netdev - Re: [PATCH net-next v2 5/5] virtio-net: keep tx interrupts disabled unless kick

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Message-ID: <72551dd3-ba91-812e-9065-d153bf8a7024@redhat.com>
Date:   Thu, 20 Apr 2017 14:17:21 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        davem@...emloft.net, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 5/5] virtio-net: keep tx interrupts disabled
 unless kick



On 2017年04月19日 04:21, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
>
> Tx napi mode increases the rate of transmit interrupts. Suppress some
> by masking interrupts while more packets are expected. The interrupts
> will be reenabled before the last packet is sent.
>
> This optimization reduces the througput drop with tx napi for
> unidirectional flows such as UDP_STREAM that do not benefit from
> cleaning tx completions in the the receive napi handler.
>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>   drivers/net/virtio_net.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index b14c82ce0032..b107ae011632 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1196,8 +1196,14 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>   	bool use_napi = sq->napi.weight;
>   
>   	/* Free up any pending old buffers before queueing new ones. */
> -	if (!use_napi)
> +	if (use_napi) {
> +		if (kick)
> +			virtqueue_enable_cb_delayed(sq->vq);
> +		else
> +			virtqueue_disable_cb(sq->vq);

Since virtqueue_disable_cb() do nothing for event idx. I wonder whether 
or not just calling enable_cb_dealyed() is ok here.

Btw, it does not disable interrupt at all, I propose a patch in the past 
which can do more than this:

https://patchwork.kernel.org/patch/6472601/

Thanks



> +	} else {
>   		free_old_xmit_skbs(sq);
> +	}
>   
>   	/* timestamp packet in software */
>   	skb_tx_timestamp(skb);