Subject: Re: [PATCH net-next v2 3/3] net: stmmac: Optimize cache prefetch in RX path

From: Joe Damato <[email-masked]>
Date: Tue, 14 Jan 2025 15:31:05 -0800

On Mon, Jan 13, 2025 at 10:20:31PM +0800, Furong Xu wrote:
> Current code prefetches cache lines for the received frame first, and
> then dma_sync_single_for_cpu() against this frame, this is wrong.
> Cache prefetch should be triggered after dma_sync_single_for_cpu().
> 
> This patch brings ~2.8% driver performance improvement in a TCP RX
> throughput test with iPerf tool on a single isolated Cortex-A65 CPU
> core, 2.84 Gbits/sec increased to 2.92 Gbits/sec.
> 
> Signed-off-by: Furong Xu <[email-masked]>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index ca340fd8c937..b60f2f27140c 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -5500,10 +5500,6 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>  
>  		/* Buffer is good. Go on. */
>  
> -		prefetch(page_address(buf->page) + buf->page_offset);
> -		if (buf->sec_page)
> -			prefetch(page_address(buf->sec_page));
> -
>  		buf1_len = stmmac_rx_buf1_len(priv, p, status, len);
>  		len += buf1_len;
>  		buf2_len = stmmac_rx_buf2_len(priv, p, status, len);
> @@ -5525,6 +5521,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>  
>  			dma_sync_single_for_cpu(priv->device, buf->addr,
>  						buf1_len, dma_dir);
> +			prefetch(page_address(buf->page) + buf->page_offset);

Minor nit: I've seen in other drivers authors using net_prefetch.
Probably not worth a re-roll just for something this minor.