netdev - Re: [PATCH net-next 09/11] sfc: Prefetch RX skb header area

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Date:	Wed, 19 Jun 2013 23:39:17 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net-next 09/11] sfc: Prefetch RX skb header area

On Wed, 2013-06-19 at 15:36 -0700, Eric Dumazet wrote:
> On Wed, 2013-06-19 at 23:20 +0100, Ben Hutchings wrote:
> > When we pre-allocate skbs, the sk_buff structures themselves will be
> > cache-hot (having just been filled in) but the header area will be
> > cache-cold.  The skb cache is small and we're likely to use each skb
> > quite soon, so prefetch the start of the header area, where we will
> > copy the headers, and the skb_shared_info structure, where we will
> > store page fragments.
> > 
> > Signed-off-by: Ben Hutchings <bhutchings@...arflare.com>
> > ---
> >  drivers/net/ethernet/sfc/rx.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
> > index a28f2fe..0dc4a80 100644
> > --- a/drivers/net/ethernet/sfc/rx.c
> > +++ b/drivers/net/ethernet/sfc/rx.c
> > @@ -106,11 +106,17 @@ void efx_rx_config_page_split(struct efx_nic *efx)
> >  
> >  static void efx_refill_skb_cache(struct efx_rx_queue *rx_queue)
> >  {
> > +	struct sk_buff *skb;
> >  	int i;
> > +
> >  	for (i = 0; i < SKB_CACHE_SIZE; ++i) {
> > -		rx_queue->skb_cache[i] =
> > -			netdev_alloc_skb(rx_queue->efx->net_dev,
> > -					 EFX_SKB_HEADERS + EFX_PAGE_SKB_ALIGN);
> > +		skb = netdev_alloc_skb(rx_queue->efx->net_dev,
> > +				       EFX_SKB_HEADERS + EFX_PAGE_SKB_ALIGN);
> > +		rx_queue->skb_cache[i] = skb;
> > +		if (skb) {
> > +			prefetch(skb->data);
> 
> This is really suspect.
> 
> This part will be dirtied anyway by the NIC once frame is received.

This is not used for DMA.

> > +			prefetch(skb_shinfo(skb));
> > +		}
> >  	}
> >  	rx_queue->skb_cache_next_unused = 0;
> >  }
> > 
> > 
> 
> Quite frankly this cache sounds suspect to me.
> 
> Why is it needed only for sfc ?
> 
> Have you tried build_skb() instead ?

You can't use build_skb() if you put multiple RX buffers in a page.  See
<1365805319.2791.18.camel@...-desktop.uk.solarflarecom.com>.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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