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

Bobby Eshleman
2026-01-16T06:54:38Z

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).

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>

Changes in v10:
- add new tests for edge cases
- add new binding->users to binding for tracking socket/rxq users
- remove rx binding count (use xarray instead)
- Link to v9: https://lore.kernel.org/r/20260109-scratch-bobbyeshleman-devmem-tcp-token-upstream-v9-0-8042930d00d7@meta.com

Changes in v9:
- fixed build with NET_DEVMEM=n
- fixed bug in rx bindings count logic
- Link to v8: https://lore.kernel.org/r/20260107-scratch-bobbyeshleman-devmem-tcp-token-upstream-v8-0-92c968631496@meta.com

Changes in v8:
- change static branch logic (only set when enabled, otherwise just
always revert back to disabled)
- fix missing tests
- Link to v7: https://lore.kernel.org/r/20251119-scratch-bobbyeshleman-devmem-tcp-token-upstream-v7-0-1abc8467354c@meta.com

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

Changes in v6:
- renamed 'net: devmem: use niov array for token management' to refer to
optionality of new config
- added documentation and tests
- make autorelease flag per-socket sockopt instead of binding
field / sysctl
- many per-patch changes (see Changes sections per-patch)
- Link to v5: https://lore.kernel.org/r/20251023-scratch-bobbyeshleman-devmem-tcp-token-upstream-v5-0-47cb85f5259e@meta.com

Changes in v5:
- add sysctl to opt-out of performance benefit, back to old token release
- Link to v4: https://lore.kernel.org/all/20250926-scratch-bobbyeshleman-devmem-tcp-token-upstream-v4-0-39156563c3ea@meta.com

Changes in v4:
- rebase to net-next
- Link to v3: https://lore.kernel.org/r/20250926-scratch-bobbyeshleman-devmem-tcp-token-upstream-v3-0-084b46bda88f@meta.com

Changes in v3:
- make urefs per-binding instead of per-socket, reducing memory
footprint
- fallback to cleaning up references in dmabuf unbind if socket
leaked tokens
- drop ethtool patch
- Link to v2: https://lore.kernel.org/r/20250911-scratch-bobbyeshleman-devmem-tcp-token-upstream-v2-0-c80d735bd453@meta.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 (5):
net: devmem: rename tx_vec to vec in dmabuf binding
net: devmem: refactor sock_devmem_dontneed for autorelease split
net: devmem: implement autorelease token management
net: devmem: document NETDEV_A_DMABUF_AUTORELEASE netlink attribute
selftests: drv-net: devmem: add autorelease tests

Documentation/netlink/specs/netdev.yaml | 12 ++
Documentation/networking/devmem.rst | 73 +++++++++++
include/net/netmem.h | 1 +
include/net/sock.h | 7 +-
include/uapi/linux/netdev.h | 1 +
net/core/devmem.c | 148 ++++++++++++++++++----
net/core/devmem.h | 66 +++++++++-
net/core/netdev-genl-gen.c | 5 +-
net/core/netdev-genl.c | 10 +-
net/core/sock.c | 103 +++++++++++----
net/ipv4/tcp.c | 87 ++++++++++---
net/ipv4/tcp_ipv4.c | 15 ++-
net/ipv4/tcp_minisocks.c | 3 +-
tools/include/uapi/linux/netdev.h | 1 +
tools/testing/selftests/drivers/net/hw/devmem.py | 98 +++++++++++++-
tools/testing/selftests/drivers/net/hw/ncdevmem.c | 68 +++++++++-
16 files changed, 611 insertions(+), 87 deletions(-)
---
base-commit: d4596891e72cbf155d61798a81ce9d36b69bfaf4
change-id: 20250829-scratch-bobbyeshleman-devmem-tcp-token-upstream-292be174d503

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