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

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Message-Id: <20200630.125802.533305649716945637.davem@davemloft.net>
Date:   Tue, 30 Jun 2020 12:58:02 -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: Tue, 30 Jun 2020 08:39:58 +0200

> On Mon Jun 29, 2020 at 3:07 PM CEST, David Miller wrote:
>> 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.
> 
> Maybe Andy could chime in here, but I think the ->tx_work construction
> is load bearing. It seems to me like that is the only thing stopping
> us from trying to process non-existing queues on older versions of the
> silicon which only has a single queue.

Then iterate over "actually existing" queues.

My primary point still stands.