Fwd: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample
|Assignee:||Dave Täht||% Done:||
|Category:||Linux Kernel||Spent time:||0.50 hour|
|Target version:||1st Public Cerowrt release|
---------- Forwarded message ----------
From: David Miller <firstname.lastname@example.org>
Date: Tue, Apr 10, 2012 at 11:47 AM
Subject: Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample
Cc: email@example.com, firstname.lastname@example.org, email@example.com,
From: Neal Cardwell <firstname.lastname@example.org>
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 <email@example.com>