net: Extend NAPI threaded polling to allow kthread based busy polling

Add a new state NAPI_STATE_THREADED_BUSY_POLL to the NAPI state enum to
enable and disable threaded busy polling.

When threaded busy polling is enabled for a NAPI, enable
NAPI_STATE_THREADED also.

When the threaded NAPI is scheduled, set NAPI_STATE_IN_BUSY_POLL to
signal napi_complete_done not to rearm interrupts.

Whenever NAPI_STATE_THREADED_BUSY_POLL is unset, the
NAPI_STATE_IN_BUSY_POLL will be unset, napi_complete_done unsets the
NAPI_STATE_SCHED_THREADED bit also, which in turn will make the kthread
go to sleep.

Signed-off-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Martin Karsten <mkarsten@uwaterloo.ca>
Tested-by: Martin Karsten <mkarsten@uwaterloo.ca>
Link: https://patch.msgid.link/20251028203007.575686-2-skhawaja@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 49 insertions, 1 deletions

diff --git a/Documentation/networking/napi.rst b/Documentation/networking/napi.rst
index 7dd60366f4ff..4e008efebb35 100644
--- a/Documentation/networking/napi.rst
+++ b/Documentation/networking/napi.rst
@@ -263,7 +263,9 @@ are not well known).
 Busy polling is enabled by either setting ``SO_BUSY_POLL`` on
 selected sockets or using the global ``net.core.busy_poll`` and
 ``net.core.busy_read`` sysctls. An io_uring API for NAPI busy polling
-also exists.
+also exists. Threaded polling of NAPI also has a mode to busy poll for
+packets (:ref:`threaded busy polling<threaded_busy_poll>`) using the NAPI
+processing kthread.
 
 epoll-based busy polling
 ------------------------
@@ -426,6 +428,52 @@ Therefore, setting ``gro_flush_timeout`` and ``napi_defer_hard_irqs`` is
 the recommended usage, because otherwise setting ``irq-suspend-timeout``
 might not have any discernible effect.
 
+.. _threaded_busy_poll:
+
+Threaded NAPI busy polling
+--------------------------
+
+Threaded NAPI busy polling extends threaded NAPI and adds support to do
+continuous busy polling of the NAPI. This can be useful for forwarding or
+AF_XDP applications.
+
+Threaded NAPI busy polling can be enabled on per NIC queue basis using Netlink.
+
+For example, using the following script:
+
+.. code-block:: bash
+
+  $ ynl --family netdev --do napi-set \
+            --json='{"id": 66, "threaded": "busy-poll"}'
+
+The kernel will create a kthread that busy polls on this NAPI.
+
+The user may elect to set the CPU affinity of this kthread to an unused CPU
+core to improve how often the NAPI is polled at the expense of wasted CPU
+cycles. Note that this will keep the CPU core busy with 100% usage.
+
+Once threaded busy polling is enabled for a NAPI, PID of the kthread can be
+retrieved using Netlink so the affinity of the kthread can be set up.
+
+For example, the following script can be used to fetch the PID:
+
+.. code-block:: bash
+
+  $ ynl --family netdev --do napi-get --json='{"id": 66}'
+
+This will output something like following, the pid `258` is the PID of the
+kthread that is polling this NAPI.
+
+.. code-block:: bash
+
+  $ {'defer-hard-irqs': 0,
+     'gro-flush-timeout': 0,
+     'id': 66,
+     'ifindex': 2,
+     'irq-suspend-timeout': 0,
+     'pid': 258,
+     'threaded': 'busy-poll'}
+
 .. _threaded:
 
 Threaded NAPI