Re: [net-next RFC V3 PATCH 4/6] tuntap: multiqueue support

From: Jason Wang
Date: Mon Jun 25 2012 - 23:40:27 EST

On 06/25/2012 04:25 PM, Michael S. Tsirkin wrote:
On Mon, Jun 25, 2012 at 02:10:18PM +0800, Jason Wang wrote:
This patch adds multiqueue support for tap device. This is done by abstracting
each queue as a file/socket and allowing multiple sockets to be attached to the
tuntap device (an array of tun_file were stored in the tun_struct). Userspace
could write and read from those files to do the parallel packet
sending/receiving.

Unlike the previous single queue implementation, the socket and device were
loosely coupled, each of them were allowed to go away first. In order to let the
tx path lockless, netif_tx_loch_bh() is replaced by RCU/NETIF_F_LLTX to
synchronize between data path and system call.
Don't use LLTX/RCU. It's not worth it.
Use something like netif_set_real_num_tx_queues.

The tx queue selecting is first based on the recorded rxq index of an skb, it
there's no such one, then choosing based on rx hashing (skb_get_rxhash()).

Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
Interestingly macvtap switched to hashing first:
ef0002b577b52941fb147128f30bd1ecfdd3ff6d
(the commit log is corrupted but see what it
does in the patch).
Any idea why?

Yes, so tap should be changed to behave same as macvtap. I remember the reason we do that is to make sure the packet of a single flow to be queued to a fixed socket/virtqueues. As 10g cards like ixgbe choose the rx queue for a flow based on the last tx queue where the packets of that flow comes. So if we are using recored rx queue in macvtap, the queue index of a flow would change as vhost thread moves amongs processors.

But during test tun/tap, one interesting thing I find is that even ixgbe has recorded the queue index during rx, it seems be lost when tap tries to transmit skbs to userspace.

---
drivers/net/tun.c | 371 +++++++++++++++++++++++++++++++++--------------------
1 files changed, 232 insertions(+), 139 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 8233b0a..5c26757 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -107,6 +107,8 @@ struct tap_filter {
unsigned char addr[FLT_EXACT_COUNT][ETH_ALEN];
};

+#define MAX_TAP_QUEUES (NR_CPUS< 16 ? NR_CPUS : 16)
Why the limit? I am guessing you copied this from macvtap?
This is problematic for a number of reasons:
- will not play well with migration
- will not work well for a large guest

Yes, macvtap needs to be fixed too.

I am guessing what it is trying to prevent is queueing
up a huge number of packets?
So just divide the default tx queue limit by the # of queues.

And by the way, for MQ applications maybe we can finally
ignore tx queue altogether and limit the total number
of bytes queued?
To avoid regressions we can make it large like 64M/# queues.
Could be a separate patch I think, and for a single queue
might need a compatible mode though I am not sure.

+
struct tun_file {
struct sock sk;
struct socket socket;
@@ -114,16 +116,18 @@ struct tun_file {
int vnet_hdr_sz;
struct tap_filter txflt;
atomic_t count;
- struct tun_struct *tun;
+ struct tun_struct __rcu *tun;
struct net *net;
struct fasync_struct *fasync;
unsigned int flags;
+ u16 queue_index;
};

struct tun_sock;

struct tun_struct {
- struct tun_file *tfile;
+ struct tun_file *tfiles[MAX_TAP_QUEUES];
+ unsigned int numqueues;
unsigned int flags;
uid_t owner;
gid_t group;
@@ -138,80 +142,159 @@ struct tun_struct {
#endif
};

-static int tun_attach(struct tun_struct *tun, struct file *file)
+static DEFINE_SPINLOCK(tun_lock);
+
+/*
+ * tun_get_queue(): calculate the queue index
+ * - if skbs comes from mq nics, we can just borrow
+ * - if not, calculate from the hash
+ */
+static struct tun_file *tun_get_queue(struct net_device *dev,
+ struct sk_buff *skb)
{
- struct tun_file *tfile = file->private_data;
- int err;
+ struct tun_struct *tun = netdev_priv(dev);
+ struct tun_file *tfile = NULL;
+ int numqueues = tun->numqueues;
+ __u32 rxq;

- ASSERT_RTNL();
+ BUG_ON(!rcu_read_lock_held());

- netif_tx_lock_bh(tun->dev);
+ if (!numqueues)
+ goto out;

