Subject: Re: Kernel 4.19 network performance - forwarding/routing normal users traffic

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 2 Nov 2018 12:40:37 +0100

On Fri, 2 Nov 2018 13:23:56 +0800
Aaron Lu <[email-masked]> wrote:

> On Thu, Nov 01, 2018 at 08:23:19PM +0000, Saeed Mahameed wrote:
> > On Thu, 2018-11-01 at 23:27 +0800, Aaron Lu wrote:  
> > > On Thu, Nov 01, 2018 at 10:22:13AM +0100, Jesper Dangaard Brouer
> > > wrote:
> > > ... ...  
> > > > Section copied out:
> > > > 
> > > >   mlx5e_poll_tx_cq
> > > >   |          
> > > >    --16.34%--napi_consume_skb
> > > >              |          
> > > >              |--12.65%--__free_pages_ok
> > > >              |          |          
> > > >              |           --11.86%--free_one_page
> > > >              |                     |          
> > > >              |                     |--10.10%
> > > > --queued_spin_lock_slowpath
> > > >              |                     |          
> > > >              |                      --0.65%--_raw_spin_lock  
> > > 
> > > This callchain looks like it is freeing higher order pages than order
> > > 0:
> > > __free_pages_ok is only called for pages whose order are bigger than
> > > 0.  
> > 
> > mlx5 rx uses only order 0 pages, so i don't know where these high order
> > tx SKBs are coming from..   
> 
> Perhaps here:
> __netdev_alloc_skb(), __napi_alloc_skb(), __netdev_alloc_frag() and
> __napi_alloc_frag() will all call page_frag_alloc(), which will use
> __page_frag_cache_refill() to get an order 3 page if possible, or fall
> back to an order 0 page if order 3 page is not available.
> 
> I'm not sure if your workload will use the above code path though.

TL;DR: this is order-0 pages (code-walk trough proof below)

To Aaron, the network stack *can* call __free_pages_ok() with order-0
pages, via:

static void skb_free_head(struct sk_buff *skb)
{
	unsigned char *head = skb->head;

	if (skb->head_frag)
		skb_free_frag(head);
	else
		kfree(head);
}

static inline void skb_free_frag(void *addr)
{
	page_frag_free(addr);
}

/*
 * Frees a page fragment allocated out of either a compound or order 0 page.
 */
void page_frag_free(void *addr)
{
	struct page *page = virt_to_head_page(addr);

	if (unlikely(put_page_testzero(page)))
		__free_pages_ok(page, compound_order(page));
}
EXPORT_SYMBOL(page_frag_free);

Notice for the mlx5 driver it support several RX-memory models, so it
can be hard to follow, but from the perf report output we can see that
is uses mlx5e_skb_from_cqe_linear, which use build_skb.

--13.63%--mlx5e_skb_from_cqe_linear
          |          
           --5.02%--build_skb
                     |          
                      --1.85%--__build_skb
                                |          
                                 --1.00%--kmem_cache_alloc

/* build_skb() is wrapper over __build_skb(), that specifically
 * takes care of skb->head and skb->pfmemalloc
 * This means that if @frag_size is not zero, then @data must be backed
 * by a page fragment, not kmalloc() or vmalloc()
 */
struct sk_buff *build_skb(void *data, unsigned int frag_size)
{
	struct sk_buff *skb = __build_skb(data, frag_size);

	if (skb && frag_size) {
		skb->head_frag = 1;
		if (page_is_pfmemalloc(virt_to_head_page(data)))
			skb->pfmemalloc = 1;
	}
	return skb;
}
EXPORT_SYMBOL(build_skb);

It still doesn't prove, that the @data is backed by by a order-0 page.
For the mlx5 driver is uses mlx5e_page_alloc_mapped ->
page_pool_dev_alloc_pages(), and I can see perf report using
__page_pool_alloc_pages_slow().

The setup for page_pool in mlx5 uses order=0.

	/* Create a page_pool and register it with rxq */
	pp_params.order     = 0;
	pp_params.flags     = 0; /* No-internal DMA mapping in page_pool */
	pp_params.pool_size = pool_size;
	pp_params.nid       = cpu_to_node(c->cpu);
	pp_params.dev       = c->pdev;
	pp_params.dma_dir   = rq->buff.map_dir;

	/* page_pool can be used even when there is no rq->xdp_prog,
	 * given page_pool does not handle DMA mapping there is no
	 * required state to clear. And page_pool gracefully handle
	 * elevated refcnt.
	 */
	rq->page_pool = page_pool_create(&pp_params);
	if (IS_ERR(rq->page_pool)) {
		err = PTR_ERR(rq->page_pool);
		rq->page_pool = NULL;
		goto err_free;
	}
	err = xdp_rxq_info_reg_mem_model(&rq->xdp_rxq,
					 MEM_TYPE_PAGE_POOL, rq->page_pool);

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