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

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Message-Id: <20200629.130731.932623918439489841.davem@davemloft.net>
Date:   Mon, 29 Jun 2020 13:07:31 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     tobias@...dekranz.com
Cc:     netdev@...r.kernel.org, fugang.duan@....com
Subject: Re: [PATCH net] net: ethernet: fec: prevent tx starvation under
 high rx load

From: Tobias Waldekranz <tobias@...dekranz.com>
Date: Mon, 29 Jun 2020 21:16:01 +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.
> 
> Fixes: 4d494cdc92b3 ("net: fec: change data structure to support multiqueue")
> Signed-off-by: Tobias Waldekranz <tobias@...dekranz.com>

I don't see how this can happen since you process the TX queue
unconditionally every NAPI pass, regardless of what bits you see
set in the IEVENT register.

Or don't you?  Oh, I see, you don't:

	for_each_set_bit(queue_id, &fep->work_tx, FEC_ENET_MAX_TX_QS) {

That's the problem.  Just unconditionally process the TX work regardless
of what is in IEVENT.  That whole ->tx_work member and the code that
uses it can just be deleted.  fec_enet_collect_events() can just return
a boolean saying whether there is any RX or TX work at all.

Than you're performance and latency will be even better in this situation.