- err = -EINVAL;
- if (tfile->tun)
+ if (numqueues == 1) {
+ tfile = rcu_dereference(tun->tfiles[0]);
Instead of hacks like this, you can ask for an MQ
flag to be set in SETIFF. Then you won't need to
handle attach/detach at random times.
And most of the scary num_queues checks can go away.
You can then also ask userspace about the max # of queues
to expect if you want to save some memory.


goto out;
+ }

- err = -EBUSY;
- if (tun->tfile)
+ if (likely(skb_rx_queue_recorded(skb))) {
+ rxq = skb_get_rx_queue(skb);
+
+ while (unlikely(rxq>= numqueues))
+ rxq -= numqueues;
+
+ tfile = rcu_dereference(tun->tfiles[rxq]);
goto out;
+ }

- err = 0;
- tfile->tun = tun;
- tun->tfile = tfile;
- netif_carrier_on(tun->dev);
- dev_hold(tun->dev);
- sock_hold(&tfile->sk);
- atomic_inc(&tfile->count);
+ /* Check if we can use flow to select a queue */
+ rxq = skb_get_rxhash(skb);
+ if (rxq) {
+ u32 idx = ((u64)rxq * numqueues)>> 32;
This completely confuses me. What's the logic here?
How do we even know it's in range?

+ tfile = rcu_dereference(tun->tfiles[idx]);
+ goto out;
+ }

+ tfile = rcu_dereference(tun->tfiles[0]);
out:
- netif_tx_unlock_bh(tun->dev);
- return err;
+ return tfile;
}

-static void __tun_detach(struct tun_struct *tun)
+static int tun_detach(struct tun_file *tfile, bool clean)
{
- struct tun_file *tfile = tun->tfile;
- /* Detach from net device */
- netif_tx_lock_bh(tun->dev);
- netif_carrier_off(tun->dev);
- tun->tfile = NULL;
- netif_tx_unlock_bh(tun->dev);
-
- /* Drop read queue */
- skb_queue_purge(&tfile->socket.sk->sk_receive_queue);
-
- /* Drop the extra count on the net device */
- dev_put(tun->dev);
-}
+ struct tun_struct *tun;
+ struct net_device *dev = NULL;
+ bool destroy = false;

-static void tun_detach(struct tun_struct *tun)
-{
- rtnl_lock();
- __tun_detach(tun);
- rtnl_unlock();
-}
+ spin_lock(&tun_lock);

-static struct tun_struct *__tun_get(struct tun_file *tfile)
-{
- struct tun_struct *tun = NULL;
+ tun = rcu_dereference_protected(tfile->tun,
+ lockdep_is_held(&tun_lock));
+ if (tun) {
+ u16 index = tfile->queue_index;
+ BUG_ON(index>= tun->numqueues);
+ dev = tun->dev;
+
+ rcu_assign_pointer(tun->tfiles[index],
+ tun->tfiles[tun->numqueues - 1]);
+ tun->tfiles[index]->queue_index = index;
+ rcu_assign_pointer(tfile->tun, NULL);
+ --tun->numqueues;
+ sock_put(&tfile->sk);

- if (atomic_inc_not_zero(&tfile->count))
- tun = tfile->tun;
+ if (tun->numqueues == 0&& !(tun->flags& TUN_PERSIST))
+ destroy = true;
Please don't use flags like that. Use dedicated labels and goto there on error.


+ }

- return tun;
+ spin_unlock(&tun_lock);
+
+ synchronize_rcu();
+ if (clean)
+ sock_put(&tfile->sk);
+
+ if (destroy) {
+ rtnl_lock();
+ if (dev->reg_state == NETREG_REGISTERED)
+ unregister_netdevice(dev);
+ rtnl_unlock();
+ }
+
+ return 0;
}

-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));
+ BUG_ON(!tfile);
+ wake_up_all(&tfile->wq.wait);
+ tfile_list[j++] = tfile;
+ rcu_assign_pointer(tfile->tun, NULL);
+ --tun->numqueues;
+ }
+ BUG_ON(tun->numqueues != 0);
+ /* guarantee that any future tun_attach will fail */
+ tun->numqueues = MAX_TAP_QUEUES;
+ spin_unlock(&tun_lock);
+
+ synchronize_rcu();
+ for (--j; j>= 0; j--)
+ sock_put(&tfile_list[j]->sk);
}

-static void tun_put(struct tun_struct *tun)
+static int tun_attach(struct tun_struct *tun, struct file *file)
{
- struct tun_file *tfile = tun->tfile;
+ struct tun_file *tfile = file->private_data;
+ int err;
+
+ ASSERT_RTNL();
+
+ spin_lock(&tun_lock);

