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Date: Sat, 5 Dec 2020 11:45:13 -0800 From: Jakub Kicinski <kuba@...nel.org> To: Lars Everbrand <lars.everbrand@...tonmail.com> Cc: linux-kernel@...r.kernel.org, Jay Vosburgh <j.vosburgh@...il.com>, Veaceslav Falico <vfalico@...il.com>, Andy Gospodarek <andy@...yhouse.net>, "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org Subject: Re: [PATCH net-next] bonding: correct rr balancing during link failure On Wed, 02 Dec 2020 20:55:57 +0000 Lars Everbrand wrote: > This patch updates the sending algorithm for roundrobin to avoid > over-subscribing interface(s) when one or more interfaces in the bond is > not able to send packets. This happened when order was not random and > more than 2 interfaces were used. > > Previously the algorithm would find the next available interface > when an interface failed to send by, this means that most often it is > current_interface + 1. The problem is that when the next packet is to be > sent and the "normal" algorithm then continues with interface++ which > then hits that same interface again. > > This patch updates the resending algorithm to update the global counter > of the next interface to use. > > Example (prior to patch): > > Consider 6 x 100 Mbit/s interfaces in a rr bond. The normal order of links > being used to send would look like: > 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 ... > > If, for instance, interface 2 where unable to send the order would have been: > 1 3 3 4 5 6 1 3 3 4 5 6 1 3 3 4 5 6 ... > > The resulting speed (for TCP) would then become: > 50 + 0 + 100 + 50 + 50 + 50 = 300 Mbit/s > instead of the expected 500 Mbit/s. > > If interface 3 also would fail the resulting speed would be half of the > expected 400 Mbit/s (33 + 0 + 0 + 100 + 33 + 33). > > Signed-off-by: Lars Everbrand <lars.everbrand@...tonmail.com> Thanks for the patch! Looking at the code in question it feels a little like we're breaking abstractions if we bump the counter directly in get_slave_by_id. For one thing when the function is called for IGMP packets the counter should not be incremented at all. But also if packets_per_slave is not 1 we'd still be hitting the same leg multiple times (packets_per_slave / 2). So it seems like we should round the counter up somehow? For IGMP maybe we don't have to call bond_get_slave_by_id() at all, IMHO, just find first leg that can TX. Then we can restructure bond_get_slave_by_id() appropriately for the non-IGMP case. > diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c > index e0880a3840d7..e02d9c6d40ee 100644 > --- a/drivers/net/bonding/bond_main.c > +++ b/drivers/net/bonding/bond_main.c > @@ -4107,6 +4107,7 @@ static struct slave *bond_get_slave_by_id(struct bonding *bond, > if (--i < 0) { > if (bond_slave_can_tx(slave)) > return slave; > + bond->rr_tx_counter++; > } > } > > @@ -4117,6 +4118,7 @@ static struct slave *bond_get_slave_by_id(struct bonding *bond, > break; > if (bond_slave_can_tx(slave)) > return slave; > + bond->rr_tx_counter++; > } > /* no slave that can tx has been found */ > return NULL;
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