linux-kernel - Re: [net-next PATCH v2 2/3] net: TCP thin linear timeouts

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Date:	Wed, 10 Feb 2010 18:33:28 +0100
From:	Damian Lukowski <damian@....rwth-aachen.de>
To:	Andreas Petlund <apetlund@...ula.no>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Arnd Hannemann <hannemann@...s.rwth-aachen.de>,
	LKML <linux-kernel@...r.kernel.org>, shemminger@...tta.com,
	David Miller <davem@...emloft.net>,
	william.allen.simpson@...il.com
Subject: Re: [net-next PATCH v2 2/3] net: TCP thin linear timeouts

Am 09.02.2010, 17:40 Uhr, schrieb Andreas Petlund <apetlund@...ula.no>:

> On 02/08/2010 09:50 PM, Damian Lukowski wrote:
>>>  out_reset_timer:
>>> -	icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
>>> +	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
>>> +	 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
>>> +	 * might be increased if the stream oscillates between thin and  
>>> thick,
>>> +	 * thus the old value might already be too high compared to the value
>>> +	 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
>>> +	 * backoff. Limit to TCP_THIN_LT_RETRIES before initiating  
>>> exponential
>>> +	 * backoff behaviour to avoid continue hammering linear-timeout
>>> +	 * retransmissions into a black hole*/
>>> +	if ((tp->thin_lt || sysctl_tcp_force_thin_linear_timeouts) &&
>>> +	    tcp_stream_is_thin(sk) && sk->sk_state == TCP_ESTABLISHED &&
>>> +	    icsk->icsk_retransmits <= TCP_THIN_LT_RETRIES) {
>>> +		icsk->icsk_backoff = 0;
>>
>> Hi,
>> I think, this value should be at least 1, as icsk_backoff
>> might be decreased to -1 and used for bit-shifting in tcp_v4_err().
>> A lower boundary check might be even better.
>
> Hi
>
> Thanks for the feedback.
>
> As far as I can see, the check a couple of lines above the decrementation
> stops the icsk->icsk_backoff from being decremented if already zero.
> Beyond that I cannot find any more places where this situation may arise.
> Please correct me if I'm wrong and a boundary check is indeed warranted.

Oops, you are right, of course ...
I just had in mind, that a thin stream might also be a candidate for
backoff reversion when connectivity breaks down, that's why I said
"at least 1". And I really have forgotten the already existing check,  
sorry.
So, setting icsk_backoff = 0 will prevent a backoff reversion, but that's
ok, as the RTO is not doubled in the first place.

It might have been an issue, if you had not used __tcp_set_rto() but left
the value unchanged *and* a non-thin stream became thin at some point in
the RTO retransmission phase (if that is even possible).

Damian
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