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

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Date:	Sun, 14 Aug 2011 02:05:25 -0400 (EDT)
From:	Jason Wang <jasowang@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.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



----- Original Message -----
> 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;
> 

Sure.

> 
> 
> > -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;
> 

Yes, thanks for catching this.

> > + 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.

Sure I would go to take a look at this.

> 
> 
> --
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