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[PATCH net-next 1/2] net: mvneta: add xmit_more support

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  [PATCH net-next 1/2] net: mvneta: add xmit_more support Marcin Wojtas <mw@semihalf.com> - 2016-09-13 09:10 +0200
    Re: [PATCH net-next 1/2] net: mvneta: add xmit_more support Eric Dumazet <eric.dumazet@gmail.com> - 2016-09-13 16:40 +0200
      Re: [PATCH net-next 1/2] net: mvneta: add xmit_more support Eric Dumazet <eric.dumazet@gmail.com> - 2016-09-13 16:50 +0200

#1482182 — [PATCH net-next 1/2] net: mvneta: add xmit_more support

From: Simon Guinot <simon.guinot@sequanux.org>

Basing on xmit_more flag of the skb, TX descriptors can be concatenated
before flushing. This commit delay Tx descriptor flush if the queue is
running and if there is more skb's to send.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
---
 drivers/net/ethernet/marvell/mvneta.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
index d41c28d..b9dccea 100644
--- a/drivers/net/ethernet/marvell/mvneta.c
+++ b/drivers/net/ethernet/marvell/mvneta.c
@@ -512,6 +512,7 @@ struct mvneta_tx_queue {
 	 * descriptor ring
 	 */
 	int count;
+	int pending;
 	int tx_stop_threshold;
 	int tx_wake_threshold;
 
@@ -802,8 +803,9 @@ static void mvneta_txq_pend_desc_add(struct mvneta_port *pp,
 	/* Only 255 descriptors can be added at once ; Assume caller
 	 * process TX desriptors in quanta less than 256
 	 */
-	val = pend_desc;
+	val = pend_desc + txq->pending;
 	mvreg_write(pp, MVNETA_TXQ_UPDATE_REG(txq->id), val);
+	txq->pending = 0;
 }
 
 /* Get pointer to next TX descriptor to be processed (send) by HW */
@@ -2357,11 +2359,14 @@ out:
 		struct netdev_queue *nq = netdev_get_tx_queue(dev, txq_id);
 
 		txq->count += frags;
-		mvneta_txq_pend_desc_add(pp, txq, frags);
-
 		if (txq->count >= txq->tx_stop_threshold)
 			netif_tx_stop_queue(nq);
 
+		if (!skb->xmit_more || netif_xmit_stopped(nq))
+			mvneta_txq_pend_desc_add(pp, txq, frags);
+		else
+			txq->pending += frags;
+
 		u64_stats_update_begin(&stats->syncp);
 		stats->tx_packets++;
 		stats->tx_bytes  += len;
-- 
1.8.3.1

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#1482548

On Tue, 2016-09-13 at 09:00 +0200, Marcin Wojtas wrote:
> From: Simon Guinot <simon.guinot@sequanux.org>
> 
> Basing on xmit_more flag of the skb, TX descriptors can be concatenated
> before flushing. This commit delay Tx descriptor flush if the queue is
> running and if there is more skb's to send.
> 
> Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
> ---
>  drivers/net/ethernet/marvell/mvneta.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index d41c28d..b9dccea 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -512,6 +512,7 @@ struct mvneta_tx_queue {
>  	 * descriptor ring
>  	 */
>  	int count;
> +	int pending;
>  	int tx_stop_threshold;
>  	int tx_wake_threshold;
>  
> @@ -802,8 +803,9 @@ static void mvneta_txq_pend_desc_add(struct mvneta_port *pp,
>  	/* Only 255 descriptors can be added at once ; Assume caller
>  	 * process TX desriptors in quanta less than 256
>  	 */

Hi Marcin

Well, given the above comment, and fact that MVNETA_MAX_TXD == 532, it
looks like you might add a bug if more than 256 skb are given to your
ndo_start_xmit() with skb->xmit_more = 1

I therefore suggest you make sure it does not happen.

txq->pending += frags;
if (!skb->xmit_more ||
    txq->pending > 256 - MVNETA_MAX_SKB_DESCS ||
    netif_xmit_stopped(nq))
	mvneta_txq_pend_desc_add(pp, txq)

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#1482554

On Tue, 2016-09-13 at 07:33 -0700, Eric Dumazet wrote:

> Hi Marcin
> 
> Well, given the above comment, and fact that MVNETA_MAX_TXD == 532, it
> looks like you might add a bug if more than 256 skb are given to your
> ndo_start_xmit() with skb->xmit_more = 1
> 
> I therefore suggest you make sure it does not happen.
> 
> txq->pending += frags;
> if (!skb->xmit_more ||
>     txq->pending > 256 - MVNETA_MAX_SKB_DESCS ||
>     netif_xmit_stopped(nq))
> 	mvneta_txq_pend_desc_add(pp, txq)
> 

Another solution would be to test the potential overflow in mvneta_tx()
and force a mvneta_txq_pend_desc_add(pp, txq) _before_ adding the desc
of the "about to be cooked" TSO packet.

(This is because MVNETA_MAX_SKB_DESCS is 217, so 255-217 leaves few room
for xmit_more to show its power)

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