linux-kernel - Re: [PATCH net v2 3/3] net: nixge: fix tx queue handling

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Date:   Wed, 16 Nov 2022 19:52:34 +0800
From:   Zhang Changzhong <zhangchangzhong@...wei.com>
To:     Francois Romieu <romieu@...zoreil.com>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <mdf@...nel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v2 3/3] net: nixge: fix tx queue handling

On 2022/11/16 18:27, Francois Romieu wrote:
> Zhang Changzhong <zhangchangzhong@...wei.com> :
>> On 2022/11/16 7:04, Francois Romieu wrote:
>>> Zhang Changzhong <zhangchangzhong@...wei.com> :
> [...]
>>>> diff --git a/drivers/net/ethernet/ni/nixge.c b/drivers/net/ethernet/ni/nixge.c
>>>> index 91b7ebc..3776a03 100644
>>>> --- a/drivers/net/ethernet/ni/nixge.c
>>>> +++ b/drivers/net/ethernet/ni/nixge.c
>>> [...]
>>>>  static netdev_tx_t nixge_start_xmit(struct sk_buff *skb,
>>>> @@ -518,10 +523,15 @@ static netdev_tx_t nixge_start_xmit(struct sk_buff *skb,
>>>>  	cur_p = &priv->tx_bd_v[priv->tx_bd_tail];
>>>>  	tx_skb = &priv->tx_skb[priv->tx_bd_tail];
>>>>  
>>>> -	if (nixge_check_tx_bd_space(priv, num_frag + 1)) {
>>>> -		if (!netif_queue_stopped(ndev))
>>>> -			netif_stop_queue(ndev);
>>>> -		return NETDEV_TX_OK;
>>>> +	if (unlikely(nixge_check_tx_bd_space(priv, num_frag + 1))) {
>>>> +		/* Should not happen as last start_xmit call should have
>>>> +		 * checked for sufficient space and queue should only be
>>>> +		 * woken when sufficient space is available.
>>>> +		 */
>>>
>>> Almost. IRQ triggering after nixge_start_xmit::netif_stop_queue and
>>> before nixge_start_xmit::smp_mb may wrongly wake queue.
>>>
>>
>> I don't know what you mean by "wronly wake queue". The queue is woken
>> only when there is sufficient for next packet.
> 
> Between nixge_start_xmit::netif_stop_queue and nixge_start_xmit::smp_mb,
> "next" packet is current packet in hard_start_xmit. However said current
> packet may not be accounted for in the IRQ context transmit completion
> handler.
> 
> [nixge_start_xmit]
> 
>         ++priv->tx_bd_tail;
>         priv->tx_bd_tail %= TX_BD_NUM;
> 
> 	/* Stop queue if next transmit may not have space */
> 	if (nixge_check_tx_bd_space(priv, MAX_SKB_FRAGS + 1)) {
>                 netif_stop_queue(ndev);
> 
> Which value does [nixge_start_xmit_done] read as priv->tx_bd_tail at
> this point ? The value set a few lines above or some older value ?
> 
> 		/* Matches barrier in nixge_start_xmit_done */
> 		smp_mb();
> 
> 		/* Space might have just been freed - check again */
> 		if (!nixge_check_tx_bd_space(priv, MAX_SKB_FRAGS + 1))
> 			netif_wake_queue(ndev);
> 	}
> 

Got it! Thanks a lot for your detailed explanation.

Best Regards,
Changzhong