[RFC PATCH net-next] tun: support retrieving multiple packets in a single read with IFF_MULTI_READ

Alex Gartrell
2014-12-05T04:00:10Z

This patch adds the IFF_MULTI_READ flag. This has the following behavior.

1) If a read is too short for a packet, a single stripped packet will be read

2) If a read is long enough for multiple packets, as many *full* packets
will be read as possible. We will not return a stripped packet, so even if
there are many, many packets, we may get a short read.

In casual performance testing with a simple test program that simply reads
and counts packets, IFF_MULTI_READ conservatively yielded a 30% CPU win, as
measured by top. Load was being driven by a bunch of hpings running on a
server on the same L2 network (single hop through a top-of-rack switch).

Signed-off-by: Alex Gartrell <agartrell@fb.com>
---
drivers/net/tun.c | 66 ++++++++++++++++++++++++++++++++++++++-------
include/uapi/linux/if_tun.h | 3 +++
2 files changed, 60 insertions(+), 9 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 6d44da1..f57d618 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1228,6 +1228,26 @@ static ssize_t tun_chr_write_iter(struct kiocb *iocb, struct iov_iter *from)
return result;
}

+static inline size_t tun_calc_max_put_len(const struct tun_struct *tun)
+{
+ size_t len = 0;
+
+ /* It's a pain to peek the skb, so let's assume the worst:
+ * 1) That skb->len = mtu
+ * 2) That there is a vlan_tx_tag present
+ */
+
+ len += tun->dev->mtu + VLAN_HLEN;
+
+ if (tun->flags & TUN_VNET_HDR)
+ len += tun->vnet_hdr_sz;
+
+ if (!(tun->flags & TUN_NO_PI))
+ len += sizeof(struct tun_pi);
+
+ return len;
+}
+
/* Put packet to the user space buffer */
static ssize_t tun_put_user(struct tun_struct *tun,
struct tun_file *tfile,
@@ -1343,8 +1363,10 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
struct iov_iter *to,
int noblock)
{
+ const size_t max_put_len = tun_calc_max_put_len(tun);
struct sk_buff *skb;
- ssize_t ret;
+ ssize_t ret = 0;
+ ssize_t put_ret = 0;
int peeked, err, off = 0;

tun_debug(KERN_INFO, tun, "tun_do_read\n");
@@ -1355,14 +1377,31 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
if (tun->dev->reg_state != NETREG_REGISTERED)
return -EIO;

- /* Read frames from queue */
- skb = __skb_recv_datagram(tfile->socket.sk, noblock ? MSG_DONTWAIT : 0,
- &peeked, &off, &err);
- if (!skb)
- return 0;
+ while (!ret || ((tun->flags & TUN_MULTI_READ) &&
+ iov_iter_count(to) >= max_put_len)) {
+ /* Read frames from queue */
+ skb = __skb_recv_datagram(tfile->socket.sk,
+ noblock ? MSG_DONTWAIT : 0,
+ &peeked, &off, &err);
+ if (skb) {
+ put_ret = tun_put_user(tun, tfile, skb, to);
+ kfree_skb(skb);
+ if (put_ret < 0) {
+ ret = put_ret;
+ break;
+ }
+ ret += put_ret;
+ } else {
+ if (!ret)
+ ret = err;
+ break;
+ }

- ret = tun_put_user(tun, tfile, skb, to);
- kfree_skb(skb);
+ /* Now that we've received a datagram, noblock for the
+ * rest
+ */
+ noblock = 1;
+ }

return ret;
}
@@ -1537,6 +1576,9 @@ static int tun_flags(struct tun_struct *tun)
if (tun->flags & TUN_PERSIST)
flags |= IFF_PERSIST;

+ if (tun->flags & TUN_MULTI_READ)
+ flags |= IFF_MULTI_READ;
+
return flags;
}

@@ -1720,6 +1762,11 @@ 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_MULTI_READ)
+ tun->flags |= TUN_MULTI_READ;
+ else
+ tun->flags &= ~TUN_MULTI_READ;
+
/* Make sure persistent devices do not get stuck in
* xoff state.
*/
@@ -1883,7 +1930,8 @@ 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_MULTI_READ,
(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..aaf9ddc 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_MULTI_READ 0x0800

/* Ioctl defines */
#define TUNSETNOCSUM _IOW('T', 200, int)
@@ -74,6 +75,8 @@
#define IFF_PERSIST 0x0800
#define IFF_NOFILTER 0x1000

+#define IFF_MULTI_READ 0x2000
+
/* Socket options */
#define TUN_TX_TIMESTAMP 1

--
Alex Gartrell <agartrell@fb.com>