netdev - Re: [RFC v1 00/22] Large rx buffer support for zcrx

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Message-ID: <aIj3wEHU251DXu18@mini-arch>
Date: Tue, 29 Jul 2025 09:33:04 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
	io-uring@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, andrew+netdev@...n.ch,
	horms@...nel.org, davem@...emloft.net, sdf@...ichev.me,
	almasrymina@...gle.com, dw@...idwei.uk, michael.chan@...adcom.com,
	dtatulea@...dia.com, ap420073@...il.com
Subject: Re: [RFC v1 00/22] Large rx buffer support for zcrx

On 07/28, Pavel Begunkov wrote:
> On 7/28/25 23:06, Stanislav Fomichev wrote:
> > On 07/28, Pavel Begunkov wrote:
> > > On 7/28/25 21:21, Stanislav Fomichev wrote:
> > > > On 07/28, Pavel Begunkov wrote:
> > > > > On 7/28/25 18:13, Stanislav Fomichev wrote:
> > > ...>>> Supporting big buffers is the right direction, but I have the same
> > > > > > feedback:
> > > > > 
> > > > > Let me actually check the feedback for the queue config RFC...
> > > > > 
> > > > > it would be nice to fit a cohesive story for the devmem as well.
> > > > > 
> > > > > Only the last patch is zcrx specific, the rest is agnostic,
> > > > > devmem can absolutely reuse that. I don't think there are any
> > > > > issues wiring up devmem?
> > > > 
> > > > Right, but the patch number 2 exposes per-queue rx-buf-len which
> > > > I'm not sure is the right fit for devmem, see below. If all you
> > > 
> > > I guess you're talking about uapi setting it, because as an
> > > internal per queue parameter IMHO it does make sense for devmem.
> > > 
> > > > care is exposing it via io_uring, maybe don't expose it from netlink for
> > > 
> > > Sure, I can remove the set operation.
> > > 
> > > > now? Although I'm not sure I understand why you're also passing
> > > > this per-queue value via io_uring. Can you not inherit it from the
> > > > queue config?
> > > 
> > > It's not a great option. It complicates user space with netlink.
> > > And there are convenience configuration features in the future
> > > that requires io_uring to parse memory first. E.g. instead of
> > > user specifying a particular size, it can say "choose the largest
> > > length under 32K that the backing memory allows".
> > 
> > Don't you already need a bunch of netlink to setup rss and flow
> 
> Could be needed, but there are cases where configuration and
> virtual queue selection is done outside the program. I'll need
> to ask which option we currently use.

If the setup is done outside, you can also setup rx-buf-len outside, no?

> > steering? And if we end up adding queue api, you'll have to call that
> > one over netlink also.
> 
> There is already a queue api, even though it's cropped IIUC.
> What kind of extra setup you have in mind?

I'm talking about allocating the queues. Currently the zc/devmem setup is
a bit complicated, we need to partition the queues and rss+flow
steer into a subset of zerocopy ones. In the future we might add some apis
to request a new dedicated queue for the specific flow(s). That should
hopefully simplify the design (and make the cleanup of the queues more
robust if the application dies).

> > > > If we assume that at some point niov can be backed up by chunks larger
> > > > than PAGE_SIZE, the assumed workflow for devemem is:
> > > > 1. change rx-buf-len to 32K
> > > >     - this is needed only for devmem, but not for CPU RAM, but we'll have
> > > >       to refill the queues from the main memory anyway
> > > 
> > > Urgh, that's another reason why I prefer to just pass it through
> > > zcrx and not netlink. So maybe you can just pass the len to devmem
> > > on creation, and internally it sets up its queues with it.
> > 
> > But you still need to solve MAX_PAGE_ORDER/PAGE_ALLOC_COSTLY_ORDER I
> > think? We don't want the drivers to do PAGE_ALLOC_COSTLY_ORDER costly
> > allocation presumably?
> 
> #define PAGE_ALLOC_COSTLY_ORDER 3
> 
> It's "costly" for the page allocator and not a custom specially
> cooked memory providers. Nobody should care as long as the length
> applies to the given provider only. MAX_PAGE_ORDER also seems to
> be a page allocator thing.

By custom memory providers you mean page pool? Thinking about it more,
maybe it's fine as is as long as we have ndo_queue_cfg_validate that
enforces sensible ranges..