linux-kernel - Re: [PATCH v2 1/2] net: ravb: Always process TX descriptor ring

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Date: Wed, 3 Apr 2024 10:02:04 +0100
From: Paul Barker <paul.barker.ct@...renesas.com>
To: Ratheesh Kannoth <rkannoth@...vell.com>
Cc: Sergey Shtylyov <s.shtylyov@....ru>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Niklas Söderlund <niklas.soderlund+renesas@...natech.se>,
 netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] net: ravb: Always process TX descriptor ring

On 03/04/2024 04:39, Ratheesh Kannoth wrote:
> On 2024-04-02 at 20:23:04, Paul Barker (paul.barker.ct@...renesas.com) wrote:
>> The TX queue should be serviced each time the poll function is called,
>> even if the full RX work budget has been consumed. This prevents
>> starvation of the TX queue when RX bandwidth usage is high.
>>
>> Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
>> Signed-off-by: Paul Barker <paul.barker.ct@...renesas.com>
>> ---
>> Changes from v1:
>>   * Use the correct 'Fixes' tag.
>>   * Call the new variable 'unmask' and drop the unnecessary initializer,
>>     as requested by Sergey.
>>
>>  drivers/net/ethernet/renesas/ravb_main.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
>> index d1be030c8848..48803050abdb 100644
>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>> @@ -1324,12 +1324,12 @@ static int ravb_poll(struct napi_struct *napi, int budget)
>>  	int q = napi - priv->napi;
>>  	int mask = BIT(q);
>>  	int quota = budget;
>> +	bool unmask;
>>
>>  	/* Processing RX Descriptor Ring */
>>  	/* Clear RX interrupt */
>>  	ravb_write(ndev, ~(mask | RIS0_RESERVED), RIS0);
>> -	if (ravb_rx(ndev, &quota, q))
>> -		goto out;
>> +	unmask = !ravb_rx(ndev, &quota, q);
>>
>>  	/* Processing TX Descriptor Ring */
> AFAIU, TX is processed without any budget. This wont result in rx work starvation if
> TX traffic is more ?

Quoting the docs in Documentation/networking/napi.rst:

    The method takes a ``budget`` argument - drivers can process
    completions for any number of Tx packets but should only process up
    to ``budget`` number of Rx packets.

    skb Tx processing should happen regardless of the ``budget``

I take that to mean that the RX work budget running out should not stop
processing of TX packets.

Other drivers with a combined TX/RX poll function follow the same
pattern of processing TX packets regardless of RX budget exhaustion, for
example see ixgbe_poll() which processes TX packets first (in
drivers/net/ethernet/intel/ixgbe/ixgbe_main.c).

Thanks,

-- 
Paul Barker
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