netdev - Re: [PATCH 2/6 bpf-next] Clean up xsk reuseq API

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Date:   Mon, 1 Jul 2019 11:58:12 +0200
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Network Development <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        kernel-team@...com
Subject: Re: [PATCH 2/6 bpf-next] Clean up xsk reuseq API

On Fri, Jun 28, 2019 at 11:09 PM Jonathan Lemon
<jonathan.lemon@...il.com> wrote:
>
>
>
> On 28 Jun 2019, at 13:41, Jakub Kicinski wrote:
>
> > On Thu, 27 Jun 2019 19:31:26 -0700, Jonathan Lemon wrote:
> >> On 27 Jun 2019, at 15:38, Jakub Kicinski wrote:
> >>
> >>> On Thu, 27 Jun 2019 15:08:32 -0700, Jonathan Lemon wrote:
> >>>> The reuseq is actually a recycle stack, only accessed from the kernel
> >>>> side.
> >>>> Also, the implementation details of the stack should belong to the
> >>>> umem
> >>>> object, and not exposed to the caller.
> >>>>
> >>>> Clean up and rename for consistency in preparation for the next
> >>>> patch.
> >>>>
> >>>> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> >>>
> >>> Prepare/swap is to cater to how drivers should be written - being able
> >>> to allocate resources independently of those currently used.  Allowing
> >>> for changing ring sizes and counts on the fly.  This patch makes it
> >>> harder to write drivers in the way we are encouraging people to.
> >>>
> >>> IOW no, please don't do this.
> >>
> >> The main reason I rewrote this was to provide the same type
> >> of functionality as realloc() - no need to allocate/initialize a new
> >> array if the old one would still end up being used.  This would seem
> >> to be a win for the typical case of having the interface go up/down.
> >>
> >> Perhaps I should have named the function differently?
> >
> > Perhaps add a helper which calls both parts to help poorly architected
> > drivers?
>
> Still ends up taking more memory.
>
> There are only 3 drivers in the tree which do AF_XDP: i40e, ixgbe, and mlx5.
>
> All of these do the same thing:
>     reuseq = xsk_reuseq_prepare(n)
>     if (!reuseq)
>        error
>     xsk_reuseq_free(xsk_reuseq_swap(umem, reuseq));
>
> I figured simplifying was a good thing.
>
> But I do take your point that some future driver might want to allocate
> everything up front before performing a commit of the resources.

Jonathan, can you come up with a solution that satisfies both these
goals: providing a lower level API that Jakub can and would like to
use for his driver and a higher level helper that can be used by
today's driver to make the AF_XDP part smaller and easier to
implement? I like the fact that you are simplifying the AF_XDP enabled
drivers that are out there today, but at the same time I do not want
to hinder Jakub from, hopefully, in the future upstreaming his
support.

Thanks: Magnus

> --
> Jonathan