netdev - Re: [PATCH] net: Cache route in IP tunnels

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Date:	Thu, 12 Dec 2013 14:54:13 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: Cache route in IP tunnels

On Thu, Dec 12, 2013 at 1:11 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2013-12-12 at 12:56 -0800, Tom Herbert wrote:
>
>> Reverting that patch doesn't fix the problem. We are hitting the
>> spinlock in gro_cells_receive a lot though.
>
> Well, something broke then.
>

The test I'm running is over a single tunnel between two hosts, so all
packets will be hitting the same RX queue since we only steer by
3-tuple hash. Clearing or not clearing hash will have no effect.

Nevertheless, I don't see how code how gro_cells_receive (called via
ip_tunnel_rcv) could have ever have worked right for tunnels. By the
time this function is called, we've probably already spread the
packets from RX queue across multiple CPUs via RPS/RFS. So putting
them back on a queue based on RX queue would be a source of
contention.  It seems like we should be using another per CPU backlog.

> I was able to reach line rate at the time I did this work.
>
>
>
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