[dpdk-dev] [PATCH v3 7/7] app/testpmd: adjust ethdev port ownership

[Matan Azrad]

Gaetan, Konstantin, Thomas

Any response to my suggestion below?

From: Matan Azrad
> Hi
> 
> From: Gaëtan Rivet [mailto:gaetan.rivet at 6wind.com] <snip>
> > > > > > > Look,
> > > > > > > > Testpmd initiates some of its internal databases depends
> > > > > > > > on specific port iteration, In some time someone may take
> > > > > > > > ownership of Testpmd ports and testpmd will continue to
> > > > > > > > touch
> > them.
> > > > > >
> > > > > > But if someone will take the ownership (assign new owner_id)
> > > > > > that port will not appear in RTE_ETH_FOREACH_DEV() any more.
> > > > > >
> > > > >
> > > > > Yes, but testpmd sometimes depends on previous iteration using
> > internal database.
> > > > > So it uses internal database that was updated by old iteration.
> > > >
> > > > That sounds like just a bug in testpmd that need to be fixed, no?
> > >
> > > If Testpmd already took ownership for these ports(like I did), it is ok.
> > >
> >
> > Have you tested using the default iterator (NO_OWNER)?
> > It worked until now with the bare minimal device tagging using
> > DEV_DEFERRED. Testpmd did not seem to mind having to skip this port.
> >
> > I'm sure there were places where this was overlooked, but overall, I'd
> > think everything should be fixable using only the NO_OWNER iteration.
> 
> I don't think so.
> 
> > Can you point to a specific scenario (command line, chain of event)
> > that would lead to a problem?
> >
> 
> I didn't construct a race test to catch testpmd issue, but I think without this
> patch, there is a lot of issues.
> Go to the testpmd code (before ownership) and find usage of the old
> iterator(after the first iteration in main), Ask yourself what should happen if
> exactly in this time, a new port is created by fail-safe(plug in event).
> 
> > > > Any particular places where outdated device info is used?
> > >
> > > For example, look for the stream management in testpmd(I think I saw
> > > it
> > there).
> > >
> >
> > The stream management is certainly shaky, but it happens after the EAL
> > initial port creation, and is not able to update itself for new
> > hotplugged ports (unless something changed).
> >
> 
> Yes, but conceptually someone in the future may take the port(because it
> ownerless).
> 
> > > > > > > If I look back on the fail-safe, its sole purpose is to have
> > > > > > > seamless hotplug with existing applications.
> > > > > > >
> > > > > > > Port ownership is a genericization of some functions
> > > > > > > introduced by the fail-safe, that could structure DPDK
> > > > > > > further. It should allow applications to have a seamless
> > > > > > > integration with subsystems using port ownership. Without
> > > > > > > this,
> > port ownership cannot be used.
> > > > > > >
> > > > > > > Testpmd should be fixed, but follow the most common design
> > > > > > > patterns of DPDK applications. Going with port ownership
> > > > > > > seems like a paradigm shift.
> > > > > > >
> > > > > > > > In addition
> > > > > > > > Using the old iterator in some places in testpmd will
> > > > > > > > cause a race for run-
> > > > > > time new ports(can be created by failsafe or any hotplug code):
> > > > > > > > - testpmd finds an ownerless port(just now created) by the
> > > > > > > > old iterator and start traffic there,
> >
> > How does testpmd start traffic there? Testpmd has only a callback for
> > displaying that it received an event for a new port. It has no concept
> > of hotplugging beyond that.
> >
> 
> Yes, so no traffic just some control command.
> 
> > Testpmd will not start using any new port probed using the hotplug API
> > on its own, again, unless something has drastically changed.
> >
> 
> Every iterator using in testpmd is exposed to race.
> 
> > > > > > > > - failsafe takes ownership of this new port and start traffic there.
> > > > > > > > Problem!
> > > > > >
> > > > > > Could you shed a bit more light here - it would be race
> > > > > > condition between whom and whom?
> > > > >
> > > > > Sure.
> > > > >
> > > > > > As I remember in testpmd all control ops are done within one
> > > > > > thread (main lcore).
> > > > >
> > > > > But other dpdk entity can use another thread, for example:
> > > > > Failsafe uses the host thread(using alarm callback) to create a
> > > > > new port and
> > > > to take ownership of a port.
> > > >
> > > > Hm, and you create new ports inside failsafe PMD, right and then
> > > > set new owner_id for it?
> > >
> > > Yes.
> > >
> > > > And all this in alarm in interrupt thread?
> > >
> > > Yes.
> > >
