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

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Date: Mon, 10 Jun 2024 02:12:24 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: David Ahern <dsahern@...nel.org>, Christoph Hellwig <hch@...radead.org>,
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Subject: Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for
 custom page providers

On 6/7/24 17:59, Mina Almasry wrote:
> 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.

That's the easiest part as io_uring only reusing call points
you added for devmem and thus doesn't add anything new on top
to hot paths.

-- 
Pavel Begunkov