netdev - Re: [RFC PATCH] net: introduce HW Rate Limiting Driver API

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Message-ID: <db51b7ccff835dd5a96293fb84d527be081de062.camel@redhat.com>
Date: Fri, 10 May 2024 13:05:41 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>, Jiri Pirko
 <jiri@...nulli.us>, Madhu Chittim <madhu.chittim@...el.com>, Sridhar
 Samudrala <sridhar.samudrala@...el.com>, Simon Horman <horms@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, Sunil Kovvuri Goutham
 <sgoutham@...vell.com>,  Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [RFC PATCH] net: introduce HW Rate Limiting Driver API

On Thu, 2024-05-09 at 18:17 +0200, Andrew Lunn wrote:
> > > Now the question is, how do i get between these two states? It is not
> > > possible to mix WRR and strict priority. Any kAPI which only modifies
> > > one queue at once will go straight into an invalid state, and the
> > > driver will need to return -EOPNOTSUPP. So it seems like there needs
> > > to be an atomic set N queue configuration at once, so i can cleanly go
> > > from strict priority across 8 queues to WRR across 8 queues. Is that
> > > foreseen?
> > 
> > You could delete all the WRR shapers and then create/add SP based ones.
> 
> But that does not match the hardware. I cannot delete the hardware. It
> will either do strict priority or WRR. If i delete the software
> representation of the shaper, the hardware shaper will keep on doing
> what it was doing. So i don't see this as a good model. I think the
> driver will create shapers to represent the hardware, and you are not
> allowed to delete them or add more of them, because that is what the
> hardware is. All you can do is configure the shapers that exist.
> 
> > The 'create' op is just an abstraction to tell the NIC to switch from
> > the default configuration to the specified one.
> 
> Well, the hardware default is i think WRR for the queues, and line
> rate. That will be what the software representation of the shapers
> will be set to when the driver probes and creates the shapers
> representors.

If I read correctly, allowing each NIC to expose it's own different
starting configuration still will not solve the problem for this H/W to
switch from WRR to SP (and vice versa).

AFAICS, what would be needed there is an atomic set of operations:
'set_many' (and e.v. 'delete_many', 'create_many') that will allow
changing all the shapers at once. 

With such operations, that H/W could still fit the expected 'no-op'
default, as WRR on the queue shapers is what we expect. I agree with
Jakub, handling the complexity of arbitrary starting configuration
would pose a lot of trouble to the user/admin.

If all the above stands together, I think we have a few options (in
random order):

- add both set of operations: the ones operating on a single shaper and
the ones operating on multiple shapers
- use only the multiple shapers ops.

And the latter looks IMHO the simple/better. At that point I would
probably drop the 'add' op and would rename 'delete' as
'reset':

int (*set)(struct net_device *dev, int how_many, const u32 *handles,
	   const struct net_shaper_info *shapers,
           struct netlink_ext_ack *extack);
int (*reset)(struct net_device *dev, int how_many, const u32 *handles,
             struct netlink_ext_ack *extack);
int (*move)(struct net_device *dev, int how_many, const u32 *handles,
            const u32 *new_parent_handles,
	    struct netlink_ext_ack *extack);

An NIC with 'static' shapers can implement a dummy move always
returning EOPNOTSUPP and eventually filling a detailed extack.

NIC without any constraints on mixing and matching different kind of
shapers could implement the above as a loop over whatever they will do
for the corresponding 'single shaper op'

NIC with constrains alike the one you pointed out could validate the
final state before atomically applying the specified operation.

After a successful  'reset' operation, the kernel could drop any data
it retains/caches for the relevant shapers - the current idea is to
keep a copy of all successfully configured shaper_info in a xarray,
using the 'handle' as the index.

Side note: the move() operation could look like a complex artifact, but
it's the simplest instrument I could think of to support scenarios
where the user creates/configures/sets a queue group and 'move' some
queue under the newly created group

WDYT?

Thanks,

Paolo