Re: [PATCH v3 3/4] bonding: take queue spinlock in rx/tx burst functions

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Feb 15, 2017 at 06:01:45PM +0000, Ferruh Yigit wrote:
> On 6/16/2016 7:38 PM, thomas.monjalon at 6wind.com (Thomas Monjalon) wrote:
> > 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.
> > 
> 
> Hi,
> 
> These patches are sitting in the patchwork for a long time. Discussion
> never concluded and patches kept deferred each release.
> 
> I think we should give a decision about them:
> 
> 1- We can merge them in this release, they are fixing a valid problem,
> and patches are already acked.
> 
> 2- We can reject them, if not having them for more than six months not
> caused a problem, perhaps they are not really that required. And if
> somebody needs them in the future, we can resurrect them from patchwork.
> 
> I vote for option 2, any comments?
> 
+1 on option 2. There are obviously not badly needed if nobody is asking
for them for over six months.

	/Bruce