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

Mina Almasry
2026-01-22T04:21:50Z

On Tue, Jan 20, 2026 at 5:07 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Thu, 15 Jan 2026 21:02:11 -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).
>
> IDK if the cmsg approach is still right for this flow TBH.
> IIRC when Stan talked about this a while back we were considering doing
> this via Netlink. Anything that proves that the user owns the binding
> would work. IIUC the TCP socket in this design just proves that socket
> has received a token from a given binding right?

Doesn't 'doing this via netlink' imply it's a control path operation
that acquires rtnl_lock or netdev_lock or some heavy lock expecting
you to do some config change? Returning tokens is a data-path
operation, IIRC we don't even lock the socket to do it in the
setsockopt.

Is there precedent/path to doing fast data-path operations via netlink?

There may be value in not biting more than we can chew in one series.
Maybe an alternative non-setsockopt dontneeding scheme should be its
own patch series.

--
Thanks,
Mina