Subject: Re: [PATCH net v3] ixgbe: fix double clean of tx descriptors with xdp

From: Maciej Fijalkowski <[email-masked]>
Date: Mon, 26 Aug 2019 15:40:42 +0200

On Thu, 22 Aug 2019 20:12:37 +0300
Ilya Maximets <[email-masked]> wrote:

> Tx code doesn't clear the descriptors' status after cleaning.
> So, if the budget is larger than number of used elems in a ring, some
> descriptors will be accounted twice and xsk_umem_complete_tx will move
> prod_tail far beyond the prod_head breaking the completion queue ring.
> 
> Fix that by limiting the number of descriptors to clean by the number
> of used descriptors in the tx ring.
> 
> 'ixgbe_clean_xdp_tx_irq()' function refactored to look more like
> 'ixgbe_xsk_clean_tx_ring()' since we're allowed to directly use
> 'next_to_clean' and 'next_to_use' indexes.
> 
> Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support")
> Signed-off-by: Ilya Maximets <[email-masked]>
> ---
> 
> Version 3:
>   * Reverted some refactoring made for v2.
>   * Eliminated 'budget' for tx clean.
>   * prefetch returned.
> 
> Version 2:
>   * 'ixgbe_clean_xdp_tx_irq()' refactored to look more like
>     'ixgbe_xsk_clean_tx_ring()'.
> 
>  drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 29 ++++++++------------
>  1 file changed, 11 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> index 6b609553329f..a3b6d8c89127 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> @@ -633,19 +633,17 @@ static void ixgbe_clean_xdp_tx_buffer(struct ixgbe_ring *tx_ring,
>  bool ixgbe_clean_xdp_tx_irq(struct ixgbe_q_vector *q_vector,
>  			    struct ixgbe_ring *tx_ring, int napi_budget)

While you're at it, can you please as well remove the 'napi_budget' argument?
It wasn't used at all even before your patch.

I'm jumping late in, but I was really wondering and hesitated with taking
part in discussion since the v1 of this patch - can you elaborate why simply
clearing the DD bit wasn't sufficient?

Maciej

>  {
> +	u16 ntc = tx_ring->next_to_clean, ntu = tx_ring->next_to_use;
>  	unsigned int total_packets = 0, total_bytes = 0;
> -	u32 i = tx_ring->next_to_clean, xsk_frames = 0;
> -	unsigned int budget = q_vector->tx.work_limit;
>  	struct xdp_umem *umem = tx_ring->xsk_umem;
>  	union ixgbe_adv_tx_desc *tx_desc;
>  	struct ixgbe_tx_buffer *tx_bi;
> -	bool xmit_done;
> +	u32 xsk_frames = 0;
>  
> -	tx_bi = &tx_ring->tx_buffer_info[i];
> -	tx_desc = IXGBE_TX_DESC(tx_ring, i);
> -	i -= tx_ring->count;
> +	tx_bi = &tx_ring->tx_buffer_info[ntc];
> +	tx_desc = IXGBE_TX_DESC(tx_ring, ntc);
>  
> -	do {
> +	while (ntc != ntu) {
>  		if (!(tx_desc->wb.status & cpu_to_le32(IXGBE_TXD_STAT_DD)))
>  			break;
>  
> @@ -661,22 +659,18 @@ bool ixgbe_clean_xdp_tx_irq(struct ixgbe_q_vector *q_vector,
>  
>  		tx_bi++;
>  		tx_desc++;
> -		i++;
> -		if (unlikely(!i)) {
> -			i -= tx_ring->count;
> +		ntc++;
> +		if (unlikely(ntc == tx_ring->count)) {
> +			ntc = 0;
>  			tx_bi = tx_ring->tx_buffer_info;
>  			tx_desc = IXGBE_TX_DESC(tx_ring, 0);
>  		}
>  
>  		/* issue prefetch for next Tx descriptor */
>  		prefetch(tx_desc);
> +	}
>  
> -		/* update budget accounting */
> -		budget--;
> -	} while (likely(budget));
> -
> -	i += tx_ring->count;
> -	tx_ring->next_to_clean = i;
> +	tx_ring->next_to_clean = ntc;
>  
>  	u64_stats_update_begin(&tx_ring->syncp);
>  	tx_ring->stats.bytes += total_bytes;
> @@ -688,8 +682,7 @@ bool ixgbe_clean_xdp_tx_irq(struct ixgbe_q_vector *q_vector,
>  	if (xsk_frames)
>  		xsk_umem_complete_tx(umem, xsk_frames);
>  
> -	xmit_done = ixgbe_xmit_zc(tx_ring, q_vector->tx.work_limit);
> -	return budget > 0 && xmit_done;
> +	return ixgbe_xmit_zc(tx_ring, q_vector->tx.work_limit);
>  }
>  
>  int ixgbe_xsk_async_xmit(struct net_device *dev, u32 qid)