netdev - Re: [PATCH net-next 03/13] devlink: Support add and delete devlink port

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Date:   Wed, 18 Nov 2020 12:36:26 -0700
From:   David Ahern <dsahern@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Parav Pandit <parav@...dia.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Jiri Pirko <jiri@...dia.com>,
        "dledford@...hat.com" <dledford@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Vu Pham <vuhuong@...dia.com>
Subject: Re: [PATCH net-next 03/13] devlink: Support add and delete devlink
 port

On 11/18/20 11:38 AM, Jason Gunthorpe wrote:
> On Wed, Nov 18, 2020 at 11:03:24AM -0700, David Ahern wrote:
> 
>> With Connectx-4 Lx for example the netdev can have at most 63 queues
> 
> What netdev calls a queue is really a "can the device deliver
> interrupts and packets to a given per-CPU queue" and covers a whole
> spectrum of smaller limits like RSS scheme, # of available interrupts,
> ability of the device to create queues, etc.
> 
> CX4Lx can create a huge number of queues, but hits one of these limits
> that mean netdev's specific usage can't scale up. Other stuff like
> RDMA doesn't have the same limits, and has tonnes of queues.
> 
> What seems to be needed is a resource controller concept like cgroup
> has for processes. The system is really organized into a tree:
> 
>            physical device
>               mlx5_core
>         /      |      \      \                        (aux bus)
>      netdev   rdma    vdpa   SF  etc
>                              |                        (aux bus)
>                            mlx5_core
>                           /      \                    (aux bus)
>                        netdev   vdpa
> 
> And it does make a lot of sense to start to talk about limits at each
> tree level.
> 
> eg the top of the tree may have 128 physical interrupts. With 128 CPU
> cores that isn't enough interrupts to support all of those things
> concurrently.
> 
> So the user may want to configure:
>  - The first level netdev only gets 64,
>  - 3rd level mlx5_core gets 32 
>  - Final level vdpa gets 8
> 
> Other stuff has to fight it out with the remaining shared interrupts.
> 
> In netdev land # of interrupts governs # of queues
> 
> For RDMA # of interrupts limits the CPU affinities for queues
> 
> VPDA limits the # of VMs that can use VT-d
> 
> The same story repeats for other less general resources, mlx5 also
> has consumption of limited BAR space, and consumption of some limited
> memory elements. These numbers are much bigger and may not need
> explicit governing, but the general concept holds.
> 
> It would be very nice if the limit could be injected when the aux
> device is created but before the driver is bound. I'm not sure how to
> manage that though..
> 
> I assume other devices will be different, maybe some devices have a
> limit on the number of total queues, or a limit on the number of
> VDPA or RDMA devices.
> 
> Jason
> 

A lot of low level resource details that need to be summarized into a
nicer user / config perspective to specify limits / allocations.

Thanks for the detailed response.