netdev - Re: [PATCH net-next v10 0/5] net: devmem: improve cpu cost of RX token management

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Message-ID: <CAHS8izMZxM6rcF+7Lfw=KFv4dmbHGSUrQBPmxO+sYj=V3TRuwQ@mail.gmail.com>
Date: Wed, 21 Jan 2026 20:21:36 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Bobby Eshleman <bobbyeshleman@...il.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Willem de Bruijn <willemb@...gle.com>, 
	Neal Cardwell <ncardwell@...gle.com>, David Ahern <dsahern@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>, Shuah Khan <shuah@...nel.org>, 
	Donald Hunter <donald.hunter@...il.com>, Stanislav Fomichev <sdf@...ichev.me>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	asml.silence@...il.com, matttbe@...nel.org, skhawaja@...gle.com, 
	Bobby Eshleman <bobbyeshleman@...a.com>
Subject: Re: [PATCH net-next v10 0/5] net: devmem: improve cpu cost of RX
 token management

On Tue, Jan 20, 2026 at 5:07 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 15 Jan 2026 21:02:11 -0800 Bobby Eshleman wrote:
> > This series improves the CPU cost of RX token management by adding an
> > attribute to NETDEV_CMD_BIND_RX that configures sockets using the
> > binding to avoid the xarray allocator and instead use a per-binding niov
> > array and a uref field in niov.
> >
> > Improvement is ~13% cpu util per RX user thread.
> >
> > Using kperf, the following results were observed:
> >
> > Before:
> >       Average RX worker idle %: 13.13, flows 4, test runs 11
> > After:
> >       Average RX worker idle %: 26.32, flows 4, test runs 11
> >
> > Two other approaches were tested, but with no improvement. Namely, 1)
> > using a hashmap for tokens and 2) keeping an xarray of atomic counters
> > but using RCU so that the hotpath could be mostly lockless. Neither of
> > these approaches proved better than the simple array in terms of CPU.
> >
> > The attribute NETDEV_A_DMABUF_AUTORELEASE is added to toggle the
> > optimization. It is an optional attribute and defaults to 0 (i.e.,
> > optimization on).
>
> IDK if the cmsg approach is still right for this flow TBH.
> IIRC when Stan talked about this a while back we were considering doing
> this via Netlink. Anything that proves that the user owns the binding
> would work. IIUC the TCP socket in this design just proves that socket
> has received a token from a given binding right?

Doesn't 'doing this via netlink' imply it's a control path operation
that acquires rtnl_lock or netdev_lock or some heavy lock expecting
you to do some config change? Returning tokens is a data-path
operation, IIRC we don't even lock the socket to do it in the
setsockopt.

Is there precedent/path to doing fast data-path operations via netlink?

There may be value in not biting more than we can chew in one series.
Maybe an alternative non-setsockopt dontneeding scheme should be its
own patch series.

-- 
Thanks,
Mina