linux-kernel - Re: [PATCH v2] ntb_netdev: Use dev_kfree_skb

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Date:   Thu, 8 Dec 2022 17:20:32 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     epilmore@...aio.com, netdev@...r.kernel.org,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        ntb@...ts.linux.dev, allenbh@...il.com, jdmason@...zu.us
Subject: Re: [PATCH v2] ntb_netdev: Use dev_kfree_skb_any() in interrupt
 context



On 12/8/2022 5:06 PM, epilmore@...aio.com wrote:
> From: Eric Pilmore <epilmore@...aio.com>
> 
> TX/RX callback handlers (ntb_netdev_tx_handler(),
> ntb_netdev_rx_handler()) can be called in interrupt
> context via the DMA framework when the respective
> DMA operations have completed. As such, any calls
> by these routines to free skb's, should use the
> interrupt context safe dev_kfree_skb_any() function.
> 
> Previously, these callback handlers would call the
> interrupt unsafe version of dev_kfree_skb(). This has
> not presented an issue on Intel IOAT DMA engines as
> that driver utilizes tasklets rather than a hard
> interrupt handler, like the AMD PTDMA DMA driver.
> On AMD systems, a kernel WARNING message is
> encountered, which is being issued from
> skb_release_head_state() due to in_hardirq()
> being true.
> 
> Besides the user visible WARNING from the kernel,
> the other symptom of this bug was that TCP/IP performance
> across the ntb_netdev interface was very poor, i.e.
> approximately an order of magnitude below what was
> expected. With the repair to use dev_kfree_skb_any(),
> kernel WARNINGs from skb_release_head_state() ceased
> and TCP/IP performance, as measured by iperf, was on
> par with expected results, approximately 20 Gb/s on
> AMD Milan based server. Note that this performance
> is comparable with Intel based servers.
> 
> Fixes: 765ccc7bc3d91 ("ntb_netdev: correct skb leak")
> Fixes: 548c237c0a997 ("net: Add support for NTB virtual ethernet device")
> Signed-off-by: Eric Pilmore <epilmore@...aio.com>

Reviewed-by: Dave Jiang <dave.jiang@...el.com>

As an FYI for future. Typically you would add the patch revision change 
log under the "---" line just as an FYI for reviewers on what you've 
changed and who suggested the change.

> ---

i.e.

v2:
- Use dev_kfree_skb_any() instead of dev_kfree_skb_irq(). (DaveJ)


>   drivers/net/ntb_netdev.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ntb_netdev.c b/drivers/net/ntb_netdev.c
> index 80bdc07f2cd3..59250b7accfb 100644
> --- a/drivers/net/ntb_netdev.c
> +++ b/drivers/net/ntb_netdev.c
> @@ -137,7 +137,7 @@ static void ntb_netdev_rx_handler(struct ntb_transport_qp *qp, void *qp_data,
>   enqueue_again:
>   	rc = ntb_transport_rx_enqueue(qp, skb, skb->data, ndev->mtu + ETH_HLEN);
>   	if (rc) {
> -		dev_kfree_skb(skb);
> +		dev_kfree_skb_any(skb);
>   		ndev->stats.rx_errors++;
>   		ndev->stats.rx_fifo_errors++;
>   	}
> @@ -192,7 +192,7 @@ static void ntb_netdev_tx_handler(struct ntb_transport_qp *qp, void *qp_data,
>   		ndev->stats.tx_aborted_errors++;
>   	}
>   
> -	dev_kfree_skb(skb);
> +	dev_kfree_skb_any(skb);
>   
>   	if (ntb_transport_tx_free_entry(dev->qp) >= tx_start) {
>   		/* Make sure anybody stopping the queue after this sees the new