- if (atomic_dec_and_test(&tfile->count))
- tun_detach(tfile->tun);
+ err = -EINVAL;
+ if (rcu_dereference_protected(tfile->tun, lockdep_is_held(&tun_lock)))
+ goto out;
+
+ err = -EBUSY;
+ if (!(tun->flags& TUN_TAP_MQ)&& tun->numqueues == 1)
+ goto out;
+
+ if (tun->numqueues == MAX_TAP_QUEUES)
+ goto out;
+
+ err = 0;
+ tfile->queue_index = tun->numqueues;
+ rcu_assign_pointer(tfile->tun, tun);
+ rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
+ sock_hold(&tfile->sk);
+ tun->numqueues++;
+
+ if (tun->numqueues == 1)
+ netif_carrier_on(tun->dev);
+
+ /* device is allowed to go away first, so no need to hold extra
+ * refcnt. */
+
+out:
+ spin_unlock(&tun_lock);
+ return err;
}

/* TAP filtering */
@@ -331,16 +414,7 @@ static const struct ethtool_ops tun_ethtool_ops;
/* Net device detach from fd. */
static void tun_net_uninit(struct net_device *dev)
{
- struct tun_struct *tun = netdev_priv(dev);
- struct tun_file *tfile = tun->tfile;
-
- /* Inform the methods they need to stop using the dev.
- */
- if (tfile) {
- wake_up_all(&tfile->wq.wait);
- if (atomic_dec_and_test(&tfile->count))
- __tun_detach(tun);
- }
+ tun_detach_all(dev);
}

/* Net device open. */
@@ -360,10 +434,10 @@ static int tun_net_close(struct net_device *dev)
/* Net device start xmit */
static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
{
- struct tun_struct *tun = netdev_priv(dev);
- struct tun_file *tfile = tun->tfile;
+ struct tun_file *tfile = NULL;

- tun_debug(KERN_INFO, tun, "tun_net_xmit %d\n", skb->len);
+ rcu_read_lock();
+ tfile = tun_get_queue(dev, skb);

/* Drop packet if interface is not attached */
if (!tfile)
@@ -381,7 +455,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)

if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
>= dev->tx_queue_len) {
- if (!(tun->flags& TUN_ONE_QUEUE)) {
+ if (!(tfile->flags& TUN_ONE_QUEUE)&&
Which patch moved flags from tun to tfile?

+ !(tfile->flags& TUN_TAP_MQ)) {
/* Normal queueing mode. */
/* Packet scheduler handles dropping of further packets. */
netif_stop_queue(dev);
@@ -390,7 +465,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
* error is more appropriate. */
dev->stats.tx_fifo_errors++;
} else {
- /* Single queue mode.
+ /* Single queue mode or multi queue mode.
* Driver handles dropping of all packets itself. */
Please don't do this. Stop the queue on overrun as appropriate.
ONE_QUEUE is a legacy hack.

BTW we really should stop queue before we start dropping packets,
but that can be a separate patch.

goto drop;
}
@@ -408,9 +483,11 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
kill_fasync(&tfile->fasync, SIGIO, POLL_IN);
wake_up_interruptible_poll(&tfile->wq.wait, POLLIN |
POLLRDNORM | POLLRDBAND);
+ rcu_read_unlock();
return NETDEV_TX_OK;

drop:
+ rcu_read_unlock();
dev->stats.tx_dropped++;
kfree_skb(skb);
return NETDEV_TX_OK;
@@ -527,16 +604,22 @@ static void tun_net_init(struct net_device *dev)
static unsigned int tun_chr_poll(struct file *file, poll_table * wait)
{
struct tun_file *tfile = file->private_data;
- struct tun_struct *tun = __tun_get(tfile);
+ struct tun_struct *tun = NULL;
struct sock *sk;
unsigned int mask = 0;

- if (!tun)
+ if (!tfile)
return POLLERR;

- sk = tfile->socket.sk;
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
+ if (!tun) {
+ rcu_read_unlock();
+ return POLLERR;
+ }
+ rcu_read_unlock();

- tun_debug(KERN_INFO, tun, "tun_chr_poll\n");
+ sk =&tfile->sk;

poll_wait(file,&tfile->wq.wait, wait);

@@ -548,10 +631,12 @@ static unsigned int tun_chr_poll(struct file *file, poll_table * wait)
sock_writeable(sk)))
mask |= POLLOUT | POLLWRNORM;

