticed earlier). Besides trying with 2.6.32, it might also be 
> worth of the effort to try that in 2.6.31 to see if that also gives those 
> EAGAIN things or if it's a new issue. It is quite insignificant whether 
> to previous fix patch is included or not though perhaps easier to know 
> that the problem really happened if a stall is experiencable vs. the fixed 
> kernel.

I made 3 test this time. Unlike the other, I put them appart in a new directory : 
http://www.starox.org/pub/scp_stall/eagain/

The first was made with 2.6.31 kernel.
I disconnected the ethernet cable for about 30s.
There was some eagain error.

The two other were made with 2.6.32, without the fix.
I didn't disconnected anything as I know it will fails quickly :)
The first try has an eagain error.

> 
> -- 
>  i.
> 
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index fcd278a..c29aed0 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1894,6 +1894,8 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
>  		icsk->icsk_mtup.probe_size = 0;
>  	}
>  
> +	printk("sk: %p wm_a: %u wm_q: %u sbuf: %u\n", sk,
> +		atomic_read(&sk->sk_wmem_alloc), sk->sk_wmem_queued, sk->sk_sndbuf);
>  	/* Do not sent more than we queued. 1/4 is reserved for possible
>  	 * copying overhead: fragmentation, tunneling, mangling etc.
>  	 */
> @@ -1986,6 +1988,7 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
>  		 */
>  		TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt;
>  	}
> +	printk("sk: %p E %u", sk, err);
                           ^
                      missing \n ;-)
>  	return err;
>  }

-- 
Frédéric Leroy
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