linux-kernel - Re: [PATCH net-next v7 0/5] net: devmem: improve cpu cost of RX token management

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Message-ID: <aTh9/waV23uRZc9E@devvm11784.nha0.facebook.com>
Date: Tue, 9 Dec 2025 11:52:31 -0800
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, 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>,
	Mina Almasry <almasrymina@...gle.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Stanislav Fomichev <sdf@...ichev.me>,
	Bobby Eshleman <bobbyeshleman@...a.com>
Subject: Re: [PATCH net-next v7 0/5] net: devmem: improve cpu cost of RX
 token management

On Wed, Nov 19, 2025 at 07:37:07PM -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).
> 

[...]
> 
> Changes in v7:
> - use netlink instead of sockopt (Stan)
> - restrict system to only one mode, dmabuf bindings can not co-exist
>   with different modes (Stan)
> - use static branching to enforce single system-wide mode (Stan)
> - Link to v6: https://lore.kernel.org/r/20251104-scratch-bobbyeshleman-devmem-tcp-token-upstream-v6-0-ea98cf4d40b3@meta.com
> 

Mina, I was wondering if you had any feedback on this approach?

Best,
Bobby