netdev - Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

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Message-Id: <20120410.144737.1047745906309843732.davem@davemloft.net>
Date:	Tue, 10 Apr 2012 14:47:37 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ncardwell@...gle.com
Cc:	netdev@...r.kernel.org, nanditad@...gle.com, ycheng@...gle.com,
	edumazet@...gle.com, therbert@...gle.com
Subject: Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT
 sample

From: Neal Cardwell <ncardwell@...gle.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@...gle.com>

Applied, thanks.
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