netdev - Re: [PATCH net-next 2/4] net: page_pool: add bulk support for ptr

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Date:   Thu, 29 Oct 2020 11:13:29 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        ilias.apalodimas@...aro.org, brouer@...hat.com
Subject: Re: [PATCH net-next 2/4] net: page_pool: add bulk support for
 ptr_ring

On Tue, 27 Oct 2020 20:04:08 +0100
Lorenzo Bianconi <lorenzo@...nel.org> wrote:

> +void page_pool_put_page_bulk(struct page_pool *pool, void **data,
> +			     int count)
> +{
> +	struct page *page_ring[XDP_BULK_QUEUE_SIZE];

Maybe we could reuse the 'data' array instead of creating a new array
(2 cache-lines long) for the array of pages?

> +	int i, len = 0;
> +
> +	for (i = 0; i < count; i++) {
> +		struct page *page = virt_to_head_page(data[i]);
> +
> +		if (unlikely(page_ref_count(page) != 1 ||
> +			     !pool_page_reusable(pool, page))) {
> +			page_pool_release_page(pool, page);
> +			put_page(page);
> +			continue;
> +		}
> +
> +		if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> +			page_pool_dma_sync_for_device(pool, page, -1);

Here we sync the entire DMA area (-1), which have a *huge* cost for
mvneta (especially on EspressoBin HW).  For this xdp_frame->len is
unfortunately not enough.  We will need the *maximum* length touch by
(1) CPU and (2) remote device DMA engine.  DMA-TX completion knows the
length for (2).  The CPU length (1) is max of original xdp_buff size
and xdp_frame->len, because BPF-helpers could have shrinked the size.
(tricky part is that xdp_frame->len isn't correct in-case of header
adjustments, thus like mvneta_run_xdp we to calc dma_sync size, and
store this in xdp_frame, maybe via param to xdp_do_redirect). Well, not
sure if it is too much work to transfer this info, for this use-case.

> +
> +		page_ring[len++] = page;

> +	}
> +
> +	page_pool_ring_lock(pool);
> +	for (i = 0; i < len; i++) {
> +		if (__ptr_ring_produce(&pool->ring, page_ring[i]))
> +			page_pool_return_page(pool, page_ring[i]);
> +	}
> +	page_pool_ring_unlock(pool);
> +}
> +EXPORT_SYMBOL(page_pool_put_page_bulk);

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