Re: [RFC PATCH v2 02/11] netdev: implement netlink api to bind dma-buf to netdevice

[David Ahern <dsahern@kernel.org>]

[ sorry for the delayed response; very busy 2 days ]

On 8/16/23 10:12 AM, Willem de Bruijn wrote:
> Jakub Kicinski wrote:
>> On Sun, 13 Aug 2023 19:10:35 -0600 David Ahern wrote:
>>> Also, this suggests that the Rx queue is unique to the flow. I do not
>>> recall a netdev API to create H/W queues on the fly (only a passing
>>> comment from Kuba), so how is the H/W queue (or queue set since a
>>> completion queue is needed as well) created for the flow?
>>> And in turn if it is unique to the flow, what deletes the queue if
>>> an app does not do a proper cleanup? If the queue sticks around,
>>> the dmabuf references stick around.
>>
>> Let's start sketching out the design for queue config.
>> Without sliding into scope creep, hopefully.
>>
>> Step one - I think we can decompose the problem into:
>>  A) flow steering
>>  B) object lifetime and permissions
>>  C) queue configuration (incl. potentially creating / destroying queues)
>>
>> These come together into use scenarios like:
>>  #1 - partitioning for containers - when high perf containers share
>>       a machine each should get an RSS context on the physical NIC
>>       to have predictable traffic<>CPU placement, they may also have
>>       different preferences on how the queues are configured, maybe
>>       XDP, too?

subfunctions are a more effective and simpler solution for containers, no?

>>  #2 - fancy page pools within the host (e.g. huge pages)
>>  #3 - very fancy page pools not within the host (Mina's work)
>>  #4 - XDP redirect target (allowing XDP_REDIRECT without installing XDP
>>       on the target)
>>  #5 - busy polling - admittedly a bit theoretical, I don't know of
>>       anyone busy polling in real life, but one of the problems today
>>       is that setting it up requires scraping random bits of info from
>>       sysfs and a lot of hoping.
>>
>> Flow steering (A) is there today, to a sufficient extent, I think,
>> so we can defer on that. Sooner or later we should probably figure
>> out if we want to continue down the unruly path of TC offloads or
>> just give up and beef up ethtool.

Flow steering to TC offloads -- more details on what you were thinking here?

>>
>> I don't have a good sense of what a good model for cleanup and
>> permissions is (B). All I know is that if we need to tie things to
>> processes netlink can do it, and we shouldn't have to create our
>> own FS and special file descriptors...

From my perspective the main sticking point that has not been handled is
flushing buffers from the RxQ, but there is 100% tied to queue
management and a process' ability to effect a flush or queue tear down -
and that is the focus of your list below:

>>
>> And then there's (C) which is the main part to talk about.
>> The first step IMHO is to straighten out the configuration process.
>> Currently we do:
>>
>>  user -> thin ethtool API --------------------> driver
>>                               netdev core <---'
>>
>> By "straighten" I mean more of a:
>>
>>  user -> thin ethtool API ---> netdev core ---> driver
>>
>> flow. This means core maintains the full expected configuration,
>> queue count and their parameters and driver creates those queues
>> as instructed.
>>
>> I'd imagine we'd need 4 basic ops:
>>  - queue_mem_alloc(dev, cfg) -> queue_mem
>>  - queue_mem_free(dev, cfg, queue_mem)
>>  - queue_start(dev, queue info, cfg, queue_mem) -> errno
>>  - queue_stop(dev, queue info, cfg)
>>
>> The mem_alloc/mem_free takes care of the commonly missed requirement to
>> not take the datapath down until resources are allocated for new config.

sounds reasonable.

>>
>> Core then sets all the queues up after ndo_open, and tears down before
>> ndo_stop. In case of an ethtool -L / -G call or enabling / disabling XDP
>> core can handle the entire reconfiguration dance.