- if (tun->dev->reg_state != NETREG_REGISTERED)
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
+ if (!tun || tun->dev->reg_state != NETREG_REGISTERED)
mask = POLLERR;
+ rcu_read_unlock();

- tun_put(tun);
return mask;
}

@@ -708,9 +793,12 @@ static ssize_t tun_get_user(struct tun_file *tfile,
skb_shinfo(skb)->gso_segs = 0;
}

- tun = __tun_get(tfile);
- if (!tun)
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
+ if (!tun) {
+ rcu_read_unlock();
return -EBADFD;
+ }

switch (tfile->flags& TUN_TYPE_MASK) {
case TUN_TUN_DEV:
@@ -720,26 +808,30 @@ static ssize_t tun_get_user(struct tun_file *tfile,
skb->protocol = eth_type_trans(skb, tun->dev);
break;
}
-
- netif_rx_ni(skb);
tun->dev->stats.rx_packets++;
tun->dev->stats.rx_bytes += len;
- tun_put(tun);
+ rcu_read_unlock();
+
+ netif_rx_ni(skb);
+
return count;

err_free:
count = -EINVAL;
kfree_skb(skb);
err:
- tun = __tun_get(tfile);
- if (!tun)
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
+ if (!tun) {
+ rcu_read_unlock();
return -EBADFD;
+ }

if (drop)
tun->dev->stats.rx_dropped++;
if (error)
tun->dev->stats.rx_frame_errors++;
- tun_put(tun);
+ rcu_read_unlock();
return count;
}

@@ -833,12 +925,13 @@ static ssize_t tun_put_user(struct tun_file *tfile,
skb_copy_datagram_const_iovec(skb, 0, iv, total, len);
total += skb->len;

- tun = __tun_get(tfile);
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
if (tun) {
tun->dev->stats.tx_packets++;
tun->dev->stats.tx_bytes += len;
- tun_put(tun);
}
+ rcu_read_unlock();

return total;
}
@@ -869,28 +962,31 @@ static ssize_t tun_do_read(struct tun_file *tfile,
break;
}

- tun = __tun_get(tfile);
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
if (!tun) {
- ret = -EIO;
+ ret = -EBADFD;
BADFD is for when you get passed something like -1 fd.
Here fd is OK, it's just in a bad state so you can not do IO.


+ rcu_read_unlock();
break;
}
if (tun->dev->reg_state != NETREG_REGISTERED) {
ret = -EIO;
- tun_put(tun);
+ rcu_read_unlock();
break;
}
- tun_put(tun);
+ rcu_read_unlock();

/* Nothing to read, let's sleep */
schedule();
continue;
}

- tun = __tun_get(tfile);
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
if (tun) {
netif_wake_queue(tun->dev);
- tun_put(tun);
}
+ rcu_read_unlock();

ret = tun_put_user(tfile, skb, iv, len);
kfree_skb(skb);
@@ -1038,6 +1134,9 @@ static int tun_flags(struct tun_struct *tun)
if (tun->flags& TUN_VNET_HDR)
flags |= IFF_VNET_HDR;

+ if (tun->flags& TUN_TAP_MQ)
+ flags |= IFF_MULTI_QUEUE;
+
return flags;
}

@@ -1097,8 +1196,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
err = tun_attach(tun, file);
if (err< 0)
return err;
- }
- else {
+ } else {
char *name;
unsigned long flags = 0;

@@ -1142,6 +1240,8 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
TUN_USER_FEATURES;
dev->features = dev->hw_features;
+ if (ifr->ifr_flags& IFF_MULTI_QUEUE)
+ dev->features |= NETIF_F_LLTX;

err = register_netdevice(tun->dev);
if (err< 0)
@@ -1154,7 +1254,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)

err = tun_attach(tun, file);
if (err< 0)
- goto failed;
+ goto err_free_dev;
}

tun_debug(KERN_INFO, tun, "tun_set_iff\n");
@@ -1174,6 +1274,11 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
else
tun->flags&= ~TUN_VNET_HDR;

+ if (ifr->ifr_flags& IFF_MULTI_QUEUE)
+ tun->flags |= TUN_TAP_MQ;
+ else
+ tun->flags&= ~TUN_TAP_MQ;
+
/* Cache flags from tun device */
tfile->flags = tun->flags;
/* Make sure persistent devices do not get stuck in
@@ -1187,7 +1292,6 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)

err_free_dev:
free_netdev(dev);
-failed:
return err;
}

@@ -1264,38 +1368,40 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
(unsigned int __user*)argp);
}

