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

[Damian Lukowski]

Am 09.02.2010, 17:40 Uhr, schrieb Andreas Petlund <apetlund@xxxxxxxxx>:

> 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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