netdev - Re: [PATCH bpf-next 3/3] veth: Support bulk XDP

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Message-ID: <87zhnd1kg9.fsf@toke.dk>
Date:   Thu, 23 May 2019 13:25:42 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Cc:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        netdev@...r.kernel.org, xdp-newbies@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 3/3] veth: Support bulk XDP_TX

Toshiaki Makita <makita.toshiaki@....ntt.co.jp> writes:

> This improves XDP_TX performance by about 8%.
>
> Here are single core XDP_TX test results. CPU consumptions are taken
> from "perf report --no-child".
>
> - Before:
>
>   7.26 Mpps
>
>   _raw_spin_lock  7.83%
>   veth_xdp_xmit  12.23%
>
> - After:
>
>   7.84 Mpps
>
>   _raw_spin_lock  1.17%
>   veth_xdp_xmit   6.45%
>
> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
> ---
>  drivers/net/veth.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 52110e5..4edc75f 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -442,6 +442,23 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
>  	return ret;
>  }
>  
> +static void veth_xdp_flush_bq(struct net_device *dev)
> +{
> +	struct xdp_tx_bulk_queue *bq = this_cpu_ptr(&xdp_tx_bq);
> +	int sent, i, err = 0;
> +
> +	sent = veth_xdp_xmit(dev, bq->count, bq->q, 0);

Wait, veth_xdp_xmit() is just putting frames on a pointer ring. So
you're introducing an additional per-cpu bulk queue, only to avoid lock
contention around the existing pointer ring. But the pointer ring is
per-rq, so if you have lock contention, this means you must have
multiple CPUs servicing the same rq, no? So why not just fix that
instead?

-Toke