Re: [PATCH net-next v4] rtnetlink: Support fine-grained netdevice bulk deletion

On Sun, Dec 05, 2021 at 02:11:00PM +0200, Lahav Schlesinger wrote:

On Sun, Dec 05, 2021 at 11:53:03AM +0200, Ido Schimmel wrote:

quoted

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On Thu, Dec 02, 2021 at 07:45:02PM +0200, Lahav Schlesinger wrote:

quoted

Under large scale, some routers are required to support tens of thousands
of devices at once, both physical and virtual (e.g. loopbacks, tunnels,
vrfs, etc).
At times such routers are required to delete massive amounts of devices
at once, such as when a factory reset is performed on the router (causing
a deletion of all devices), or when a configuration is restored after an
upgrade, or as a request from an operator.

Currently there are 2 means of deleting devices using Netlink:
1. Deleting a single device (either by ifindex using ifinfomsg::ifi_index,
or by name using IFLA_IFNAME)
2. Delete all device that belong to a group (using IFLA_GROUP)

Deletion of devices one-by-one has poor performance on large scale of
devices compared to "group deletion":
After all device are handled, netdev_run_todo() is called which
calls rcu_barrier() to finish any outstanding RCU callbacks that were
registered during the deletion of the device, then wait until the
refcount of all the devices is 0, then perform final cleanups.

However, calling rcu_barrier() is a very costly operation, each call
taking in the order of 10s of milliseconds.

When deleting a large number of device one-by-one, rcu_barrier()
will be called for each device being deleted.
As an example, following benchmark deletes 10K loopback devices,
all of which are UP and with only IPv6 LLA being configured:

1. Deleting one-by-one using 1 thread : 243 seconds
2. Deleting one-by-one using 10 thread: 70 seconds
3. Deleting one-by-one using 50 thread: 54 seconds
4. Deleting all using "group deletion": 30 seconds

Note that even though the deletion logic takes place under the rtnl
lock, since the call to rcu_barrier() is outside the lock we gain
some improvements.

But, while "group deletion" is the fastest, it is not suited for
deleting large number of arbitrary devices which are unknown a head of
time. Furthermore, moving large number of devices to a group is also a
costly operation.

These are the number I get in a VM running on my laptop.

Moving 16k dummy netdevs to a group:

# time -p ip -b group.batch
real 1.91
user 0.04
sys 0.27

Deleting the group:

# time -p ip link del group 10
real 6.15
user 0.00
sys 3.02

Hi Ido, in your tests in which state the dummy devices are before
deleting/changing group?
When they are DOWN I get similar numbers to yours (16k devices):

# time ip -b group_16000_batch
real	0m0.640s
user	0m0.152s
sys	0m0.478s

# time ip link delete group 100
real	0m5.324s
user	0m0.017s
sys	0m4.991s

But when the devices are in state UP, I get:

# time ip -b group_16000_batch
real	0m48.605s
user	0m0.218s
sys	0m48.244s

# time ip link delete group 100
real	1m13.219s
user	0m0.010s
sys	1m9.117s

And for completeness, setting the devices to DOWN prior to deleting them
is as fast as deleting them in the first place while they're UP.

Also, while this is probably a minuscule issue, changing the group of
10ks+ of interfaces will result in a storm of netlink events that will
make any userspace program listening on link events to spend time
handling these events.  This will result in twice as many events
compared to directly deleting the devices.

Yes, in my setup the netdevs were down. Looking at the code, I think the
reason for the 75x increase in latency is the fact that netlink
notifications are not generated when the netdev is down. See
netdev_state_change().

In your specific case, it is quite useless for the kernel to generate
16k notifications when moving the netdevs to a group since the entire
reason they are moved to a group is so that they could be deleted in a
batch.

I assume that there are other use cases where having the kernel suppress
notifications can be useful. Did you consider adding such a flag to the
request? I think such a mechanism is more generic/useful than an ad-hoc
API to delete a list of netdevs and should allow you to utilize the
existing group deletion mechanism.

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IMO, these numbers do not justify a new API. Also, your user space can
be taught to create all the netdevs in the same group to begin with:

# ip link add name dummy1 group 10 type dummy
# ip link show dev dummy1
10: dummy1: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group 10 qlen 1000
    link/ether 12:b6:7d:ff:48:99 brd ff:ff:ff:ff:ff:ff

Moreover, unlike the list API that is specific to deletion, the group
API also lets you batch set operations:

# ip link set group 10 mtu 2000
# ip link show dev dummy1
10: dummy1: <BROADCAST,NOARP> mtu 2000 qdisc noop state DOWN mode
DEFAULT group 10 qlen 1000
    link/ether 12:b6:7d:ff:48:99 brd ff:ff:ff:ff:ff:ff

The list API can be extended to support other operations as well
(similar to group set operations, we can call do_setlink() for each
device specified in an IFLA_IFINDEX).
I didn't implement it in this patch because we don't have a use for it
currently.

quoted

If you are using namespaces, then during "factory reset" you can delete
the namespace which should trigger batch deletion of the netdevs inside
it.

In some scenarios we are required to delete only a subset of devices
(e.g. when a physical link becomes DOWN, we need to delete all the VLANs
and any tunnels configured on that device).  Furthermore, a user is
allowed to load a new configuration in which he deletes only some of the
devices (e.g. delete all of the loopbacks in the system), while not
touching the other devices.

quoted

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This patch adds support for passing an arbitrary list of ifindex of
devices to delete with a new IFLA_IFINDEX attribute. A single message
may contain multiple instances of this attribute).
This gives a more fine-grained control over which devices to delete,
while still resulting in rcu_barrier() being called only once.
Indeed, the timings of using this new API to delete 10K devices is
the same as using the existing "group" deletion.

Signed-off-by: Lahav Schlesinger <redacted>