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Subject[PATCH v2] tuntap: add flow control to support back pressure
Added optional per queue flow control support using IFF_FLOW_CONTROL. When the IFF_FLOW_CONTROL TUNSETIFF flag is specified it will set a per queue flag to indicate that the queue should be stopped using netif_tx_stop_queue(), rather than discarding frames once full. After reading a frame from the respective stopped queue, a netif_tx_wake_queue() is issued to signal resource availability.

The per queue TUN_FLOW_CONTROL flag is stored in struct tun_file. This provides the flexibility to enable flow control on all, none or some queues when using IFF_MULTI_QUEUE. When not using IFF_MULTI_QUEUE, IFF_FLOW_CONTROL will apply to the single queue. No changes were made to the default drop frame policy.

This change adds support for back pressure use cases.

Reported-by: Brian Adamson <brian.adamson@nrl.navy.mil>
Tested-by: Joseph Giovatto <jgiovatto@djacentlink.com>
Signed-off-by: Steven Galgano <sgalgano@adjacentlink.com>
---
Previous version of patch did not respect individual queues when applying flow control using netif_tx_stop_all_queues()/netif_tx_wake_all_queues().

drivers/net/tun.c | 32 ++++++++++++++++++++++++++++----
include/uapi/linux/if_tun.h | 2 ++
2 files changed, 30 insertions(+), 4 deletions(-)
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index ee328ba..3d09f5a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -137,7 +137,7 @@ struct tun_file {
struct tun_struct __rcu *tun;
struct net *net;
struct fasync_struct *fasync;
- /* only used for fasnyc */
+ /* used for fasnyc and flow control */
unsigned int flags;
union {
u16 queue_index;
@@ -783,8 +783,19 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
* number of queues.
*/
if (skb_queue_len(&tfile->socket.sk->sk_receive_queue) * numqueues
- >= dev->tx_queue_len)
- goto drop;
+ >= dev->tx_queue_len) {
+ if (tfile->flags & TUN_FLOW_CONTROL) {
+ /* Resources unavailable stop queue */
+ netif_tx_stop_queue(netdev_get_tx_queue(dev, txq));
+
+ /* We won't see all dropped packets individually, so
+ * over run error is more appropriate.
+ */
+ dev->stats.tx_fifo_errors++;
+ } else {
+ goto drop;
+ }
+ }

if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
goto drop;
@@ -1333,6 +1344,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
DECLARE_WAITQUEUE(wait, current);
struct sk_buff *skb;
ssize_t ret = 0;
+ struct netdev_queue *ntxq;

tun_debug(KERN_INFO, tun, "tun_do_read\n");

@@ -1362,6 +1374,12 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
continue;
}

+ ntxq = netdev_get_tx_queue(tun->dev, tfile->queue_index);
+
+ if (tfile->flags & TUN_FLOW_CONTROL &&
+ netif_tx_queue_stopped(ntxq))
+ netif_tx_wake_queue(ntxq);
+
ret = tun_put_user(tun, tfile, skb, iv, len);
kfree_skb(skb);
break;
@@ -1732,6 +1750,11 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
else
tun->flags &= ~TUN_TAP_MQ;

+ if (ifr->ifr_flags & IFF_FLOW_CONTROL)
+ tfile->flags |= TUN_FLOW_CONTROL;
+ else
+ tfile->flags &= ~TUN_FLOW_CONTROL;
+
/* Make sure persistent devices do not get stuck in
* xoff state.
*/
@@ -1900,7 +1923,8 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
* This is needed because we never checked for invalid flags on
* TUNSETIFF. */
return put_user(IFF_TUN | IFF_TAP | IFF_NO_PI | IFF_ONE_QUEUE |
- IFF_VNET_HDR | IFF_MULTI_QUEUE,
+ IFF_VNET_HDR | IFF_MULTI_QUEUE |
+ IFF_FLOW_CONTROL,
(unsigned int __user*)argp);
} else if (cmd == TUNSETQUEUE)
return tun_set_queue(file, &ifr);
diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
index e9502dd..bcf2790 100644
--- a/include/uapi/linux/if_tun.h
+++ b/include/uapi/linux/if_tun.h
@@ -36,6 +36,7 @@
#define TUN_PERSIST 0x0100
#define TUN_VNET_HDR 0x0200
#define TUN_TAP_MQ 0x0400
+#define TUN_FLOW_CONTROL 0x0800

/* Ioctl defines */
#define TUNSETNOCSUM _IOW('T', 200, int)
@@ -70,6 +71,7 @@
#define IFF_MULTI_QUEUE 0x0100
#define IFF_ATTACH_QUEUE 0x0200
#define IFF_DETACH_QUEUE 0x0400
+#define IFF_FLOW_CONTROL 0x0010
/* read-only flag */
#define IFF_PERSIST 0x0800
#define IFF_NOFILTER 0x1000

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