`ethtool -L/-G` and `ip link set {up/down}` pertain to the "general OS"
queues managed by a driver for generic workloads and networking
management (e.g., neigh discovery, icmp, etc). The discussions here
pertains to processes wanting to use their own memory or GPU memory in a
queue. Processes will come and go and the queue management needs to
align with that need without affecting all of the other queues managed
by the driver.


>>
>> The cfg object needs to contain all queue configuration, including 
>> the page pool parameters.
>>
>> If we have an abstract model of the configuration in the core we can
>> modify it much more easily, I hope. I mean - the configuration will be
>> somewhat detached from what's instantiated in the drivers.
>>
>> I'd prefer to go as far as we can without introducing a driver callback
>> to "check if it can support a config change", and try to rely on
>> (static) capabilities instead. This allows more of the validation to
>> happen in the core and also lends itself naturally to exporting the
>> capabilities to the user.
>>
>> Checking the use cases:
>>
>>  #1 - partitioning for containers - storing the cfg in the core gives
>>       us a neat ability to allow users to set the configuration on RSS
>>       context
>>  #2, #3 - page pools - we can make page_pool_create take cfg and read whatever
>>       params we want from there, memory provider, descriptor count, recycling
>>       ring size etc. Also for header-data-split we may want different settings
>>       per queue so again cfg comes in handy
>>  #4 - XDP redirect target - we should spawn XDP TX queues independently from
>>       the XDP configuration
>>
>> That's all I have thought up in terms of direction.
>> Does that make sense? What are the main gaps? Other proposals?
> 
> More on (A) and (B):
> 
> I expect most use cases match the containerization that you mention.
> Where a privileged process handles configuration.
> 
> For that, the existing interfaces of ethtool -G/-L-/N/-K/-X suffice.
> 
> A more far-out approach could infer the ntuple 5-tuple connection or
> 3-tuple listener rule from a socket itself, no ethtool required. But
> let's ignore that for now.
> 
> Currently we need to use ethtool -X to restrict the RSS indirection
> table to a subset of queues. It is not strictly necessary to
> reconfigure the device on each new container, if pre-allocation a
> sufficient set of non-RSS queues.

This is an interesting approach: This scheme here is along the lines of
you have N cpus in the server, so N queue sets (or channels). The
indirection table means M queue sets are used for RSS leaving N-M queues
for flows with "fancy memory providers". Such a model can work but it is
quite passive, needs careful orchestration and has a lot of moving,
disjointed pieces - with some race conditions around setup vs first data
packet arriving.

I was thinking about a more generic design where H/W queues are created
and destroyed - e.g., queues unique to a process which makes the cleanup
so much easier.

> 
> Then only ethtool -N is needed to drive data towards one of the
> non-RSS queues. Or one of the non context 0 RSS contexts if that is
> used.
> 
> The main part that is missing is memory allocation. Memory is stranded
> on unused queues, and there is no explicit support for special
> allocators.
> 
> A poor man's solution might be to load a ring with minimal sized
> buffers (assuming devices accept that, say a zero length buffer),
> attach a memory provider before inserting an ntuple rule, and refill
> from the memory provider. This requires accepting that a whole ring of
> packets is lost before refilled slots get filled..
> 
> (I'm messing with that with AF_XDP right now: a process that xsk_binds
>  before filling the FILL queue..)
> 
> Ideally, we would have a way to reconfigure a single queue, without
> having to down/up the entire device..
> 
> I don't know if the kernel needs an explicit abstract model, or can
> leave that to the userspace privileged daemon that presses the ethtool
> buttons.

The kernel has that in the IB verbs S/W APIs. Yes, I realize that
comment amounts to profanity on this mailing list, but it should not be.
There are existing APIs for creating, managing and destroying queues -
open source, GPL'ed, *software* APIs that are open for all to use.

That said, I have no religion here. If the netdev stack wants new APIs
to manage queues - including supplying buffers - drivers will have APIs
that can be adapted to some new ndo to create, configure, and destroy
queues. The ethtool API can be updated to manage that. Ultimately I
believe anything short of dynamic queue management will be a band-aid
approach that will have a lot of problems.