> > > > If so I wonder how you can guarantee that no-one else will set
> > > > different owner_id between
> > > > rte_eth_dev_allocate() and rte_eth_dev_owner_set()?
> > >
> > > I check it (see failsafe patch to this series - V5).
> > > Function: fs_bus_init.
> > >
> > > > Could you point me to that place (I am not really familiar with
> > > > familiar with failsafe code)?
> > > >
> > > > >
> > > > > The race:
> > > > > Testpmd iterates over all ports by the master thread.
> > > > > Failsafe takes ownership of a port by the host thread and start using
> it.
> > > > > => The two dpdk entities may use the device at same time!
> > > >
> >
> > When can this happen? Fail-safe creates its initial pool of ports
> > during EAL init, before testpmd scans eth_dev ports and configure its
> streams.
> > At that point, it has taken ownership, from the master lcore context.
> >
> > After this point, new ports could be detected and hotplugged by fail-safe.
> > However, even if testpmd had a callback to capture those new ports and
> > reconfigure its streams, it would be executed from within the
> > intr-thread, same as failsafe. If the thread was interrupted, by a
> > dataplane-lcore for example, streams would not have been reconfigured.
> > The fail-safe would execute its callback and set the owner-id before
> > the callback chains goes to the application.
> >
> 
> Some iterator may be invoked in plug out process by other thread in testpmd
> and causes to control command
> 
> > And that would only be if testpmd had any callback for hotplugging
> > ports and reconfiguring its streams, which it hasn't, as far as I know.
> >
> 
> We don't need to implement it in testpmd.
> 
> > > > Ok, if failsafe really assigns its owner_id(s) to ports that are
> > > > already in use by the app, then how such scheme supposed to work
> > > > at
> > all?
> > >
> > > If the app works well (with the new rules) it already took ownership
> > > and
> > failsafe will see it and will wait until the application release it.
> > > Every dpdk entity should know which port it wants to manage, If 2
> > > entities want to manage the same device -  it can be ok and port
> > > ownership
> > can synchronize the usage.
> > >
> > > Probably, application which will run fail-safe wants to manage only
> > > the fail-
> > safe port and therefor to take ownership only for it.
> > >
> > > > I.E. application has a port - it assigns some owner_id != 0 to it,
> > > > then PMD tries to set its owner_id tot the same port.
> > > > Obviously failsafe's set_owner() will always fail in such case.
> > > >
> > > Yes, and will try again after some time.
> > >
> > > > From what I hear we need to introduce a concept of 'default owner id'.
> > > > I.E. when failsafe PMD is created - user assigns some owner_id to
> > > > it
> > (default).
> > > > Then failsafe PMD generates it's own owner_id and assigns it only
> > > > to the ports whose current owner_id is equal either 0 or 'default'
> owner_id.
> > > >
> > >
> > > It is a suggestion and we need to think about it more (I'm talking
> > > about it
> > with Gaetan in another thread).
> > > Actually I think, if we want a generic solution to the generic
> > > problem the
> > current solution is ok.
> > >
> >
> > We could as well conclude this other thread there.
> >
> > The only solution would be to have a default relationship between
> > owners, something that goes beyond the scope assigned by Thomas to
> > your evolution, but would be necessary for this API to be properly
> > used by existing applications.
> >
> > I think it's the only way to have a sane default behavior with your
> > API, but I also think this goes beyong the scope of the DPDK altogether.
> >
> > But even with those considerations that could be ironed out later (API
> > is still experimental anyway), in the meantime, I think we should
> > strive not to break "userland" as much as possible. Meaning that
> > unless you have a specific situation creating a bug, you shouldn't
> > have to modify testpmd, and if an issues arises, you need to try to
> > improve your API before resorting to changing the resource management
> model of all existing applications.
> >
> 
> I understand it.
> Suggestion:
> 
> 2 system owners.
> APP_OWNER - 1.
> NO_OWNER - 0.
> 
> And allowing for more owners as now.
> 
> 1. Every port creation will set the owner for NO_OWNER (as now).
> 2. There is option for all dpdk entities to take owner of  NO_OWNER ports all
> the time(as now).
> 3. In some point in the end of EAL init: set all the NO_OWNER to
> APP_OWNER(for V6).
> 4. Change the old iterator to iterate over APP_OWNER ports(for V6).
> 
> What do you think?
> 
> <snip>