Re: [PATCH net-next v6 1/3] net: devmem: allow rx-page-size > PAGE_SIZE per dmabuf binding

[Nikolay Aleksandrov <razor@blackwall.org>]

On Fri, Jul 24, 2026 at 02:21:15PM -0700, Bobby Eshleman wrote:
> From: Bobby Eshleman <bobbyeshleman@meta.com>
> 
> Every devmem dmabuf binding today hands the page_pool PAGE_SIZE niovs.
> This caps a single RX descriptor at PAGE_SIZE, burning CPU on buffer
> churn for large flows.
> 
> Add a bind-time netlink attribute, NETDEV_A_DMABUF_RX_PAGE_SIZE, that
> lets userspace request a larger niov size. The value must be a power of
> two >= PAGE_SIZE.
> 
> Measurements:
> Setup: kperf in devmem RX/TX cuda mode, 4 flows, 64 MB messages, 60s,
> dctcp, num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov
> size, mlx5.
> 
> CPU Util:
> 
>    niov        net sirq %        net idle %         app sys %        app idle %
>   -----  ----------------  ----------------  ----------------  ----------------
>      4K   62.38 +/-  8.27   33.40 +/-  7.51   54.15 +/- 10.23   43.67 +/- 10.53
>     16K   58.91 +/-  5.35   35.23 +/-  5.88   41.05 +/-  8.87   56.42 +/-  9.24
>     32K   64.12 +/-  0.68   31.09 +/-  1.48   44.54 +/-  3.51   52.63 +/-  3.65
>     64K   54.69 +/-  5.54   39.67 +/-  5.81   35.47 +/-  3.11   61.97 +/-  3.27
> 
> RX app sys % drops ~19% from 4K to 64K.
> 
> Throughput:
> 
>    niov       RX dev Gbps   RX flow avg Gbps
>   -----  ----------------  -----------------
>      4K  300.63 +/- 53.21    75.16 +/- 13.30
>     16K  321.35 +/- 28.20    80.34 +/-  7.05
>     32K  347.63 +/-  2.20    86.91 +/-  0.55
>     64K  332.11 +/- 14.26    83.03 +/-  3.56
> 
> Throughput seems to increase, but the stdev is pretty wide so could just
> be noise.
> 
> kperf support (not yet merged):
> https://github.com/facebookexperimental/kperf/commit/8837577f920876bce6986ec18869ac04439ebcd2
> 
> Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
> Acked-by: Stanislav Fomichev <sdf@fomichev.me>
> Reviewed-by: Mina Almasry <almasrymina@google.com>
> ---
>  Documentation/netlink/specs/netdev.yaml | 17 ++++++++++++
>  include/uapi/linux/netdev.h             |  1 +
>  net/core/devmem.c                       | 49 +++++++++++++++++++--------------
>  net/core/devmem.h                       | 13 ++++++---
>  net/core/netdev-genl-gen.c              | 11 ++++++--
>  net/core/netdev-genl-gen.h              |  1 +
>  net/core/netdev-genl.c                  | 18 ++++++++++--
>  tools/include/uapi/linux/netdev.h       |  1 +
>  8 files changed, 83 insertions(+), 28 deletions(-)
> 

LGTM,
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>

Cheers,
 Nik