netdev - Re: [PATCH -next 2/3] bnx2: Add prefetches to rx path.

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Date:	Wed, 05 May 2010 07:45:30 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michael Chan <mchan@...adcom.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH -next 2/3] bnx2: Add prefetches to rx path.

Le mardi 04 mai 2010 à 22:21 -0700, Michael Chan a écrit :
> Add prefetches of the skb and the next rx descriptor to speed up rx path.
> 
> Signed-off-by: Michael Chan <mchan@...adcom.com>
> Signed-off-by: Matt Carlson <mcarlson@...adcom.com>
> ---
>  drivers/net/bnx2.c |   12 +++++++++---
>  drivers/net/bnx2.h |    1 +
>  2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/bnx2.c b/drivers/net/bnx2.c
> index 6ad3184..cdee29b 100644
> --- a/drivers/net/bnx2.c
> +++ b/drivers/net/bnx2.c
> @@ -2719,6 +2719,7 @@ bnx2_alloc_rx_skb(struct bnx2 *bp, struct bnx2_rx_ring_info *rxr, u16 index)
>  	}
>  
>  	rx_buf->skb = skb;
> +	rx_buf->desc = (struct l2_fhdr *) skb->data;
>  	dma_unmap_addr_set(rx_buf, mapping, mapping);
>  
>  	rxbd->rx_bd_haddr_hi = (u64) mapping >> 32;
> @@ -2941,6 +2942,7 @@ bnx2_reuse_rx_skb(struct bnx2 *bp, struct bnx2_rx_ring_info *rxr,
>  	rxr->rx_prod_bseq += bp->rx_buf_use_size;
>  
>  	prod_rx_buf->skb = skb;
> +	prod_rx_buf->desc = (struct l2_fhdr *) skb->data;
>  
>  	if (cons == prod)
>  		return;
> @@ -3086,7 +3088,7 @@ bnx2_rx_int(struct bnx2 *bp, struct bnx2_napi *bnapi, int budget)
>  	while (sw_cons != hw_cons) {
>  		unsigned int len, hdr_len;
>  		u32 status;
> -		struct sw_bd *rx_buf;
> +		struct sw_bd *rx_buf, *next_rx_buf;
>  		struct sk_buff *skb;
>  		dma_addr_t dma_addr;
>  		u16 vtag = 0;
> @@ -3097,7 +3099,11 @@ bnx2_rx_int(struct bnx2 *bp, struct bnx2_napi *bnapi, int budget)
>  
>  		rx_buf = &rxr->rx_buf_ring[sw_ring_cons];
>  		skb = rx_buf->skb;
> +		prefetch(skb);

why not a prefetchw() ?

>  
> +		next_rx_buf =
> +			&rxr->rx_buf_ring[RX_RING_IDX(NEXT_RX_BD(sw_cons))];
> +		prefetch(next_rx_buf->desc);

So cpu is allowed to start a memory transaction on next_skb->data, while
not yes DMA unmapped ?

>  		rx_buf->skb = NULL;
>  
>  		dma_addr = dma_unmap_addr(rx_buf, mapping);
> @@ -3106,7 +3112,7 @@ bnx2_rx_int(struct bnx2 *bp, struct bnx2_napi *bnapi, int budget)
>  			BNX2_RX_OFFSET + BNX2_RX_COPY_THRESH,
>  			PCI_DMA_FROMDEVICE);
>  
> -		rx_hdr = (struct l2_fhdr *) skb->data;
> +		rx_hdr = rx_buf->desc;
>  		len = rx_hdr->l2_fhdr_pkt_len;
>  		status = rx_hdr->l2_fhdr_status;
>  
> @@ -5764,7 +5770,7 @@ bnx2_run_loopback(struct bnx2 *bp, int loopback_mode)
>  	rx_buf = &rxr->rx_buf_ring[rx_start_idx];
>  	rx_skb = rx_buf->skb;
>  
> -	rx_hdr = (struct l2_fhdr *) rx_skb->data;
> +	rx_hdr = rx_buf->desc;
>  	skb_reserve(rx_skb, BNX2_RX_OFFSET);
>  
>  	pci_dma_sync_single_for_cpu(bp->pdev,
> diff --git a/drivers/net/bnx2.h b/drivers/net/bnx2.h
> index ab34a5d..dd35bd0 100644
> --- a/drivers/net/bnx2.h
> +++ b/drivers/net/bnx2.h
> @@ -6551,6 +6551,7 @@ struct l2_fhdr {
>  
>  struct sw_bd {
>  	struct sk_buff		*skb;
> +	struct l2_fhdr		*desc;
>  	DEFINE_DMA_UNMAP_ADDR(mapping);
>  };
>  

It seems there are two parts in this patch, one caching skb->data in
sw_bd, and prefetches...



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