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

Bobby Eshleman
2026-01-26T18:45:24Z

On Wed, Jan 21, 2026 at 08:21:36PM -0800, Mina Almasry wrote:
> 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.
>

I'm onboard with improving what we have since it helps all of us
currently using this API, though I'm not opposed to discussing a
redesign in another thread/RFC. I do see the attraction to locating the
core logic in one place and possibly reducing some complexity around
socket/binding relationships.

FWIW regarding nl, I do see it supports rtnl lock-free operations via
'62256f98f244 rtnetlink: add RTNL_FLAG_DOIT_UNLOCKED' and routing was
recently made lockless with that. I don't see / know of any fast path
precedent. I'm aware there are some things I'm not sure about being
relevant performance-wise, like hitting skb alloc an additional time
every release batch. I'd want to do some minimal latency comparisons
between that path and sockopt before diving head-first.

Best,
Bobby