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

From: Furong Xu <[email-masked]>
Date: Wed, 15 Jan 2025 10:33:58 +0800

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

> 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.

After switch to net_prefetch(), I get another 4.5% throughput improvement :)
Thanks! This definitely worth a v3 of this series.

pw-bot: changes-requested