netdev - RE: [EXT] [PATCH net] net: ethernet: fec: prevent tx starvation under high rx load

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Date:   Tue, 30 Jun 2020 06:28:40 +0000
From:   Andy Duan <fugang.duan@....com>
To:     Tobias Waldekranz <tobias@...dekranz.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [EXT] [PATCH net] net: ethernet: fec: prevent tx starvation under
 high rx load

From: Tobias Waldekranz <tobias@...dekranz.com> Sent: Tuesday, June 30, 2020 3:16 AM
> In the ISR, we poll the event register for the queues in need of service and
> then enter polled mode. After this point, the event register will never be read
> again until we exit polled mode.
> 
> In a scenario where a UDP flow is routed back out through the same interface,
> i.e. "router-on-a-stick" we'll typically only see an rx queue event initially.
> Once we start to process the incoming flow we'll be locked polled mode, but
> we'll never clean the tx rings since that event is never caught.
> 
> Eventually the netdev watchdog will trip, causing all buffers to be dropped and
> then the process starts over again.
> 
> By adding a poll of the active events at each NAPI call, we avoid the
> starvation.
> 
> Fixes: 4d494cdc92b3 ("net: fec: change data structure to support
> multiqueue")
> Signed-off-by: Tobias Waldekranz <tobias@...dekranz.com>

Acked-by: Fugang Duan <fugang.duan@....com>
> ---
>  drivers/net/ethernet/freescale/fec_main.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/fec_main.c
> b/drivers/net/ethernet/freescale/fec_main.c
> index 2d0d313ee7c5..f6e25c2d2c8c 100644
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c
> @@ -1617,8 +1617,17 @@ fec_enet_rx(struct net_device *ndev, int
> budget)  }
> 
>  static bool
> -fec_enet_collect_events(struct fec_enet_private *fep, uint int_events)
> +fec_enet_collect_events(struct fec_enet_private *fep)
>  {
> +       uint int_events;
> +
> +       int_events = readl(fep->hwp + FEC_IEVENT);
> +
> +       /* Don't clear MDIO events, we poll for those */
> +       int_events &= ~FEC_ENET_MII;
> +
> +       writel(int_events, fep->hwp + FEC_IEVENT);
> +
>         if (int_events == 0)
>                 return false;
> 
> @@ -1644,16 +1653,9 @@ fec_enet_interrupt(int irq, void *dev_id)  {
>         struct net_device *ndev = dev_id;
>         struct fec_enet_private *fep = netdev_priv(ndev);
> -       uint int_events;
>         irqreturn_t ret = IRQ_NONE;
> 
> -       int_events = readl(fep->hwp + FEC_IEVENT);
> -
> -       /* Don't clear MDIO events, we poll for those */
> -       int_events &= ~FEC_ENET_MII;
> -
> -       writel(int_events, fep->hwp + FEC_IEVENT);
> -       fec_enet_collect_events(fep, int_events);
> +       fec_enet_collect_events(fep);
> 
>         if ((fep->work_tx || fep->work_rx) && fep->link) {
>                 ret = IRQ_HANDLED;
> @@ -1674,6 +1676,8 @@ static int fec_enet_rx_napi(struct napi_struct
> *napi, int budget)
>         struct fec_enet_private *fep = netdev_priv(ndev);
>         int pkts;
> 
> +       fec_enet_collect_events(fep);
> +
>         pkts = fec_enet_rx(ndev, budget);
> 
>         fec_enet_tx(ndev);
> --
> 2.17.1