netdev - Re: [net-next RFC PATCH 4/7] tuntap: multiqueue support

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Date:	Fri, 12 Aug 2011 16:29:36 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, davem@...emloft.net,
	krkumar2@...ibm.com, rusty@...tcorp.com.au, qemu-devel@...gnu.org,
	kvm@...r.kernel.org, mirq-linux@...e.qmqm.pl
Subject: Re: [net-next RFC PATCH 4/7] tuntap: multiqueue support

Le vendredi 12 août 2011 à 09:55 +0800, Jason Wang a écrit :

>+       rxq = skb_get_rxhash(skb);
>+       if (rxq) {
>+               tfile = rcu_dereference(tun->tfiles[rxq % numqueues]);
>+               if (tfile)
>+                       goto out;
>+       }

You can avoid an expensive divide with following trick :

u32 idx = ((u64)rxq * numqueues) >> 32;



> -static struct tun_struct *tun_get(struct file *file)
> +static void tun_detach_all(struct net_device *dev)
>  {
> -	return __tun_get(file->private_data);
> +	struct tun_struct *tun = netdev_priv(dev);
> +	struct tun_file *tfile, *tfile_list[MAX_TAP_QUEUES];
> +	int i, j = 0;
> +
> +	spin_lock(&tun_lock);
> +
> +	for (i = 0; i < MAX_TAP_QUEUES && tun->numqueues; i++) {
> +		tfile = rcu_dereference_protected(tun->tfiles[i],
> +						lockdep_is_held(&tun_lock));
> +		if (tfile) {
> +			wake_up_all(&tfile->wq.wait);
> +			tfile_list[i++] = tfile;

	typo here, you want tfile_list[j++] = tfile;

> +			rcu_assign_pointer(tun->tfiles[i], NULL);
> +			rcu_assign_pointer(tfile->tun, NULL);
> +			--tun->numqueues;
> +		}
> +	}
> +	BUG_ON(tun->numqueues != 0);
> +	spin_unlock(&tun_lock);
> +
> +	synchronize_rcu();
> +	for(--j; j >= 0; j--)
> +		sock_put(&tfile_list[j]->sk);
>  }
>  

Could you take a look at net/packet/af_packet.c, to check how David did
the whole fanout thing ?

__fanout_unlink() 

Trick is to not leave NULL entries in the tun->tfiles[] array.

It makes things easier in hot path.


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