Subject: Re: [PATCH net-next 1/4] net: xdp: introduce bulking for xdp tx return path

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 29 Oct 2020 14:52:39 +0100

On Tue, 27 Oct 2020 20:04:07 +0100
Lorenzo Bianconi <[email-masked]> wrote:

> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 48aba933a5a8..93eabd789246 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -380,6 +380,57 @@ void xdp_return_frame_rx_napi(struct xdp_frame *xdpf)
>  }
>  EXPORT_SYMBOL_GPL(xdp_return_frame_rx_napi);
>  
> +void xdp_flush_frame_bulk(struct xdp_frame_bulk *bq)
> +{
> +	struct xdp_mem_allocator *xa = bq->xa;
> +	int i;
> +
> +	if (unlikely(!xa))
> +		return;
> +
> +	for (i = 0; i < bq->count; i++) {
> +		struct page *page = virt_to_head_page(bq->q[i]);
> +
> +		page_pool_put_full_page(xa->page_pool, page, false);
> +	}
> +	bq->count = 0;
> +}
> +EXPORT_SYMBOL_GPL(xdp_flush_frame_bulk);
> +
> +void xdp_return_frame_bulk(struct xdp_frame *xdpf,
> +			   struct xdp_frame_bulk *bq)
> +{
> +	struct xdp_mem_info *mem = &xdpf->mem;
> +	struct xdp_mem_allocator *xa, *nxa;
> +
> +	if (mem->type != MEM_TYPE_PAGE_POOL) {
> +		__xdp_return(xdpf->data, &xdpf->mem, false);
> +		return;
> +	}
> +
> +	rcu_read_lock();
> +
> +	xa = bq->xa;
> +	if (unlikely(!xa || mem->id != xa->mem.id)) {
> +		nxa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
> +		if (unlikely(!xa)) {
> +			bq->count = 0;
> +			bq->xa = nxa;
> +			xa = nxa;
> +		}
> +	}
> +
> +	if (mem->id != xa->mem.id || bq->count == XDP_BULK_QUEUE_SIZE)
> +		xdp_flush_frame_bulk(bq);
> +
> +	bq->q[bq->count++] = xdpf->data;
> +	if (mem->id != xa->mem.id)
> +		bq->xa = nxa;
> +
> +	rcu_read_unlock();
> +}
> +EXPORT_SYMBOL_GPL(xdp_return_frame_bulk);

We (Ilias my co-maintainer and I) think above code is hard to read and
understand (as a reader you need to keep too many cases in your head).

I think we both have proposals to improve this, here is mine:

/* Defers return when frame belongs to same mem.id as previous frame */
void xdp_return_frame_bulk(struct xdp_frame *xdpf,
                           struct xdp_frame_bulk *bq)
{
        struct xdp_mem_info *mem = &xdpf->mem;
        struct xdp_mem_allocator *xa;

        if (mem->type != MEM_TYPE_PAGE_POOL) {
                __xdp_return(xdpf->data, &xdpf->mem, false);
                return;
        }

        rcu_read_lock();

        xa = bq->xa;
        if (unlikely(!xa)) {
		xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
                bq->count = 0;
                bq->xa = xa;
        }

        if (bq->count == XDP_BULK_QUEUE_SIZE)
                xdp_flush_frame_bulk(bq);

        if (mem->id != xa->mem.id) {
		xdp_flush_frame_bulk(bq);
		bq->xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
        }

	bq->q[bq->count++] = xdpf->data;

        rcu_read_unlock();
}

Please review for correctness, and also for readability.

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