Re: [PATCH net-next] net: core: devlink: add new trap action HARD_DROP

On Fri, Jan 22, 2021 at 08:36:01AM +0000, Oleksandr Mazur wrote:

On Thu, 21 Jan 2021 14:21:52 +0200 Ido Schimmel wrote:

quoted

On Thu, Jan 21, 2021 at 01:29:37PM +0200, Oleksandr Mazur wrote:

quoted

Add new trap action HARD_DROP, which can be used by the
drivers to register traps, where it's impossible to get
packet reported to the devlink subsystem by the device
driver, because it's impossible to retrieve dropped packet
from the device itself.
In order to use this action, driver must also register
additional devlink operation - callback that is used
to retrieve number of packets that have been dropped by
the device.  

Are these global statistics about number of packets the hardware dropped
for a specific reason or are these per-port statistics?

It's a creative use of devlink-trap interface, but I think it makes
sense. Better to re-use an existing interface than creating yet another
one.

quoted

Not sure if I agree, if we can't trap why is it a trap?
It's just a counter.

It's just another ACTION for trap item. Action however can be switched, e.g. from HARD_DROP to MIRROR.

The thing is to be able to configure specific trap to be dropped, and provide a way for the device to report back how many packets have been dropped.
If device is able to report the packet itself, then devlink would be in charge of counting. If not, there should be a way to retrieve these statistics from the devlink.

So no need for another action. Just report these stats via
'DEVLINK_ATTR_STATS_RX_DROPPED' if the hardware supports it.

Currently you do:

+static int
+devlink_trap_hard_drop_stats_put(struct sk_buff *msg,
+				 struct devlink *devlink,
+				 const struct devlink_trap_item *trap_item)
+{
+	struct nlattr *attr;
+	u64 drops;
+	int err;
+
+	err = devlink->ops->trap_hard_drop_counter_get(devlink, trap_item->trap,
+						       &drops);
+	if (err)
+		return err;
+
+	attr = nla_nest_start(msg, DEVLINK_ATTR_STATS);
+	if (!attr)
+		return -EMSGSIZE;
+
+	if (nla_put_u64_64bit(msg, DEVLINK_ATTR_STATS_RX_DROPPED, drops,
+			      DEVLINK_ATTR_PAD))
+		goto nla_put_failure;
+
+	nla_nest_end(msg, attr);
+
+	return 0;
+
+nla_put_failure:
+	nla_nest_cancel(msg, attr);
+	return -EMSGSIZE;
+}
+
 static int devlink_nl_trap_fill(struct sk_buff *msg, struct devlink *devlink,
 				const struct devlink_trap_item *trap_item,
 				enum devlink_command cmd, u32 portid, u32 seq,

@@ -6857,7 +6889,10 @@ static int devlink_nl_trap_fill(struct sk_buff *msg, struct devlink *devlink,
 	if (err)
 		goto nla_put_failure;
 
-	err = devlink_trap_stats_put(msg, trap_item->stats);
+	if (trap_item->action == DEVLINK_TRAP_ACTION_HARD_DROP)
+		err = devlink_trap_hard_drop_stats_put(msg, devlink, trap_item);
+	else
+		err = devlink_trap_stats_put(msg, trap_item->stats);
 	if (err)
 		goto nla_put_failure;

Which means that user space will see stats come and go based on the
trap's action. That's not desirable. Instead, change:

[DEVLINK_ATTR_STATS]
	[DEVLINK_ATTR_STATS_RX_PACKETS]
	[DEVLINK_ATTR_STATS_RX_BYTES]

To:

[DEVLINK_ATTR_STATS]
	[DEVLINK_ATTR_STATS_RX_PACKETS]
	[DEVLINK_ATTR_STATS_RX_BYTES]
	[DEVLINK_ATTR_STATS_RX_DROPPED]

Where the last attribute is reported to user space for devices that
support such stats. No changes required in uAPI / iproute2.