netdev - Re: Poor TCP performance with XPS enabled after scrubbing skb

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Date:   Tue, 15 May 2018 14:08:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Flavio Leitner <fbl@...close.org>, netdev@...r.kernel.org
Cc:     Paolo Abeni <pabeni@...hat.com>
Subject: Re: Poor TCP performance with XPS enabled after scrubbing skb



On 05/15/2018 12:31 PM, Flavio Leitner wrote:
> Hi,
> 
> There is a significant throughput issue (~50% drop) for a single TCP
> stream when the skb is scrubbed and XPS is enabled.
> 
> If I turn CONFIG_XPS off, then the issue never happens and the test
> reaches line rate.  The same happens if I echo 0 to tx-*/xps_cpus.
> 
> It looks like that when the skb is scrubbed, there is no more reference
> to the struct sock, 

And this is really the problem here, since it breaks back pressure (and TCP Small queues)

I am not sure why skb_orphan() is used in this scrubbing really.