netdev - Re: [PATCH net-next-2.6] ip

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Date:	Wed, 29 Sep 2010 10:33:08 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] ip_gre: lockless xmit

On Tue, Sep 28, 2010 at 2:05 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> GRE tunnels can benefit from lockless xmits, using NETIF_F_LLTX
>
> Note: If tunnels are created with the "oseq" option, LLTX is not
> enabled :
>
> Even using an atomic_t o_seq, we would increase chance for packets being
> out of order at receiver.
>
> Bench on a 16 cpus machine (dual E5540 cpus), 16 threads sending
> 10000000 UDP frames via one gre tunnel (size:200 bytes per frame)
>
> Before patch :
> real    3m0.094s
> user    0m9.365s
> sys     47m50.103s
>
> After patch:
> real    0m29.756s
> user    0m11.097s
> sys     7m33.012s
>
> Last problem to solve is the contention on dst :
>
>
> 38660.00 21.4% __ip_route_output_key          vmlinux
> 20786.00 11.5% dst_release                    vmlinux
> 14191.00  7.8% __xfrm_lookup                  vmlinux
> 12410.00  6.9% ip_finish_output               vmlinux
>  4540.00  2.5% ip_push_pending_frames         vmlinux
>  4427.00  2.4% ip_append_data                 vmlinux
>  4265.00  2.4% __alloc_skb                    vmlinux
>  4140.00  2.3% __ip_local_out                 vmlinux
>  3991.00  2.2% dev_queue_xmit                 vmlinux
>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>

The tx lock has another use here: to break local loops.  With this
change, a misconfigured tunnel can bring down the machine with a stack
overflow.  There are clearly other ways to fix this that don't require
a lock that restricts parallelism, such as a loop counter, but that's
the way it is now.
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