netdev - Re: Upstream Homa?

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Date:   Sat, 12 Nov 2022 22:09:48 -0800
From:   John Ousterhout <ouster@...stanford.edu>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org
Subject: Re: Upstream Homa?

On Fri, Nov 11, 2022 at 11:25 AM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Fri, Nov 11, 2022 at 10:59:58AM -0800, John Ousterhout wrote:
> > The netlink and 32-bit kernel issues are new for me; I've done some digging to
> > learn more, but still have some questions.
> >
>
> > * Is the intent that netlink replaces *all* uses of /proc and ioctl? Homa
> > currently uses ioctls on sockets for I/O (its APIs aren't sockets-compatible).
> > It looks like switching to netlink would double the number of system calls that
> > have to be invoked, which would be unfortunate given Homa's goal of getting the
> > lowest possible latency. It also looks like netlink might be awkward for
> > dumping large volumes of kernel data to user space (potential for buffer
> > overflow?).
>
> I've not looked at the actually code, i'm making general comments.
>
> netlink, like ioctl, is meant for the control plain, not the data
> plain. Your statistics should be reported via netlink, for
> example. netlink is used to configure routes, setup bonding, bridges
> etc. netlink can also dump large volumes of data, it has no problems
> dumping the full Internet routing table for example.
>
> How you get real packet data between the userspace and kernel space is
> a different question. You say it is not BSD socket compatible. But
> maybe there is another existing kernel API which will work? Maybe post
> what your ideal API looks like and why sockets don't work. Eric
> Dumazet could give you some ideas about what the kernel has which
> might do what you need. This is the uAPI point that Stephen raised.

OK, will do. I'm in the middle of a major API refactor, so I'll wait
until that is
resolved before pursing this issue more.

> > * By "32 bit kernel problems" are you referring to the lack of atomic 64-bit
> > operations and using the facilities of u64_stats_sync.h, or is there a more
> > general issue with 64-bit operations?
>
> Those helpers do the real work, and should optimise to pretty much
> nothing on an 64 bit kernel, but do the right thing on 32 bit kernels.
>
> But you are right, the general point is that they are not atomic, so
> you need to be careful with threads, and any access to a 64 bit values
> needs to be protected somehow, hopefully in a way that is optimised
> out on 64bit systems.

Is it acceptable to have features that are only supported on 64-bit kernels?
This would be my first choice, since I don't think there will be much interest
in Homa on 32-bit platforms.

If that's not OK, are there any mechanisms available for helping people
test on 32-bit platforms? For example, is it possible to configure Linux to
compile in 32-bit mode so I could test that even on a 64-bit machine
(I don't have access to a 32-bit machine)?

-John-