Subject: Re: [PATCH v3 net-next 1/5] net: xdp: introduce bulking for xdp tx return path

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 4 Nov 2020 12:11:58 +0100

On Wed,  4 Nov 2020 11:22:54 +0100
Lorenzo Bianconi <[email-masked]> wrote:

> XDP bulk APIs introduce a defer/flush mechanism to return
> pages belonging to the same xdp_mem_allocator object
> (identified via the mem.id field) in bulk to optimize
> I-cache and D-cache since xdp_return_frame is usually run
> inside the driver NAPI tx completion loop.
> The bulk queue size is set to 16 to be aligned to how
> XDP_REDIRECT bulking works. The bulk is flushed when
> it is full or when mem.id changes.
> xdp_frame_bulk is usually stored/allocated on the function
> call-stack to avoid locking penalties.
> Current implementation considers only page_pool memory model.
> 
> Suggested-by: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Lorenzo Bianconi <[email-masked]>
> ---
>  include/net/xdp.h |  9 +++++++
>  net/core/xdp.c    | 61 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 70 insertions(+)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 3814fb631d52..a1f48a73e6df 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -104,6 +104,12 @@ struct xdp_frame {
>  	struct net_device *dev_rx; /* used by cpumap */
>  };
>  
> +#define XDP_BULK_QUEUE_SIZE	16
> +struct xdp_frame_bulk {
> +	int count;
> +	void *xa;
> +	void *q[XDP_BULK_QUEUE_SIZE];
> +};
>  
>  static inline struct skb_shared_info *
>  xdp_get_shared_info_from_frame(struct xdp_frame *frame)
> @@ -194,6 +200,9 @@ struct xdp_frame *xdp_convert_buff_to_frame(struct xdp_buff *xdp)
>  void xdp_return_frame(struct xdp_frame *xdpf);
>  void xdp_return_frame_rx_napi(struct xdp_frame *xdpf);
>  void xdp_return_buff(struct xdp_buff *xdp);
> +void xdp_flush_frame_bulk(struct xdp_frame_bulk *bq);
> +void xdp_return_frame_bulk(struct xdp_frame *xdpf,
> +			   struct xdp_frame_bulk *bq);
>  
>  /* When sending xdp_frame into the network stack, then there is no
>   * return point callback, which is needed to release e.g. DMA-mapping
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 48aba933a5a8..66ac275a0360 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -380,6 +380,67 @@ void xdp_return_frame_rx_napi(struct xdp_frame *xdpf)
>  }
>  EXPORT_SYMBOL_GPL(xdp_return_frame_rx_napi);
>  
> +/* XDP bulk APIs introduce a defer/flush mechanism to return
> + * pages belonging to the same xdp_mem_allocator object
> + * (identified via the mem.id field) in bulk to optimize
> + * I-cache and D-cache.
> + * The bulk queue size is set to 16 to be aligned to how
> + * XDP_REDIRECT bulking works. The bulk is flushed when

If this is connected, then why have you not redefined DEV_MAP_BULK_SIZE?

Cc. DPAA2 maintainers as they use this define in their drivers.
You want to make sure this driver is flexible enough for future changes.

Like:

diff --git a/include/net/xdp.h b/include/net/xdp.h
index 3814fb631d52..44440a36f96f 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -245,6 +245,6 @@ bool xdp_attachment_flags_ok(struct xdp_attachment_info *info,
 void xdp_attachment_setup(struct xdp_attachment_info *info,
                          struct netdev_bpf *bpf);
 
-#define DEV_MAP_BULK_SIZE 16
+#define DEV_MAP_BULK_SIZE XDP_BULK_QUEUE_SIZE
 
 #endif /* __LINUX_NET_XDP_H__ */

> + * it is full or when mem.id changes.
> + * xdp_frame_bulk is usually stored/allocated on the function
> + * call-stack to avoid locking penalties.
> + */
> +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;
> +
> +	if (mem->type != MEM_TYPE_PAGE_POOL) {
> +		__xdp_return(xdpf->data, &xdpf->mem, false);
> +		return;
> +	}
>

I cannot make up my mind: It would be a micro-optimization to move
this if-statement to include/net/xdp.h, but it will make code harder to
read/follow, and the call you replace xdp_return_frame() is also in
xdp.c with same call to _xdp_return().  Let keep it as-is. (we can
followup with micro-optimizations)

> +	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();
> +}
> +EXPORT_SYMBOL_GPL(xdp_return_frame_bulk);
> +
>  void xdp_return_buff(struct xdp_buff *xdp)
>  {
>  	__xdp_return(xdp->data, &xdp->rxq->mem, true);

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