Re: [PATCH] net/bonding: send arp in interval if no active slave

From: Uwe Koziolek
Date: Tue Sep 01 2015 - 19:25:06 EST

On Tue, Sep 01, 2015 at 05:41 PM +0200, Andy Gospodarek wrote:
On Mon, Aug 17, 2015 at 10:51:27PM +0200, Uwe Koziolek wrote:
On Mon, Aug 17, 2015 at 09:14PM +0200, Jay Vosburgh wrote:
Uwe Koziolek <uwe.koziolek@xxxxxxxxxxx> wrote:

On2015-08-17 07:12 PM,Jarod Wilson wrote:
On 2015-08-17 12:55 PM, Veaceslav Falico wrote:
On Mon, Aug 17, 2015 at 12:23:03PM -0400, Jarod Wilson wrote:
From: Uwe Koziolek <uwe.koziolek@xxxxxxxxxxx>

With some very finicky switch hardware, active backup bonding can get
a situation where we play ping-pong between interfaces, trying to get
to come up as the active slave. There seems to be an issue with the
switch's arp replies either taking too long, or simply getting lost,
so we
wind up unable to get any interface up and active. Sometimes, the issue
sorts itself out after a while, sometimes it doesn't.

Testing with num_grat_arp has proven fruitless, but sending an
arp on curr_arp_slave if we're still in the arp_interval timeslice in
bond_ab_arp_probe(), has shown to produce 100% reliability in testing
this hardware combination.
Sorry, I don't understand the logic of why it works, and what exactly
we fixiing here.

It also breaks completely the logic for link state management in case
of no
current active slave for 2*arp_interval.

Could you please elaborate what exactly is fixed here, and how it
works? :)
I can either duplicate some information from the bug, or Uwe can, to
illustrate the exact nature of the problem.

p.s. num_grat_arp maybe could help?
That was my thought as well, but as I understand it, that route was
explored, and it didn't help any. I don't actually have a reproducer
setup of my own, unfortunately, so I'm kind of caught in the middle

Uwe, can you perhaps further enlighten us as to what num_grat_arp
settings were tried that didn't help? I'm still of the mind that if
num_grat_arp *didn't* help, we probably need to do something keyed off
The bonding slaves are connected to high available switches, each of the
slaves is connected to a different switch. If the bond is starting, only
the selected slave sends one arp-request. If a matching arp_response was
received, this slave and the bond is going into state up, sending the
gratitious arps...
But if you got no arp reply the next slave was selected.
With most of the newer switches, not overloaded, or with other software
bugs, or with a single switch configuration, you would get a arp response
on the first arp request.
But in case of high availability configuration with non perfect switches
like HP ProCurve 54xx, also with some Cisco models, you may not get a
response on the first arp request.

I have seen network snoops, there the switches are not responding to the
first arp request on slave 1, the second arp request was sent on slave 2
but the response was received on slave one, and all following arp
requests are anwsered on the wrong slave for a longer time.
Could you elaborate on the exact "high availability
configuration" here, including the model(s) of switch(es) involved?

Is this some kind of race between the switch or switches
updating the forwarding tables and the bond flip flopping between the
slaves? E.g., source MAC from ARP sent on slave 1 is used to populate
the forwarding table, but (for whatever reason) there is no reply. ARP
on slave 2 is sent (using the same source MAC, unless you set
fail_over_mac), but forwarding tables still send that MAC to slave 1, so
reply is sent there.
High availability:
2 managed switches with routing capabilities have an interconnect.
One slave of a bonding interface is connected to the first switch, the
second slave is connected to the other switch.
The switch models are HP ProCurve 5406 and HP ProCurve 5412. As far as i
remember also HP E 3500 and E 3800 are also
affected, for the affected Cisco models I can't answer today.
Affected single switch configurations was not seen.

Yes, race conditions with delayed upgrades of the forwarding tables is a
well matching explanation for the problem.

The proposed change sents up to 3 arp requests on a down bond using the
same slave, delayed by arp_interval.
Using problematic switches i have seen the the arp response on the right
slave at latest on the second arp request. So the bond is going into state

How does it works:
The bonds in up state are handled on the beginning of bond_ab_arp_probe
procedure, the other part of this procedure is handling the slave change.
The proposed change is bypassing the slave change for 2 additional calls
of bond_ab_arp_probe.
Now the retries are not only for an up bond available, they are also
implemented for a down bond.
Does this delay failover or bringup on switches that are not
"problematic"? I.e., if arp_interval is, say, 1000 (1 second), will
this impact failover / recovery times?

It depends.
failover times are not impacted, this is handled different.
Only the transition from a down bonding interface (bond and all slaves are
down) to the state up can be increased by up to 2 times arp_interval,
If the selected interface did not came up .If well working switches are
used, and everything other is also ok, there are no impacts.
So I'm not a huge fan of workarounds like these, but I also understand
from a practical standpoint that this is useful. My only issue with the
patch would be to please include a small comment (1-2 lines) in the code
that describes the behavior. I know we have the changelog entries for
this, but I would feel better about having an exception like this in the
code for those reading it and wondering:

"Why would we wait 2 intervals before failing over to the next interface
when there are no active interfaces?"

diff -up a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
--- a/drivers/net/bonding/bond_main.c 2015-08-30 20:34:09.000000000 +0200
+++ b/drivers/net/bonding/bond_main.c 2015-09-02 00:39:10.000298202 +0200
@@ -2795,6 +2795,16 @@ static bool bond_ab_arp_probe(struct bon
return should_notify_rtnl;

+ /* sometimes the forwarding tables of the switches are not updated fast enough
+ * the first arp response after a slave change is received on the wrong slave.
+ * the arp requests will be retried 2 times on the same slave
+ */
+ if (bond_time_in_interval(bond, curr_arp_slave->last_link_up, 2)) {
+ bond_arp_send_all(bond, curr_arp_slave);
+ return should_notify_rtnl;
+ }
bond_set_slave_inactive_flags(curr_arp_slave, BOND_SLAVE_NOTIFY_LATER);

bond_for_each_slave_rcu(bond, slave, iter) {

The num_grat_arp has no chance to solve the problem. The num_grat_arp is
only used, if a different slave is going active.
But in our case, the bonding slaves are not going into the state active
for a longer time.
[jarod: manufacturing of changelog]
CC: Jay Vosburgh <j.vosburgh@xxxxxxxxx>
CC: Veaceslav Falico <vfalico@xxxxxxxxx>
CC: Andy Gospodarek <gospo@xxxxxxxxxxxxxxxxxxx>
CC: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Uwe Koziolek <uwe.koziolek@xxxxxxxxxxx>
Signed-off-by: Jarod Wilson <jarod@xxxxxxxxxx>
drivers/net/bonding/bond_main.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/net/bonding/bond_main.c
index 0c627b4..60b9483 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -2794,6 +2794,11 @@ static bool bond_ab_arp_probe(struct bonding
return should_notify_rtnl;

+ if (bond_time_in_interval(bond, curr_arp_slave->last_link_up, 2))
+ bond_arp_send_all(bond, curr_arp_slave);
+ return should_notify_rtnl;
+ }

bond_for_each_slave_rcu(bond, slave, iter) {
-Jay Vosburgh, jay.vosburgh@xxxxxxxxxxxxx

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