Re: [PATCH net-next] net: ethernet: fec: prevent tx starvation under high rx load

[David Miller <davem@davemloft.net>]

From: Tobias Waldekranz <tobias@waldekranz.com>
Date: Thu, 25 Jun 2020 10:57:28 +0200

> In the ISR, we poll the event register for the queues in need of
> service and then enter polled mode. After this point, the event
> register will never be read again until we exit polled mode.
> 
> In a scenario where a UDP flow is routed back out through the same
> interface, i.e. "router-on-a-stick" we'll typically only see an rx
> queue event initially. Once we start to process the incoming flow
> we'll be locked polled mode, but we'll never clean the tx rings since
> that event is never caught.
> 
> Eventually the netdev watchdog will trip, causing all buffers to be
> dropped and then the process starts over again.
> 
> By adding a poll of the active events at each NAPI call, we avoid the
> starvation.
> 
> Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>

You're losing events, which is a bug.  Therefore this is a bug fix
which should be submitted to 'net' and an appropriate "Fixes: "
tag must be added to your commit message.

Thank you.