linux-kernel - Re: [PATCH net V3] virtio-net: correctly enable callback during start

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Message-ID: <fefccbdb-9ccd-4057-743a-b23bad4e123d@redhat.com>
Date:   Tue, 17 Jan 2023 17:33:55 +0100
From:   Laurent Vivier <lvivier@...hat.com>
To:     Jason Wang <jasowang@...hat.com>, mst@...hat.com
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        xuanzhuo@...ux.alibaba.com, Stefano Brivio <sbrivio@...hat.com>
Subject: Re: [PATCH net V3] virtio-net: correctly enable callback during
 start_xmit

On 1/17/23 04:47, Jason Wang wrote:
> Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
> virtqueue callback via the following statement:
> 
>          do {
> 		if (use_napi)
> 			virtqueue_disable_cb(sq->vq);
> 
> 		free_old_xmit_skbs(sq, false);
> 
> 	} while (use_napi && kick &&
>                 unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> 
> When NAPI is used and kick is false, the callback won't be enabled
> here. And when the virtqueue is about to be full, the tx will be
> disabled, but we still don't enable tx interrupt which will cause a TX
> hang. This could be observed when using pktgen with burst enabled.
> 
> TO be consistent with the logic that tries to disable cb only for
> NAPI, fixing this by trying to enable delayed callback only when NAPI
> is enabled when the queue is about to be full.
> 
> Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
> The patch is needed for -stable.
> Changes since V2:
> - try to enabled delayed callback and schedule NAPI instead of try to
>    poll as when TX NAPI is disabled
> Changes since V1:
> - enable tx interrupt after we disable TX
> ---
>   drivers/net/virtio_net.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7723b2a49d8e..18b3de854aeb 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1877,8 +1877,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>   	 */
>   	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
>   		netif_stop_subqueue(dev, qnum);
> -		if (!use_napi &&
> -		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> +		if (use_napi) {
> +			if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
> +				virtqueue_napi_schedule(&sq->napi, sq->vq);
> +		} else if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
>   			/* More just got used, free them then recheck. */
>   			free_old_xmit_skbs(sq, false);
>   			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {

This fix works fine with my test case (netdev stream + passt)

Tested-by: Laurent Vivier <lvivier@...hat.com>

Thanks,
Laurent