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Message-Id: <20251104-scratch-bobbyeshleman-devmem-tcp-token-upstream-v6-5-ea98cf4d40b3@meta.com> Date: Tue, 04 Nov 2025 17:23:24 -0800 From: Bobby Eshleman <bobbyeshleman@...il.com> To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>, Willem de Bruijn <willemb@...gle.com>, Neal Cardwell <ncardwell@...gle.com>, David Ahern <dsahern@...nel.org>, Arnd Bergmann <arnd@...db.de>, Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>, Shuah Khan <shuah@...nel.org>, Mina Almasry <almasrymina@...gle.com> Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, Stanislav Fomichev <sdf@...ichev.me>, Bobby Eshleman <bobbyeshleman@...a.com> Subject: [PATCH net-next v6 5/6] net: devmem: document SO_DEVMEM_AUTORELEASE socket option From: Bobby Eshleman <bobbyeshleman@...a.com> Update devmem.rst documentation to describe the new SO_DEVMEM_AUTORELEASE socket option and its usage. Document the following: - The two token release modes (automatic vs manual) - How to use SO_DEVMEM_AUTORELEASE to control the behavior - Performance benefits of disabling autorelease (~10% CPU reduction) - Restrictions and caveats of manual token release - Usage examples for both getsockopt and setsockopt Signed-off-by: Bobby Eshleman <bobbyeshleman@...a.com> --- Documentation/networking/devmem.rst | 70 +++++++++++++++++++++++++++++++++++-- 1 file changed, 68 insertions(+), 2 deletions(-) diff --git a/Documentation/networking/devmem.rst b/Documentation/networking/devmem.rst index a6cd7236bfbd..1bfce686dce6 100644 --- a/Documentation/networking/devmem.rst +++ b/Documentation/networking/devmem.rst @@ -215,8 +215,8 @@ Freeing frags ------------- Frags received via SCM_DEVMEM_DMABUF are pinned by the kernel while the user -processes the frag. The user must return the frag to the kernel via -SO_DEVMEM_DONTNEED:: +processes the frag. Users should return tokens to the kernel via +SO_DEVMEM_DONTNEED when they are done processing the data:: ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_DONTNEED, &token, sizeof(token)); @@ -235,6 +235,72 @@ can be less than the tokens provided by the user in case of: (a) an internal kernel leak bug. (b) the user passed more than 1024 frags. + +Autorelease Control +~~~~~~~~~~~~~~~~~~~ + +The SO_DEVMEM_AUTORELEASE socket option controls what happens to outstanding +tokens (tokens not released via SO_DEVMEM_DONTNEED) when the socket closes:: + + int autorelease = 0; /* 0 = manual release, 1 = automatic release */ + ret = setsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_AUTORELEASE, + &autorelease, sizeof(autorelease)); + + /* Query current setting */ + int current_val; + socklen_t len = sizeof(current_val); + ret = getsockopt(client_fd, SOL_SOCKET, SO_DEVMEM_AUTORELEASE, + ¤t_val, &len); + +When autorelease is disabled (default): + +- Outstanding tokens are NOT released when the socket closes +- Outstanding tokens are only released when the dmabuf is unbound +- Provides better performance by eliminating xarray overhead (~10% CPU reduction) +- Kernel tracks tokens via atomic reference counters in net_iov structures + +When autorelease is enabled: + +- Outstanding tokens are automatically released when the socket closes +- Backwards compatible behavior +- Kernel tracks tokens in an xarray per socket + +Important: In both modes, applications should call SO_DEVMEM_DONTNEED to +return tokens as soon as they are done processing. The autorelease setting only +affects what happens to tokens that are still outstanding when close() is called. + +The autorelease setting can only be changed when the socket has no outstanding +tokens. If tokens are present, setsockopt returns -EBUSY. + + +Performance Considerations +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Disabling autorelease provides approximately ~10% CPU utilization improvement in +RX workloads by: + +- Eliminating xarray allocations and lookups for token tracking +- Using atomic reference counters instead +- Reducing lock contention on the xarray spinlock + +However, applications must ensure all tokens are released via +SO_DEVMEM_DONTNEED before closing the socket, otherwise the backing pages will +remain pinned until the dmabuf is unbound. + + +Caveats +~~~~~~~ + +- With autorelease disabled, sockets cannot switch between different dmabuf + bindings. This restriction exists because tokens in this mode do not encode + the binding information necessary to perform the token release. + +- Applications using manual release mode (autorelease=0) must ensure all tokens + are returned via SO_DEVMEM_DONTNEED before socket close to avoid resource + leaks during the lifetime of the dmabuf binding. Tokens not released before + close() will only be freed when the dmabuf is unbound. + + TX Interface ============ -- 2.47.3
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