2016-06-16 16:41, Iremonger, Bernard: > Hi Thomas, > <snip> > > 2016-06-16 15:32, Bruce Richardson: > > > On Mon, Jun 13, 2016 at 01:28:08PM +0100, Iremonger, Bernard wrote: > > > > > Why does this particular PMD need spinlocks when doing RX and TX, > > > > > while other device types do not? How is adding/removing devices > > > > > from a bonded device different to other control operations that > > > > > can be done on physical PMDs? Is this not similar to say bringing > > > > > down or hotplugging out a physical port just before an RX or TX > > operation takes place? > > > > > For all other PMDs we rely on the app to synchronise control and > > > > > data plane operation - why not here? > > > > > > > > > > /Bruce > > > > > > > > This issue arose during VM live migration testing. > > > > For VM live migration it is necessary (while traffic is running) to be > > > > able to > > remove a bonded slave device, stop it, close it and detach it. > > > > It a slave device is removed from a bonded device while traffic is > > > > running > > a segmentation fault may occur in the rx/tx burst function. The spinlock has > > been added to prevent this occurring. > > > > > > > > The bonding device already uses a spinlock to synchronise between the > > add and remove functionality and the slave_link_status_change_monitor > > code. > > > > > > > > Previously testpmd did not allow, stop, close or detach of PMD while > > > > traffic was running. Testpmd has been modified with the following > > > > patchset > > > > > > > > http://dpdk.org/dev/patchwork/patch/13472/ > > > > > > > > It now allows stop, close and detach of a PMD provided in it is not > > forwarding and is not a slave of bonded PMD. > > > > > > > I will admit to not being fully convinced, but if nobody else has any > > > serious objections, and since this patch has been reviewed and acked, > > > I'm ok to merge it in. I'll do so shortly. > > > > Please hold on. > > Seeing locks introduced in the Rx/Tx path is an alert. > > We clearly need a design document to explain where locks can be used and > > what are the responsibility of the control plane. > > If everybody agrees in this document that DPDK can have some locks in the > > fast path, then OK to merge it. > > > > So I would say NACK for 16.07 and maybe postpone to 16.11. > > Looking at the documentation for the bonding PMD. > > http://dpdk.org/doc/guides/prog_guide/link_bonding_poll_mode_drv_lib.html > > In section 10.2 it states the following: > > Bonded devices support the dynamical addition and removal of slave devices > using the rte_eth_bond_slave_add / rte_eth_bond_slave_remove APIs. > > If a slave device is added or removed while traffic is running, there is the > possibility of a segmentation fault in the rx/tx burst functions. This is > most likely to occur in the round robin bonding mode. > > This patch set fixes what appears to be a bug in the bonding PMD.
It can be fixed by removing this statement in the doc. One of the design principle of DPDK is to avoid locks. > Performance measurements have been made with this patch set applied and > without the patches applied using 64 byte packets. > > With the patches applied the following drop in performance was observed: > > % drop for fwd+io: 0.16% > % drop for fwd+mac: 0.39% > > This patch set has been reviewed and ack'ed, so I think it should be applied > in 16.07 I understand your point of view and I gave mine. Now we need more opinions from others.
