linux-kernel - Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for custom page providers

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Date: Fri, 7 Jun 2024 09:59:16 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: David Ahern <dsahern@...nel.org>, Christoph Hellwig <hch@...radead.org>, netdev@...r.kernel.org, 
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	Praveen Kaligineedi <pkaligineedi@...gle.com>
Subject: Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for
 custom page providers

On Fri, Jun 7, 2024 at 8:47 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 6/7/24 16:42, Pavel Begunkov wrote:
> > On 6/7/24 15:27, David Ahern wrote:
> >> On 6/7/24 7:42 AM, Pavel Begunkov wrote:
> >>> I haven't seen any arguments against from the (net) maintainers so
> >>> far. Nor I see any objection against callbacks from them (considering
> >>> that either option adds an if).
> >>
> >> I have said before I do not understand why the dmabuf paradigm is not
> >> sufficient for both device memory and host memory. A less than ideal
> >> control path to put hostmem in a dmabuf wrapper vs extra checks and
> >> changes in the datapath. The former should always be preferred.
> >
> > If we're talking about types of memory specifically, I'm not strictly
> > against wrapping into dmabuf in kernel, but that just doesn't give
> > anything.
>
> And the reason I don't have too strong of an opinion on that is
> mainly because it's just setup/cleanup path.
>

I agree wrapping io uring in dmabuf seems to be an unnecessary detour.
I never understood the need or upside to do that, but it could be a
lack of understanding on my part.

However, the concern that David brings up may materialize. I've had to
spend a lot of time minimizing or justifying checks to the code with
page pool benchmarks that detect even 1 cycle regressions. You may be
asked to run the same benchmarks and minimize similar overhead.

The benchmark in question is Jesper's bench_page_pool_simple. I've
forked it and applied it on top of net-next here:
https://github.com/mina/linux/commit/927596f87ab5791a8a6ba8597ba2189747396e54

As io_uring ZC comes close to merging, I suspect it would be good to
run this to understand the regression in the fast path, if any. If
there are no to little regressions, I have no concerns over io uring
memory not being wrapped in dmabufs, and David may agree as well.

--
Thanks,
Mina