Re: [PATCH net-next] mvpp2: prefetch frame header

From: David Miller
Date: Wed Oct 23 2019 - 23:28:18 EST


From: Matteo Croce <mcroce@xxxxxxxxxx>
Date: Tue, 22 Oct 2019 16:14:38 +0200

> When receiving traffic, eth_type_trans() is high up on the perf top list,
> because it's the first function which access the packet data.
>
> Move the DMA unmap a bit higher, and put a prefetch just after it, so we
> have more time to load the data into the cache.
>
> The packet rate increase is about 13% with a tc drop test: 1620 => 1830 kpps
>
> Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxx>
> ---
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> index 111b3b8239e1..17378e0d8da1 100644
> --- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> +++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> @@ -2966,6 +2966,11 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
> continue;
> }
>
> + dma_unmap_single(dev->dev.parent, dma_addr,
> + bm_pool->buf_size, DMA_FROM_DEVICE);
> +
> + prefetch(data);
> +
> if (bm_pool->frag_size > PAGE_SIZE)
> frag_size = 0;
> else

You cannot unmap it this early, because of all of the err_drop_frame
code paths that might be taken next. The DMA mapping must stay in place
in those cases.