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Subject: Re: [PATCH v3 2/4] virtio_net: move txq wakeups under tx q lock
To:     "Michael S. Tsirkin" <mst@redhat.com>, linux-kernel@vger.kernel.org
Cc:     Jakub Kicinski <kuba@kernel.org>, Wei Wang <weiwan@google.com>,
        David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
        Willem de Bruijn <willemb@google.com>,
        virtualization@lists.linux-foundation.org
References: <20210526082423.47837-1-mst@redhat.com>
 <20210526082423.47837-3-mst@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <30c69182-cc6c-3e45-1db9-5b061e43e1d6@redhat.com>
Date:   Thu, 27 May 2021 11:48:33 +0800
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?? 2021/5/26 ????4:24, Michael S. Tsirkin д??:
> We currently check num_free outside tx q lock
> which is unsafe: new packets can arrive meanwhile
> and there won't be space in the queue.
> Thus a spurious queue wakeup causing overhead
> and even packet drops.
>
> Move the check under the lock to fix that.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>


Acked-by: Jason Wang <jasowang@redhat.com>


> ---
>   drivers/net/virtio_net.c | 13 +++++++------
>   1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 12512d1002ec..c29f42d1e04f 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1434,11 +1434,12 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
>   
>   	if (__netif_tx_trylock(txq)) {
>   		free_old_xmit_skbs(sq, true);
> +
> +		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> +			netif_tx_wake_queue(txq);
> +
>   		__netif_tx_unlock(txq);
>   	}
> -
> -	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> -		netif_tx_wake_queue(txq);
>   }
>   
>   static int virtnet_poll(struct napi_struct *napi, int budget)
> @@ -1522,6 +1523,9 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>   	virtqueue_disable_cb(sq->vq);
>   	free_old_xmit_skbs(sq, true);
>   
> +	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> +		netif_tx_wake_queue(txq);
> +
>   	opaque = virtqueue_enable_cb_prepare(sq->vq);
>   
>   	done = napi_complete_done(napi, 0);
> @@ -1542,9 +1546,6 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>   		}
>   	}
>   
> -	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> -		netif_tx_wake_queue(txq);
> -
>   	return 0;
>   }
>