Bug #359

Fwd: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

Added by David Taht on Apr 10, 2012. Updated on Apr 19, 2012.
Closed Normal Dave Täht


———- Forwarded message ———-
From: David Miller davem@davemloft.net
Date: Tue, Apr 10, 2012 at 11:47 AM
Subject: Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample
To: ncardwell@google.com
Cc: netdev@vger.kernel.org, nanditad@google.com, ycheng@google.com,
edumazet@google.com, therbert@google.com

From: Neal Cardwell ncardwell@google.com
Date: Tue, 10 Apr 2012 13:59:20 -0400

> Fix a code path in tcp_rcv_rtt_update() that was comparing scaled and
> unscaled RTT samples.
> The intent in the code was to only use the ’m’ measurement if it was a
> new minimum.  However, since ’m’ had not yet been shifted left 3 bits
> but ‘new_sample’ had, this comparison would nearly always succeed,
> leading us to erroneously set our receive-side RTT estimate to the ’m’
> sample when that sample could be nearly 8x too high to use.
> The overall effect is to often cause the receive-side RTT estimate to
> be significantly too large (up to 40% too large for brief periods in
> my tests).
> Signed-off-by: Neal Cardwell ncardwell@google.com

Applied, thanks.


Updated by Dave Täht on Apr 19, 2012.
Tested in cerowrt 3.3.2-6. Will probably be fixed in 3.3.3

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