Re: [PATCH net-next] xdp: improve page_pool xdp_return performance

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi Jesper,

On Wed, Sep 21, 2022 at 07:05:32PM +0200, Jesper Dangaard Brouer wrote:
> During LPC2022 I meetup with my page_pool co-maintainer Ilias. When
> discussing page_pool code we realised/remembered certain optimizations
> had not been fully utilised.
> 
> Since commit c07aea3ef4d4 ("mm: add a signature in struct page") struct
> page have a direct pointer to the page_pool object this page was
> allocated from.
> 
> Thus, with this info it is possible to skip the rhashtable_lookup to
> find the page_pool object in __xdp_return().
> 
> The rcu_read_lock can be removed as it was tied to xdp_mem_allocator.
> The page_pool object is still safe to access as it tracks inflight pages
> and (potentially) schedules final release from a work queue.
> 
> Created a micro benchmark of XDP redirecting from mlx5 into veth with
> XDP_DROP bpf-prog on the peer veth device. This increased performance
> 6.5% from approx 8.45Mpps to 9Mpps corresponding to using 7 nanosec
> (27 cycles at 3.8GHz) less per packet.

Thanks for the detailed testing

> 
> Suggested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  net/core/xdp.c |   10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 24420209bf0e..844c9d99dc0e 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -375,19 +375,17 @@ EXPORT_SYMBOL_GPL(xdp_rxq_info_reg_mem_model);
>  void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
>  		  struct xdp_buff *xdp)
>  {
> -	struct xdp_mem_allocator *xa;
>  	struct page *page;
>  
>  	switch (mem->type) {
>  	case MEM_TYPE_PAGE_POOL:
> -		rcu_read_lock();
> -		/* mem->id is valid, checked in xdp_rxq_info_reg_mem_model() */
> -		xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
>  		page = virt_to_head_page(data);
>  		if (napi_direct && xdp_return_frame_no_direct())
>  			napi_direct = false;
> -		page_pool_put_full_page(xa->page_pool, page, napi_direct);
> -		rcu_read_unlock();
> +		/* No need to check ((page->pp_magic & ~0x3UL) == PP_SIGNATURE)
> +		 * as mem->type knows this a page_pool page
> +		 */
> +		page_pool_put_full_page(page->pp, page, napi_direct);
>  		break;
>  	case MEM_TYPE_PAGE_SHARED:
>  		page_frag_free(data);
> 
> 

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>