linux-kernel - Re: [PATCH net 3/3] net: enetc: reset xdp_tx_in

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Message-ID: <20240927143308.emkgu7x5ybjnqaty@skbuf>
Date: Fri, 27 Sep 2024 17:33:08 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Wei Fang <wei.fang@....com>
Cc: Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	Claudiu Manoil <claudiu.manoil@....com>,
	"ast@...nel.org" <ast@...nel.org>,
	"daniel@...earbox.net" <daniel@...earbox.net>,
	"hawk@...nel.org" <hawk@...nel.org>,
	"john.fastabend@...il.com" <john.fastabend@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>
Subject: Re: [PATCH net 3/3] net: enetc: reset xdp_tx_in_flight when updating
 bpf program

Hi Wei,

On Mon, Sep 23, 2024 at 04:59:56AM +0300, Wei Fang wrote:
> Okay, I have tested this solution (see changes below), and from what I observed,
> the xdp_tx_in_flight can naturally drop to 0 in every test. So if there are no other
> risks, the next version will use this solution.
> 

Sorry for the delay. I have tested this variant and it works. Just one
thing below.

> @@ -2467,10 +2469,6 @@ void enetc_start(struct net_device *ndev)
>         struct enetc_ndev_priv *priv = netdev_priv(ndev);
>         int i;
> 
> -       enetc_setup_interrupts(priv);
> -
> -       enetc_enable_tx_bdrs(priv);
> -
>         for (i = 0; i < priv->bdr_int_num; i++) {
>                 int irq = pci_irq_vector(priv->si->pdev,
>                                          ENETC_BDR_INT_BASE_IDX + i);
> @@ -2479,6 +2477,10 @@ void enetc_start(struct net_device *ndev)
>                 enable_irq(irq);
>         }
> 
> +       enetc_setup_interrupts(priv);
> +
> +       enetc_enable_tx_bdrs(priv);
> +
>         enetc_enable_rx_bdrs(priv);
> 
>         netif_tx_start_all_queues(ndev);
> @@ -2547,6 +2549,12 @@ void enetc_stop(struct net_device *ndev)
> 
>         enetc_disable_rx_bdrs(priv);
> 
> +       enetc_wait_bdrs(priv);
> +
> +       enetc_disable_tx_bdrs(priv);
> +
> +       enetc_clear_interrupts(priv);

Here, NAPI may still be scheduled. So if you clear interrupts, enetc_poll()
on another CPU might still have time to re-enable them. This makes the
call pointless.

Please move the enetc_clear_interrupts() call after the for() loop below
(AKA leave it where it is).

> +
>         for (i = 0; i < priv->bdr_int_num; i++) {
>                 int irq = pci_irq_vector(priv->si->pdev,
>                                          ENETC_BDR_INT_BASE_IDX + i);
> @@ -2555,12 +2563,6 @@ void enetc_stop(struct net_device *ndev)
>                 napi_synchronize(&priv->int_vector[i]->napi);
>                 napi_disable(&priv->int_vector[i]->napi);
>         }
> -
> -       enetc_wait_bdrs(priv);
> -
> -       enetc_disable_tx_bdrs(priv);
> -
> -       enetc_clear_interrupts(priv);
>  }
>  EXPORT_SYMBOL_GPL(enetc_stop);

FWIW, there are at least 2 other valid ways of solving this problem. One
has already been mentioned (reset the counter in enetc_free_rx_ring()):

@@ -2014,6 +2015,8 @@ static void enetc_free_rx_ring(struct enetc_bdr *rx_ring)
 		__free_page(rx_swbd->page);
 		rx_swbd->page = NULL;
 	}
+
+	rx_ring->xdp.xdp_tx_in_flight = 0;
 }

 static void enetc_free_rxtx_rings(struct enetc_ndev_priv *priv)

And the other would be to keep rescheduling NAPI until there are no more
pending XDP_TX frames.

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
index 3cff76923ab9..36520f8c49a6 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc.c
@@ -1689,6 +1689,7 @@ static int enetc_poll(struct napi_struct *napi, int budget)
 		work_done = enetc_clean_rx_ring_xdp(rx_ring, napi, budget, prog);
 	else
 		work_done = enetc_clean_rx_ring(rx_ring, napi, budget);
-	if (work_done == budget)
+	if (work_done == budget || rx_ring->xdp.xdp_tx_in_flight)
 		complete = false;
 	if (work_done)

But I like your second proposal the best. It doesn't involve adding an
unnecessary extra test in the fast path.