Re: [RFC PATCH net-next] net/core: initial support for stacked dev feature toggles

From: Jarod Wilson
Date: Fri Oct 30 2015 - 12:25:56 EST

Michal Kubecek wrote:
On Fri, Oct 23, 2015 at 10:51:09PM -0700, Alexander Duyck wrote:
On 10/23/2015 08:40 PM, Jarod Wilson wrote:
+static netdev_features_t netdev_sync_upper_features(struct net_device *lower,
+ struct net_device *upper, netdev_features_t features)
+ netdev_features_t want = upper->wanted_features& lower->hw_features;
+ if (!(upper->wanted_features& NETIF_F_LRO)
+ && (features& NETIF_F_LRO)) {
+ netdev_info(lower, "Dropping LRO, upper dev %s has it off.\n",
+ upper->name);
+ features&= ~NETIF_F_LRO;
+ } else if ((want& NETIF_F_LRO)&& !(features& NETIF_F_LRO)) {
+ netdev_info(lower, "Keeping LRO, upper dev %s has it on.\n",
+ upper->name);
+ features |= NETIF_F_LRO;
+ }
+ return features;
I'd say to drop the second half of this statement. LRO is a feature
that should be enabled explicitly per interface. If someone enables
LRO on the master they may only want it on one interface. The fact
is there are some implementations of LRO that work better than
others so you want to give the end user the option to mix and match.

Agreed. IMHO it makes sense to allow setups with LRO disabled on some
slaves and enabled on other.

Also, the logic seems to only consider the 1 upper : N lower scheme
(bond, team) but we also have N upper : 1 lower setups (vlan, macvlan).
For these, there is no way to propagate both 0 and 1 down as this would
result in a conflict.

Okay, so we're thinking do prevent lower devices turning LRO on if the upper device has it off. Or rather, if *an* upper device has it off. Probably need to rework the bit that calls this function to use netdev_for_each_upper_dev{_rcu}() to walk all of adj_list.upper here.

Rather than outright dropping the second bit though, I was thinking maybe just drop a note in dmesg along the lines of "hey, you shut off LRO, it is still enabled on upper dev foo", to placate end-users.

+static void netdev_sync_lower_features(struct net_device *upper,
+ struct net_device *lower, netdev_features_t features)
+ netdev_features_t want = features& lower->hw_features;
+ if (!(features& NETIF_F_LRO)&& (lower->features& NETIF_F_LRO)) {
+ netdev_info(upper, "Disabling LRO on lower dev %s.\n",
+ lower->name);
+ upper->wanted_features&= ~NETIF_F_LRO;
+ lower->wanted_features&= ~NETIF_F_LRO;
+ netdev_update_features(lower);
+ if (unlikely(lower->features& NETIF_F_LRO))
+ netdev_WARN(upper, "failed to disable LRO on %s!\n",
+ lower->name);
+ } else if ((want& NETIF_F_LRO)&& !(lower->features& NETIF_F_LRO)) {
+ netdev_info(upper, "Enabling LRO on lower dev %s.\n",
+ lower->name);
+ upper->wanted_features |= NETIF_F_LRO;
+ lower->wanted_features |= NETIF_F_LRO;
+ netdev_update_features(lower);
+ if (unlikely(!(lower->features& NETIF_F_LRO)))
+ netdev_WARN(upper, "failed to enable LRO on %s!\n",
+ lower->name);
+ }
Same thing here. If a lower dev has it disabled then leave it
disabled. I believe your goal is to make it so that
dev_disable_lro() can shut down LRO when it is making packets in the
data-path unusable.

This is already the case since commit fbe168ba91f7 ("net: generic
dev_disable_lro() stacked device handling"). That commit makes sure
dev_disable_lro() is propagated down the stack and also makes sure new
slaves added to a bond/team with LRO disabled have it disabled too.

What it does not do is propagating LRO disabling down if it is disabled
in ways that do not call dev_disable_lro() (e.g. via ethtool). I'm not
sure if this should be done or not, both options have their pros and

Making it work with ethtool was one of my primary goals with this change, as it was users prodding things with ethtool that prompted the "hey, this doesn't make sense" bug reports.

However, I believe enabling LRO shouldn't be propagated down.

Hm. Devices that should never have LRO enabled still won't get it enabled, so I'm not clear what harm it would cause. I tend to think you do want this sync'ing down the stack if set on an upper dev (i.e., ethtool -K bond0 lro on), for consistency's sake. You can always come back through afterwards and disable things on lower devs individually if they're really not wanted, since we're in agreement that we shouldn't prevent disabling features on lower devices.

Jarod Wilson

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