Re: [PATCH net-next v1 0/2] Reuse threaded NAPI kthread across napi_del()/napi_add().

On Mon, Jun 29, 2026 at 4:26 PM Jakub Kicinski [off-list ref] wrote:

On Mon, 29 Jun 2026 12:20:25 -0700 Shuhao Tan wrote:

quoted

These drivers destroy and recreate queues during configuration
changes. If a NAPI was threaded before destruction, during the
creation, a new kthread will be spawned for the NAPI.

Some drivers do not have this problem, e.g. netdevsim. But these
drivers and the drivers mentioned above will still lose kthread
during link flap (ndo_stop/ndo_open).

Because the kthreads before and after these configuration changes are
different, all the attributes associated with the kthread are lost.
These include CPU mask, priority, scheduler policy, etc.. If the
threaded state is preserved for a NAPI, it makes sense to want to
preserve the attributes of the thread as well.

Send a netdev Netlink notification when NAPI is re-created and
let the userspace re-apply the settings?

It feels surprising that the userspace needs to reconfigure thread
properties when changing NIC configurations unrelated to threading.
Another downside is that when userspace configures NIC configurations
in quick succession, re-application becomes messy because a previous
re-application might still be in progress when the thread is gone.

Keeping a few u32s
around is one thing but keeping a thread running and visible
in /procfs for the lifetime of a machine feels a little bit much.
IDK.

The thread will be parked, so at least it will not eat CPU cycles.
Regarding visibility, the only case that I am aware of where a thread
can be unwantedly visible requires first setting the device/queue to
be threaded and then reducing the number of queues. If certain
configurations are applied before the reduction, it makes sense to
want to restore those stuff when the queues get added back (like
irq_affinity today). Moreover, the user can stop the stale thread and
completely remove it by disabling threaded NAPI on the device. So the
thread is not completely tied to the machine's lifetime, and there is
a way to get rid of it.