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

From: Alexander Duyck <[email-masked]>
Date: Fri, 30 Oct 2015 13:02:53 -0700

On 10/30/2015 09:25 AM, Jarod Wilson wrote:
> 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.

Right.  This part sounds fine.

> 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.

I would rather not see it.  It would be mostly noise.  It is perfectly 
valid to have LRO advertised on an upper device, but not supported on a 
lower one.  It basically just means that the path will allow LRO frames 
through, it doesn't guarantee that we are going to provide them.

>>>> +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
>> cons.
>
> 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.

I'd say make it work like dev_disable_lro already does.  Disabling LRO 
propagates down, enabling LRO only enables it on the specific device.

The way to think of it is as a warning flag.  With LRO enabled this 
device may report frames larger than MTU to the stack and will mangle 
checksums.  Without LRO all of the frames received should be restricted 
to MTU.  That is why you have to force the disabling down to all lower 
devices, and why you cannot enable it if an upper device has it disabled.

>> 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

How do you define "devices that should never have LRO enabled"?  The 
fact is LRO is very messy in terms of the way it functions.  Different 
drivers handle it different ways.  Usually it results in the Rx checksum 
being mangled, it provides frames larger than MTU, and uses fraglist 
instead of frags on some drivers.

> 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.

Think of it this way.  Lets say I have a NIC that I know is problematic 
when LRO is enabled, it might cause a kernel panic due to an skb 
overrun.  So I have a bond with it and some other NIC which can run with 
LRO enabled without issues.  How do I enable LRO on the other device 
without causing a kernel panic, and without tearing apart the existing 
bond?  With the approach you have described I can't because I have to 
enable it at the bond and doing so will enable it on the NIC with the 
faulty implementation.

This is why we cannot enable LRO unless all upper devices support it, 
and why we should propagate disabling LRO down to all lower devices. 
Trying to force it on for a lower device just because the upper device 
supports it is a bad idea because there are multiple LRO implementations 
and they all behave very differently.

- Alex