From mboxrd@z Thu Jan 1 00:00:00 1970 From: Damian Lukowski Subject: Re: [net-next PATCH v2 2/3] net: TCP thin linear timeouts Date: Wed, 10 Feb 2010 18:33:28 +0100 Message-ID: References: <4B701EE2.1000006@simula.no> <4B707901.50905@tvk.rwth-aachen.de> <4B718FF4.6040906@simula.no> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes Content-Transfer-Encoding: 7BIT Cc: "netdev@vger.kernel.org" , =?utf-8?Q?Ilpo_J=C3=A4rvinen?= , Eric Dumazet , Arnd Hannemann , LKML , shemminger@vyatta.com, David Miller , william.allen.simpson@gmail.com To: Andreas Petlund Return-path: In-reply-to: <4B718FF4.6040906@simula.no> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Am 09.02.2010, 17:40 Uhr, schrieb Andreas Petlund : > 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