netdev - Re: [PATCH net-next 4/4] dpaa2-eth: use bulk enqueue in .ndo_xdp

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Message-ID: <20200422091226.774b0dd7@carbon>
Date:   Wed, 22 Apr 2020 09:12:26 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Ioana Ciornei <ioana.ciornei@....com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, brouer@...hat.com
Subject: Re: [PATCH net-next 4/4] dpaa2-eth: use bulk enqueue in
 .ndo_xdp_xmit

On Tue, 21 Apr 2020 18:21:54 +0300
Ioana Ciornei <ioana.ciornei@....com> wrote:

> Take advantage of the bulk enqueue feature in .ndo_xdp_xmit.
> We cannot use the XDP_XMIT_FLUSH since the architecture is not capable
> to store all the frames dequeued in a NAPI cycle so we instead are
> enqueueing all the frames received in a ndo_xdp_xmit call right away.
> 
> After setting up all FDs for the xdp_frames received, enqueue multiple
> frames at a time until all are sent or the maximum number of retries is
> hit.
> 
> Signed-off-by: Ioana Ciornei <ioana.ciornei@....com>
> ---
>  .../net/ethernet/freescale/dpaa2/dpaa2-eth.c  | 60 ++++++++++---------
>  1 file changed, 32 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> index 9a0432cd893c..08b4efad46fd 100644
> --- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> +++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> @@ -1933,12 +1933,12 @@ static int dpaa2_eth_xdp_xmit(struct net_device *net_dev, int n,
>  			      struct xdp_frame **frames, u32 flags)
>  {
>  	struct dpaa2_eth_priv *priv = netdev_priv(net_dev);
> +	int total_enqueued = 0, retries = 0, enqueued;
>  	struct dpaa2_eth_drv_stats *percpu_extras;
>  	struct rtnl_link_stats64 *percpu_stats;
> +	int num_fds, i, err, max_retries;
>  	struct dpaa2_eth_fq *fq;
> -	struct dpaa2_fd fd;
> -	int drops = 0;
> -	int i, err;
> +	struct dpaa2_fd *fds;
>  
>  	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
>  		return -EINVAL;
> @@ -1946,41 +1946,45 @@ static int dpaa2_eth_xdp_xmit(struct net_device *net_dev, int n,
>  	if (!netif_running(net_dev))
>  		return -ENETDOWN;
>  
> +	/* create the array of frame descriptors */
> +	fds = kcalloc(n, sizeof(*fds), GFP_ATOMIC);

I don't like that you have an allocation on the transmit fast-path.

There are a number of ways you can avoid this.

Option (1) Given we know that (currently) devmap will max bulk 16
xdp_frames, we can have a call-stack local array with struct dpaa2_fd,
that contains 16 elements, sizeof(struct dpaa2_fd)==32 bytes times 16 is
512 bytes, so it might be acceptable.  (And add code to alloc if n >
16, to be compatible with someone increasing max bulk in devmap).

Option (2) extend struct dpaa2_eth_priv with an array of 16 struct
dpaa2_fd's, that can be used as fds storage.

> +	if (!fds)
> +		return -ENOMEM;
> +

[...]
> +	kfree(fds);


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