Re: [PATCH net V2 1/2] veth: enable dev_watchdog for detecting stalled TXQs

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Jesper Dangaard Brouer <hawk@kernel.org> writes:

> The changes introduced in commit dc82a33297fc ("veth: apply qdisc
> backpressure on full ptr_ring to reduce TX drops") have been found to cause
> a race condition in production environments.
>
> Under specific circumstances, observed exclusively on ARM64 (aarch64)
> systems with Ampere Altra Max CPUs, a transmit queue (TXQ) can become
> permanently stalled. This happens when the race condition leads to the TXQ
> entering the QUEUE_STATE_DRV_XOFF state without a corresponding queue wake-up,
> preventing the attached qdisc from dequeueing packets and causing the
> network link to halt.
>
> As a first step towards resolving this issue, this patch introduces a
> failsafe mechanism. It enables the net device watchdog by setting a timeout
> value and implements the .ndo_tx_timeout callback.
>
> If a TXQ stalls, the watchdog will trigger the veth_tx_timeout() function,
> which logs a warning and calls netif_tx_wake_queue() to unstall the queue
> and allow traffic to resume.
>
> The log message will look like this:
>
>  veth42: NETDEV WATCHDOG: CPU: 34: transmit queue 0 timed out 5393 ms
>  veth42: veth backpressure stalled(n:1) TXQ(0) re-enable
>
> This provides a necessary recovery mechanism while the underlying race
> condition is investigated further. Subsequent patches will address the root
> cause and add more robust state handling.
>
> Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
> Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>