- rtnl_lock();
-
- tun = __tun_get(tfile);
- if (cmd == TUNSETIFF&& !tun) {
+ ret = 0;
+ if (cmd == TUNSETIFF) {
+ rtnl_lock();
ifr.ifr_name[IFNAMSIZ-1] = '\0';
-
ret = tun_set_iff(tfile->net, file,&ifr);
-
+ rtnl_unlock();
if (ret)
- goto unlock;
-
+ return ret;
if (copy_to_user(argp,&ifr, ifreq_len))
- ret = -EFAULT;
- goto unlock;
+ return -EFAULT;
+ return ret;
}

+ rtnl_lock();
+
+ rcu_read_lock();
+
ret = -EBADFD;
+ tun = rcu_dereference(tfile->tun);
if (!tun)
goto unlock;
+ else
+ ret = 0;

- tun_debug(KERN_INFO, tun, "tun_chr_ioctl cmd %d\n", cmd);
-
- ret = 0;
switch (cmd) {
case TUNGETIFF:
ret = tun_get_iff(current->nsproxy->net_ns, tun,&ifr);
+ rcu_read_unlock();
if (ret)
- break;
+ goto out;

if (copy_to_user(argp,&ifr, ifreq_len))
ret = -EFAULT;
- break;
+ goto out;

case TUNSETNOCSUM:
/* Disable/Enable checksum */
@@ -1357,9 +1463,10 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
/* Get hw address */
memcpy(ifr.ifr_hwaddr.sa_data, tun->dev->dev_addr, ETH_ALEN);
ifr.ifr_hwaddr.sa_family = tun->dev->type;
+ rcu_read_unlock();
if (copy_to_user(argp,&ifr, ifreq_len))
ret = -EFAULT;
- break;
+ goto out;

case SIOCSIFHWADDR:
/* Set hw address */
@@ -1375,9 +1482,9 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
}

unlock:
+ rcu_read_unlock();
+out:
rtnl_unlock();
- if (tun)
- tun_put(tun);
return ret;
}

@@ -1517,6 +1624,11 @@ out:
return ret;
}

+static void tun_sock_destruct(struct sock *sk)
+{
+ skb_queue_purge(&sk->sk_receive_queue);
+}
+
static int tun_chr_open(struct inode *inode, struct file * file)
{
struct net *net = current->nsproxy->net_ns;
@@ -1540,6 +1652,7 @@ static int tun_chr_open(struct inode *inode, struct file * file)
sock_init_data(&tfile->socket,&tfile->sk);

tfile->sk.sk_write_space = tun_sock_write_space;
+ tfile->sk.sk_destruct = tun_sock_destruct;
tfile->sk.sk_sndbuf = INT_MAX;
file->private_data = tfile;

@@ -1549,31 +1662,8 @@ static int tun_chr_open(struct inode *inode, struct file * file)
static int tun_chr_close(struct inode *inode, struct file *file)
{
struct tun_file *tfile = file->private_data;
- struct tun_struct *tun;
-
- tun = __tun_get(tfile);
- if (tun) {
- struct net_device *dev = tun->dev;
-
- tun_debug(KERN_INFO, tun, "tun_chr_close\n");
-
- __tun_detach(tun);
-
- /* If desirable, unregister the netdevice. */
- if (!(tun->flags& TUN_PERSIST)) {
- rtnl_lock();
- if (dev->reg_state == NETREG_REGISTERED)
- unregister_netdevice(dev);
- rtnl_unlock();
- }

- /* drop the reference that netdevice holds */
- sock_put(&tfile->sk);
-
- }
-
- /* drop the reference that file holds */
- sock_put(&tfile->sk);
+ tun_detach(tfile, true);

return 0;
}
@@ -1700,14 +1790,17 @@ static void tun_cleanup(void)
* holding a reference to the file for as long as the socket is in use. */
struct socket *tun_get_socket(struct file *file)
{
- struct tun_struct *tun;
+ struct tun_struct *tun = NULL;
struct tun_file *tfile = file->private_data;
if (file->f_op !=&tun_fops)
return ERR_PTR(-EINVAL);
- tun = tun_get(file);
- if (!tun)
+ rcu_read_lock();
+ tun = rcu_dereference(tfile->tun);
+ if (!tun) {
+ rcu_read_unlock();
return ERR_PTR(-EBADFD);
- tun_put(tun);
+ }
+ rcu_read_unlock();
return&tfile->socket;
}
EXPORT_SYMBOL_GPL(tun_get_socket);

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