Subject: Re: [PATCH] net: Update netdev_alloc_frag to work more efficiently with TCP and GRO

From: Alexander Duyck <[email-masked]>
Date: Fri, 29 Jun 2012 16:04:39 -0700

On 06/22/2012 05:33 AM, Eric Dumazet wrote:
> Here is the patch I tested here.
>
> Using 32768 bytes allocations is actually nice for MTU=9000 traffic,
> since we can fit 3 frames per 32KB instead of only 2 frames (using
> kmalloc-16384 slab))
>
> Also, I prefill page->_count with a high bias value, to avoid the
> get_page() we did for each allocated frag.
>
> In my profiles, the get_page() cost was dominant, because of false
> sharing with skb consumers (as they might run on different cpus)
>
> This way, when 32768 bytes are filled, we perform a single
> atomic_sub_return() and can recycle the page if we find we are the last
> user (this is what you did in your patch, when testing page->_count
> being 1)
>
> Note : If I used max(PAGE_SIZE, 32678) for MAX_NETDEV_FRAGSIZE,
> gcc was not able to optimise get_order(MAX_NETDEV_FRAGSIZE), strange...
>
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 5b21522..d31efa2 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -296,9 +296,18 @@ EXPORT_SYMBOL(build_skb);
>  struct netdev_alloc_cache {
>  	struct page *page;
>  	unsigned int offset;
> +	unsigned int pagecnt_bias;
>  };
>  static DEFINE_PER_CPU(struct netdev_alloc_cache, netdev_alloc_cache);
>  
> +#if PAGE_SIZE > 32768
> +#define MAX_NETDEV_FRAGSIZE	PAGE_SIZE
> +#else
> +#define MAX_NETDEV_FRAGSIZE	32768
> +#endif
> +
> +#define NETDEV_PAGECNT_BIAS	(MAX_NETDEV_FRAGSIZE /		\
> +				 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
>  /**
>   * netdev_alloc_frag - allocate a page fragment
>   * @fragsz: fragment size
> @@ -316,18 +325,25 @@ void *netdev_alloc_frag(unsigned int fragsz)
>  	nc = &__get_cpu_var(netdev_alloc_cache);
>  	if (unlikely(!nc->page)) {
>  refill:
> -		nc->page = alloc_page(GFP_ATOMIC | __GFP_COLD);
> +		nc->page = alloc_pages(GFP_ATOMIC | __GFP_COLD | __GFP_COMP,
> +				       get_order(MAX_NETDEV_FRAGSIZE));
> +		if (unlikely(!nc->page))
> +			goto end;
> +recycle:
> +		atomic_set(&nc->page->_count, NETDEV_PAGECNT_BIAS);
> +		nc->pagecnt_bias = NETDEV_PAGECNT_BIAS;
>  		nc->offset = 0;
>  	}
> -	if (likely(nc->page)) {
> -		if (nc->offset + fragsz > PAGE_SIZE) {
> -			put_page(nc->page);
> -			goto refill;
> -		}
> -		data = page_address(nc->page) + nc->offset;
> -		nc->offset += fragsz;
> -		get_page(nc->page);
> +	if (nc->offset + fragsz > MAX_NETDEV_FRAGSIZE) {
> +		if (!atomic_sub_return(nc->pagecnt_bias,
> +				       &nc->page->_count))
> +			goto recycle;
> +		goto refill;
>  	}
> +	data = page_address(nc->page) + nc->offset;
> +	nc->offset += fragsz;
> +	nc->pagecnt_bias--; /* avoid get_page()/get_page() false sharing */
> +end:
>  	local_irq_restore(flags);
>  	return data;
>  }
> @@ -353,7 +369,7 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev,
>  	unsigned int fragsz = SKB_DATA_ALIGN(length + NET_SKB_PAD) +
>  			      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>  
> -	if (fragsz <= PAGE_SIZE && !(gfp_mask & __GFP_WAIT)) {
> +	if (fragsz <= MAX_NETDEV_FRAGSIZE && !(gfp_mask & __GFP_WAIT)) {
>  		void *data = netdev_alloc_frag(fragsz);
>  
>  		if (likely(data)) {
>
>
I was wondering if there were any plans to clean this patch up and
submit it to net-next?  If not, I can probably work on that since this
addressed the concerns I had in my original patch.

Thanks,

Alex