linux-kernel - "tcp: refine TSO autosizing" causes performance regression on Xen

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Date:	Thu, 9 Apr 2015 16:46:30 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	<xen-devel@...ts.xensource.com>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	<christoffer.dall@...aro.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"Wei Liu" <wei.liu2@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>, <edumazet@...gle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: "tcp: refine TSO autosizing" causes performance regression on Xen

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@...gle.com>
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?

Cheers,

Stefano
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