[PATCH 1/2] net: sch_generic: add flag IFF_FIFO_QUEUE to use pfifo_fast as default scheduler

[Marc Kleine-Budde <mkl@pengutronix.de>]

There is networking hardware that isn't based on Ethernet for layers 1 and 2.

For example CAN.

CAN is a multi-master serial bus standard for connecting Electronic Control
Units [ECUs] also known as nodes. A frame on the CAN bus carries up to 8 bytes
of payload. Frame corruption is detected by a CRC. However frame loss due to
corruption is possible, but a quite unusual phenomenon.

While fq_codel works great for TCP/IP, it doesn't for CAN. There are a lot of
legacy protocols on top of CAN, which are not build with flow control or high
CAN frame drop rates in mind.

When using fq_codel, as soon as the queue reaches a certain delay based length,
skbs from the head of the queue are silently dropped. Silently meaning that the
user space using a send() or similar syscall doesn't get an error. However
TCP's flow control algorithm will detect dropped packages and adjust the
bandwidth accordingly.

When using fq_codel and sending raw frames over CAN, which is the common use
case, the user space thinks the package has been sent without problems, because
send() returned without an error. pfifo_fast will drop skbs, if the queue
length exceeds the maximum. But with this scheduler the skbs at the tail are
dropped, an error (-ENOBUFS) is propagated to user space. So that the user
space can slow down the package generation.

On distributions, where fq_codel is made default via CONFIG_DEFAULT_NET_SCH
during compile time, or set default during runtime with sysctl
net.core.default_qdisc (see [1]), we get a bad user experience. In my test case
with pfifo_fast, I can transfer thousands of million CAN frames without a frame
drop. On the other hand with fq_codel there is more then one lost CAN frame per
thousand frames.

As pointed out fq_codel is not suited for CAN hardware, so this patch
introduces a new netdev_priv_flag called "IFF_FIFO_QUEUE" (in contrast to the
existing "IFF_NO_QUEUE").

During transition of a netdev from down to up state the default queuing
discipline is attached by attach_default_qdiscs() with the help of
attach_one_default_qdisc(). This patch modifies attach_one_default_qdisc() to
attach the pfifo_fast (pfifo_fast_ops) if the "IFF_FIFO_QUEUE" flag is set.

[1] https://github.com/systemd/systemd/issues/9194

Cc: Dave Taht <dave.taht@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
 include/linux/netdevice.h | 3 +++
 net/sched/sch_generic.c   | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 166fdc0a78b4..1867e27e3369 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1498,6 +1498,7 @@ struct net_device_ops {
  * @IFF_FAILOVER: device is a failover master device
  * @IFF_FAILOVER_SLAVE: device is lower dev of a failover master device
  * @IFF_L3MDEV_RX_HANDLER: only invoke the rx handler of L3 master device
+ * @IFF_FIFO_QUEUE: device must run with FIFO qdisc attached. skb drop without NET_XMIT_DROP is fatal
  */
 enum netdev_priv_flags {
 	IFF_802_1Q_VLAN			= 1<<0,
@@ -1530,6 +1531,7 @@ enum netdev_priv_flags {
 	IFF_FAILOVER			= 1<<27,
 	IFF_FAILOVER_SLAVE		= 1<<28,
 	IFF_L3MDEV_RX_HANDLER		= 1<<29,
+	IFF_FIFO_QUEUE			= 1<<30,
 };
 
 #define IFF_802_1Q_VLAN			IFF_802_1Q_VLAN
@@ -1561,6 +1563,7 @@ enum netdev_priv_flags {
 #define IFF_FAILOVER			IFF_FAILOVER
 #define IFF_FAILOVER_SLAVE		IFF_FAILOVER_SLAVE
 #define IFF_L3MDEV_RX_HANDLER		IFF_L3MDEV_RX_HANDLER
+#define IFF_FIFO_QUEUE			IFF_FIFO_QUEUE
 
 /**
  *	struct net_device - The DEVICE structure.
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 81356ef38d1d..c309d0751cbc 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1049,6 +1049,9 @@ static void attach_one_default_qdisc(struct net_device *dev,
 	struct Qdisc *qdisc;
 	const struct Qdisc_ops *ops = default_qdisc_ops;
 
+	if (dev->priv_flags & IFF_FIFO_QUEUE)
+		ops = &pfifo_fast_ops;
+
 	if (dev->priv_flags & IFF_NO_QUEUE)
 		ops = &noqueue_qdisc_ops;
 
-- 
2.20.1