linux-kernel - [PATCH net-next v2 0/3] net: devmem: improve cpu cost of RX token management

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Message-Id: <20250911-scratch-bobbyeshleman-devmem-tcp-token-upstream-v2-0-c80d735bd453@meta.com>
Date: Thu, 11 Sep 2025 22:28:14 -0700
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>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Stanislav Fomichev <sdf@...ichev.me>, Mina Almasry <almasrymina@...gle.com>, 
 Bobby Eshleman <bobbyeshleman@...a.com>
Subject: [PATCH net-next v2 0/3] net: devmem: improve cpu cost of RX token
 management

This series improves the CPU cost of RX token management by replacing
the xarray allocator with a normal array of atomics. Similar to devmem
TX's page-index lookup scheme for niovs, RX also uses page indices to
lookup the corresponding atomic in the array.

Improvement is ~5% per RX user thread.

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.

Signed-off-by: Bobby Eshleman <bobbyeshleman@...a.com>
---
Changes in v2:
- net: ethtool: prevent user from breaking devmem single-binding rule
  (Mina)
- pre-assign niovs in binding->vec for RX case (Mina)
- remove WARNs on invalid user input (Mina)
- remove extraneous binding ref get (Mina)
- remove WARN for changed binding (Mina)
- always use GFP_ZERO for binding->vec (Mina)
- fix length of alloc for urefs
- use atomic_set(, 0) to initialize sk_user_frags.urefs
- Link to v1: https://lore.kernel.org/r/20250902-scratch-bobbyeshleman-devmem-tcp-token-upstream-v1-0-d946169b5550@meta.com

---
Bobby Eshleman (3):
      net: devmem: rename tx_vec to vec in dmabuf binding
      net: devmem: use niov array for token management
      net: ethtool: prevent user from breaking devmem single-binding rule

 include/net/sock.h       |   6 +-
 net/core/devmem.c        |  29 +++++-----
 net/core/devmem.h        |   4 +-
 net/core/sock.c          |  23 +++++---
 net/ethtool/ioctl.c      | 144 +++++++++++++++++++++++++++++++++++++++++++++++
 net/ipv4/tcp.c           | 120 ++++++++++++++++-----------------------
 net/ipv4/tcp_ipv4.c      |  45 +++++++++++++--
 net/ipv4/tcp_minisocks.c |   2 -
 8 files changed, 266 insertions(+), 107 deletions(-)
---
base-commit: dc2f650f7e6857bf384069c1a56b2937a1ee370d
change-id: 20250829-scratch-bobbyeshleman-devmem-tcp-token-upstream-292be174d503

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@...a.com>