netdev - Re: [PATCH v3 4/4] virtio_net: disable cb aggressively

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Message-ID: <18b47c22-8c8a-7699-ffaf-ccfdcbf39d37@gmail.com>
Date:   Wed, 26 May 2021 17:15:08 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Jakub Kicinski <kuba@...nel.org>, Wei Wang <weiwan@...gle.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Willem de Bruijn <willemb@...gle.com>,
        virtualization@...ts.linux-foundation.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v3 4/4] virtio_net: disable cb aggressively



On 5/26/21 10:24 AM, Michael S. Tsirkin wrote:
> There are currently two cases where we poll TX vq not in response to a
> callback: start xmit and rx napi.  We currently do this with callbacks
> enabled which can cause extra interrupts from the card.  Used not to be
> a big issue as we run with interrupts disabled but that is no longer the
> case, and in some cases the rate of spurious interrupts is so high
> linux detects this and actually kills the interrupt.
> 
> Fix up by disabling the callbacks before polling the tx vq.


It is not clear why we want to poll TX completions from ndo_start_xmit() in napi mode ?

This seems not needed, adding costs to sender thread, this might
reduce the ability to use a different cpu for tx completions.

Also this will likely conflict with BQL model if we want to use BQL at some point.

> 

This probably needs a Fixes: tag 

> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/net/virtio_net.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c29f42d1e04f..a83dc038d8af 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1433,7 +1433,10 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
>  		return;
>  
>  	if (__netif_tx_trylock(txq)) {
> -		free_old_xmit_skbs(sq, true);
> +		do {
> +			virtqueue_disable_cb(sq->vq);
> +			free_old_xmit_skbs(sq, true);
> +		} while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
>  
>  		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
>  			netif_tx_wake_queue(txq);
> @@ -1605,12 +1608,17 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
>  	bool kick = !netdev_xmit_more();
>  	bool use_napi = sq->napi.weight;
> +	unsigned int bytes = skb->len;
>  
>  	/* Free up any pending old buffers before queueing new ones. */
> -	free_old_xmit_skbs(sq, false);
> +	do {
> +		if (use_napi)
> +			virtqueue_disable_cb(sq->vq);
>  
> -	if (use_napi && kick)
> -		virtqueue_enable_cb_delayed(sq->vq);
> +		free_old_xmit_skbs(sq, false);
> +
> +	} while (use_napi && kick &&
> +	       unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
>  
>  	/* timestamp packet in software */
>  	skb_tx_timestamp(skb);
>