linux-kernel - Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

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Message-ID: <87k3rn2qwb.fsf@rustcorp.com.au>
Date:	Wed, 09 Jan 2013 10:01:32 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Wanlong Gao <gaowanlong@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Eric Dumazet <erdnetdev@...il.com>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>
Subject: Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

Wanlong Gao <gaowanlong@...fujitsu.com> writes:
>   */
>  static u16 virtnet_select_queue(struct net_device *dev, struct sk_buff *skb)
>  {
> -	int txq = skb_rx_queue_recorded(skb) ? skb_get_rx_queue(skb) :
> -		  smp_processor_id();
> +	int txq = 0;
> +
> +	if (skb_rx_queue_recorded(skb))
> +		txq = skb_get_rx_queue(skb);
> +	else if ((txq = per_cpu(vq_index, smp_processor_id())) == -1)
> +		txq = 0;

You should use __get_cpu_var() instead of smp_processor_id() here, ie:

        else if ((txq = __get_cpu_var(vq_index)) == -1)

And AFAICT, no reason to initialize txq to 0 to start with.

So:

        int txq;

        if (skb_rx_queue_recorded(skb))
		txq = skb_get_rx_queue(skb);
        else {
                txq = __get_cpu_var(vq_index);
                if (txq == -1)
                        txq = 0;
        }

Now, just to confirm, I assume this can happen even if we use vq_index,
right, because of races with virtnet_set_channels?

  	while (unlikely(txq >= dev->real_num_tx_queues))
  		txq -= dev->real_num_tx_queues;


Thanks,
Rusty.
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