"tcp: refine TSO autosizing" causes performance regression on Xen

From: Stefano Stabellini
Date: Thu Apr 09 2015 - 11:49:15 EST

Hi all,

I found a performance regression when running netperf -t TCP_MAERTS from
an external host to a Xen VM on ARM64: v3.19 and v4.0-rc4 running in the
virtual machine are 30% slower than v3.18.

Through bisection I found that the perf regression is caused by the
prensence of the following commit in the guest kernel:

commit 605ad7f184b60cfaacbc038aa6c55ee68dee3c89
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date: Sun Dec 7 12:22:18 2014 -0800

tcp: refine TSO autosizing

A simple revert would fix the issue.

Does anybody have any ideas on what could be the cause of the problem?
Suggestions on what to do to fix it?


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