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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

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?
>  #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.
> 
> 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...
> 
> 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.
> 
> 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.
> 
> 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.

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.