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

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 29 Oct 2020 11:31:48 +0100

> On Tue, 27 Oct 2020 20:04:08 +0100
> Lorenzo Bianconi <[email-masked]> 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?

I agree, I will try to reuse the data array for that

> 
> > +	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.

I was thinking about that but I guess point (1) is tricky since "cpu length"
can be changed even in the middle by devmaps or cpumaps (not just in the driver
rx napi loop). I guess we can try to address this point in a subsequent series.
Agree?

Regards,
Lorenzo

> 
> > +
> > +		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
>