[PATCH 6/6] gianfar: Revive SKB recycling

[Kumar Gopalpet-B05799]

>-----Original Message-----
>From: Anton Vorontsov [mailto:avorontsov at ru.mvista.com] 
>Sent: Wednesday, November 11, 2009 5:41 AM
>To: David Miller
>Cc: Fleming Andy-AFLEMING; Jon Loeliger; Kumar 
>Gopalpet-B05799; Lennert Buytenhek; Stephen Hemminger; 
>netdev at vger.kernel.org; linuxppc-dev at ozlabs.org
>Subject: [PATCH 6/6] gianfar: Revive SKB recycling
>
>Before calling gfar_clean_tx_ring() the driver grabs an 
>irqsave spinlock, and then tries to recycle skbs. But since
>skb_recycle_check() returns 0 with IRQs disabled, we'll never 
>recycle any skbs.
>
>It appears that gfar_clean_tx_ring() and gfar_start_xmit() are 
>mostly idependent and can work in parallel, except when they 
>modify num_txbdfree.
>
>So we can drop the lock from most sections and thus fix the 
>skb recycling.
>
>Signed-off-by: Anton Vorontsov <avorontsov at ru.mvista.com>
>---
> drivers/net/gianfar.c |   31 +++++++++++++++++++------------
> 1 files changed, 19 insertions(+), 12 deletions(-)
>
>diff --git a/drivers/net/gianfar.c b/drivers/net/gianfar.c 
>index fde430a..16def13 100644
>--- a/drivers/net/gianfar.c
>+++ b/drivers/net/gianfar.c
>@@ -1928,14 +1928,11 @@ static int gfar_start_xmit(struct 
>sk_buff *skb, struct net_device *dev)
> 	/* total number of fragments in the SKB */
> 	nr_frags = skb_shinfo(skb)->nr_frags;
> 
>-	spin_lock_irqsave(&tx_queue->txlock, flags);
>-
> 	/* check if there is space to queue this packet */
> 	if ((nr_frags+1) > tx_queue->num_txbdfree) {
> 		/* no space, stop the queue */
> 		netif_tx_stop_queue(txq);
> 		dev->stats.tx_fifo_errors++;
>-		spin_unlock_irqrestore(&tx_queue->txlock, flags);
> 		return NETDEV_TX_BUSY;
> 	}
> 
>@@ -1999,6 +1996,20 @@ static int gfar_start_xmit(struct 
>sk_buff *skb, struct net_device *dev)
> 	lstatus |= BD_LFLAG(TXBD_CRC | TXBD_READY) | skb_headlen(skb);
> 
> 	/*
>+	 * We can work in parallel with gfar_clean_tx_ring(), except
>+	 * when modifying num_txbdfree. Note that we didn't 
>grab the lock
>+	 * when we were reading the num_txbdfree and checking 
>for available
>+	 * space, that's because outside of this function it 
>can only grow,
>+	 * and once we've got needed space, it cannot suddenly 
>disappear.
>+	 *
>+	 * The lock also protects us from gfar_error(), which can modify
>+	 * regs->tstat and thus retrigger the transfers, which is why we
>+	 * also must grab the lock before setting ready bit for 
>the first
>+	 * to be transmitted BD.
>+	 */
>+	spin_lock_irqsave(&tx_queue->txlock, flags);
>+
>+	/*
> 	 * The powerpc-specific eieio() is used, as wmb() has too strong
> 	 * semantics (it requires synchronization between cacheable and
> 	 * uncacheable mappings, which eieio doesn't provide 
>and which we @@ -2225,6 +2236,8 @@ static int 
>gfar_clean_tx_ring(struct gfar_priv_tx_q *tx_queue)
> 	skb_dirtytx = tx_queue->skb_dirtytx;
> 
> 	while ((skb = tx_queue->tx_skbuff[skb_dirtytx])) {
>+		unsigned long flags;
>+
> 		frags = skb_shinfo(skb)->nr_frags;
> 		lbdp = skip_txbd(bdp, frags, base, tx_ring_size);
> 
>@@ -2269,7 +2282,9 @@ static int gfar_clean_tx_ring(struct 
>gfar_priv_tx_q *tx_queue)
> 			TX_RING_MOD_MASK(tx_ring_size);
> 
> 		howmany++;
>+		spin_lock_irqsave(&tx_queue->txlock, flags);
> 		tx_queue->num_txbdfree += frags + 1;
>+		spin_unlock_irqrestore(&tx_queue->txlock, flags);
> 	}
> 
> 	/* If we freed a buffer, we can restart transmission, 
>if necessary */ @@ -2548,7 +2563,6 @@ static int 
>gfar_poll(struct napi_struct *napi, int budget)
> 	int tx_cleaned = 0, i, left_over_budget = budget;
> 	unsigned long serviced_queues = 0;
> 	int num_queues = 0;
>-	unsigned long flags;
> 
> 	num_queues = gfargrp->num_rx_queues;
> 	budget_per_queue = budget/num_queues;
>@@ -2568,14 +2582,7 @@ static int gfar_poll(struct napi_struct 
>*napi, int budget)
> 			rx_queue = priv->rx_queue[i];
> 			tx_queue = priv->tx_queue[rx_queue->qindex];
> 
>-			/* If we fail to get the lock,
>-			 * don't bother with the TX BDs */
>-			if 
>(spin_trylock_irqsave(&tx_queue->txlock, flags)) {
>-				tx_cleaned += 
>gfar_clean_tx_ring(tx_queue);
>-				
>spin_unlock_irqrestore(&tx_queue->txlock,
>-							flags);
>-			}
>-
>+			tx_cleaned += gfar_clean_tx_ring(tx_queue);
> 			rx_cleaned_per_queue = 
>gfar_clean_rx_ring(rx_queue,
> 							
>budget_per_queue);
> 			rx_cleaned += rx_cleaned_per_queue;
>--


Anton, we tried some experiments too at our end, and removing the
spinlocks did help improve the performance and recycling was effective
although, I don't have exact numbers to specify.

But overall I agree with you in removing the spinlocks from the
gfar_poll context.


--

Thanks
Sandeep