On Wed, Jul 1, 2026 at 12:22 PM Bobby Eshleman bobbyeshleman@gmail.com 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_BUF_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 +/- 3.27RX 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 +/- 3.56Throughput 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/8837577f920876bce6986ec...
Signed-off-by: Bobby Eshleman bobbyeshleman@meta.com Acked-by: Stanislav Fomichev sdf@fomichev.me
I'm pretty happy to see most of this patch being a spot-for-spot replacement of PAGE_SIZE with a variable. FWIW:
Reviewed-by: Mina Almasry almasrymina@google.com
Documentation/netlink/specs/netdev.yaml | 8 +++++ include/uapi/linux/netdev.h | 1 + net/core/devmem.c | 55 +++++++++++++++++++-------------- net/core/devmem.h | 13 +++++--- net/core/netdev-genl-gen.c | 5 +-- net/core/netdev-genl.c | 19 ++++++++++-- tools/include/uapi/linux/netdev.h | 1 + 7 files changed, 71 insertions(+), 31 deletions(-)
diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml index 5f143da7458c..70b902008bd3 100644 --- a/Documentation/netlink/specs/netdev.yaml +++ b/Documentation/netlink/specs/netdev.yaml @@ -598,6 +598,13 @@ attribute-sets: type: u32 checks: min: 1
-name: rx-buf-sizedoc: |Size in bytes of each RX buffer the NIC writes into from the bounddmabuf. Must be a power of two and >= PAGE_SIZE; defaults toPAGE_SIZE.type: u32operations: list: @@ -812,6 +819,7 @@ operations: - ifindex - fd - queues
- rx-buf-size reply: attributes: - iddiff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h index 2f3ab75e8cc0..85e1d20c6268 100644 --- a/include/uapi/linux/netdev.h +++ b/include/uapi/linux/netdev.h @@ -219,6 +219,7 @@ enum { NETDEV_A_DMABUF_QUEUES, NETDEV_A_DMABUF_FD, NETDEV_A_DMABUF_ID,
NETDEV_A_DMABUF_RX_BUF_SIZE, __NETDEV_A_DMABUF_MAX, NETDEV_A_DMABUF_MAX = (__NETDEV_A_DMABUF_MAX - 1)diff --git a/net/core/devmem.c b/net/core/devmem.c index 957d6b96216b..3d6cf35e50f3 100644 --- a/net/core/devmem.c +++ b/net/core/devmem.c @@ -46,7 +46,7 @@ static dma_addr_t net_devmem_get_dma_addr(const struct net_iov *niov)
owner = net_devmem_iov_to_chunk_owner(niov); return owner->base_dma_addr +
((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT);
((dma_addr_t)net_iov_idx(niov) << owner->binding->niov_shift);}
static void net_devmem_dmabuf_binding_release(struct percpu_ref *ref) @@ -90,16 +90,17 @@ net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding) struct dmabuf_genpool_chunk_owner *owner; unsigned long dma_addr; struct net_iov *niov;
ssize_t offset;ssize_t index;
size_t offset;size_t index;
nit: I would keep this signed. Some of the most frustrating issues I ran into is some of the underflowing and then passing a > check or something. Although if the LLM is not complaining about this particular case, there is probably no issue with it. I also notice a lot of existing code that deals with indexes and offsets goes for signed.