Subject: Re: [PATCH net-next 6/6] mvneta: recycle buffers

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 23 Mar 2021 16:06:11 +0100

On Mon, 22 Mar 2021 18:03:01 +0100
Matteo Croce <[email-masked]> wrote:

> From: Matteo Croce <[email-masked]>
> 
> Use the new recycling API for page_pool.
> In a drop rate test, the packet rate increased di 10%,
> from 269 Kpps to 296 Kpps.
> 
> perf top on a stock system shows:
> 
> Overhead  Shared Object     Symbol
>   21.78%  [kernel]          [k] __pi___inval_dcache_area
>   21.66%  [mvneta]          [k] mvneta_rx_swbm
>    7.00%  [kernel]          [k] kmem_cache_alloc
>    6.05%  [kernel]          [k] eth_type_trans
>    4.44%  [kernel]          [k] kmem_cache_free.part.0
>    3.80%  [kernel]          [k] __netif_receive_skb_core
>    3.68%  [kernel]          [k] dev_gro_receive
>    3.65%  [kernel]          [k] get_page_from_freelist
>    3.43%  [kernel]          [k] page_pool_release_page
>    3.35%  [kernel]          [k] free_unref_page
> 
> And this is the same output with recycling enabled:
> 
> Overhead  Shared Object     Symbol
>   24.10%  [kernel]          [k] __pi___inval_dcache_area
>   23.02%  [mvneta]          [k] mvneta_rx_swbm
>    7.19%  [kernel]          [k] kmem_cache_alloc
>    6.50%  [kernel]          [k] eth_type_trans
>    4.93%  [kernel]          [k] __netif_receive_skb_core
>    4.77%  [kernel]          [k] kmem_cache_free.part.0
>    3.93%  [kernel]          [k] dev_gro_receive
>    3.03%  [kernel]          [k] build_skb
>    2.91%  [kernel]          [k] page_pool_put_page
>    2.85%  [kernel]          [k] __xdp_return
> 
> The test was done with mausezahn on the TX side with 64 byte raw
> ethernet frames.
> 
> Signed-off-by: Matteo Croce <[email-masked]>
> ---
>  drivers/net/ethernet/marvell/mvneta.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index a635cf84608a..8b3250394703 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -2332,7 +2332,7 @@ mvneta_swbm_build_skb(struct mvneta_port *pp, struct mvneta_rx_queue *rxq,
>  	if (!skb)
>  		return ERR_PTR(-ENOMEM);
>  
> -	page_pool_release_page(rxq->page_pool, virt_to_page(xdp->data));
> +	skb_mark_for_recycle(skb, virt_to_page(xdp->data), &xdp->rxq->mem);
>  
>  	skb_reserve(skb, xdp->data - xdp->data_hard_start);
>  	skb_put(skb, xdp->data_end - xdp->data);
> @@ -2344,7 +2344,7 @@ mvneta_swbm_build_skb(struct mvneta_port *pp, struct mvneta_rx_queue *rxq,
>  		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
>  				skb_frag_page(frag), skb_frag_off(frag),
>  				skb_frag_size(frag), PAGE_SIZE);
> -		page_pool_release_page(rxq->page_pool, skb_frag_page(frag));
> +		skb_mark_for_recycle(skb, skb_frag_page(frag), &xdp->rxq->mem);
>  	}
>  
>  	return skb;

This cause skb_mark_for_recycle() to set 'skb->pp_recycle=1' multiple
times, for the same SKB.  (copy-pasted function below signature to help
reviewers).

This makes me question if we need an API for setting this per page
fragment?
Or if the API skb_mark_for_recycle() need to walk the page fragments in
the SKB and set the info stored in the page for each?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer