netdev - Re: tcp: Do not apply TSO segment limit to non-TSO packets

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Message-Id: <20150102.153655.1853692198479011402.davem@davemloft.net>
Date:	Fri, 02 Jan 2015 15:36:55 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	herbert@...dor.apana.org.au, thomas.jarosch@...ra2net.com,
	netdev@...r.kernel.org, edumazet@...gle.com,
	steffen.klassert@...unet.com, bhutchings@...arflare.com
Subject: Re: tcp: Do not apply TSO segment limit to non-TSO packets

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 02 Jan 2015 10:24:00 -0800

> On Thu, 2015-01-01 at 00:42 +1100, Herbert Xu wrote:
>> Firstly not many people test non-TSO code paths anymore so bugs
>> are likely to persist for a long time there.  Perhaps it's time
>> to remove the non-TSO code path altogether? The GSO code path
>> should provide enough speed-up in terms of boosting the effective
>> MTU to offset the cost of copying.
> 
>> Secondly why are we dealing with hardware TSO segment limits
>> by limiting the size of the TSO packet in the TCP stack? Surely
>> in this case GSO is free since there won't be any copying?
> 
> It might depends on the device capabilities.
> 
> Non TSO/GSO path is known to be better for devices unable to perform TX
> checksumming, as we compute the checksum at the time we copy data from
> user to kernel (csum_and_copy_from_user() from tcp_sendmsg())).

Non-SG capable devices suffer in this scenerio as well.
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