netdev - Re: [PATCH net-next] virtio-net: enable multiqueue by default

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Date:   Fri, 25 Nov 2016 06:43:08 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hannes Frederic Sowa <hannes@...hat.com>,
        Neil Horman <nhorman@...hat.com>,
        Jeremy Eder <jeder@...hat.com>,
        Marko Myllynen <myllynen@...hat.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>
Subject: Re: [PATCH net-next] virtio-net: enable multiqueue by default

On Fri, Nov 25, 2016 at 12:37:26PM +0800, Jason Wang wrote:
> We use single queue even if multiqueue is enabled and let admin to
> enable it through ethtool later. This is used to avoid possible
> regression (small packet TCP stream transmission). But looks like an
> overkill since:
> 
> - single queue user can disable multiqueue when launching qemu
> - brings extra troubles for the management since it needs extra admin
>   tool in guest to enable multiqueue
> - multiqueue performs much better than single queue in most of the
>   cases
> 
> So this patch enables multiqueue by default: if #queues is less than or
> equal to #vcpu, enable as much as queue pairs; if #queues is greater
> than #vcpu, enable #vcpu queue pairs.
> 
> Cc: Hannes Frederic Sowa <hannes@...hat.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: Neil Horman <nhorman@...hat.com>
> Cc: Jeremy Eder <jeder@...hat.com>
> Cc: Marko Myllynen <myllynen@...hat.com>
> Cc: Maxime Coquelin <maxime.coquelin@...hat.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>

OK at some level but all uses of num_online_cpus()
like this are racy versus hotplug.
I know we already have this bug but shouldn't we fix it
before we add more?


> ---
>  drivers/net/virtio_net.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index d4ac7a6..a21d93a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1886,8 +1886,11 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> -	/* Use single tx/rx queue pair as default */
> -	vi->curr_queue_pairs = 1;
> +	/* Enable multiqueue by default */
> +	if (num_online_cpus() >= max_queue_pairs)
> +		vi->curr_queue_pairs = max_queue_pairs;
> +	else
> +		vi->curr_queue_pairs = num_online_cpus();
>  	vi->max_queue_pairs = max_queue_pairs;
>  
>  	/* Allocate/initialize the rx/tx queues, and invoke find_vqs */
> @@ -1918,6 +1921,8 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		goto free_unregister_netdev;
>  	}
>  
> +	virtnet_set_affinity(vi);
> +
>  	/* Assume link up if device can't report link status,
>  	   otherwise get link status from config. */
>  	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
> -- 
> 2.7.4