netdev - Re: [PATCH] net: mvneta: fix handling of the Tx descriptor counter

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Date:   Sat, 11 Nov 2017 18:45:04 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     simon.guinot@...uanux.org
Cc:     thomas.petazzoni@...e-electrons.com, netdev@...r.kernel.org,
        musv@....de, andreas.tobler@...udguard.ch,
        gregory.clement@...e-electrons.com,
        antoine.tenart@...e-electrons.com, mw@...ihalf.com,
        stable@...r.kernel.org
Subject: Re: [PATCH] net: mvneta: fix handling of the Tx descriptor counter

From: Simon Guinot <simon.guinot@...uanux.org>
Date: Wed,  8 Nov 2017 17:58:35 +0100

> @@ -2413,8 +2416,7 @@ static int mvneta_tx(struct sk_buff *skb, struct net_device *dev)
>  		if (txq->count >= txq->tx_stop_threshold)
>  			netif_tx_stop_queue(nq);
>  
> -		if (!skb->xmit_more || netif_xmit_stopped(nq) ||
> -		    txq->pending + frags > MVNETA_TXQ_DEC_SENT_MASK)
> +		if (!skb->xmit_more || netif_xmit_stopped(nq))
>  			mvneta_txq_pend_desc_add(pp, txq, frags);
>  		else
>  			txq->pending += frags;

As David Laight said, you should not allow unlimited amounts of
->xmit_more frames to be queued without a TX doorbell update.

Therefore, please keep some kind of limit here otherwise latency
will spike in some circumstances.